How to export DataTable to Excel

How can I export a DataTable to Excel in C#? I am using Windows Forms. The DataTable is associated with a DataGridView control. I have to export records of DataTable to Excel.

I would recommend ClosedXML -

You can turn a DataTable into an Excel worksheet with some very readable code:

XLWorkbook wb = new XLWorkbook();
DataTable dt = GetDataTableOrWhatever();
wb.Worksheets.Add(dt,"WorksheetName");

The developer is responsive and helpful. The project is actively developed, and the documentation is superb.

Try simple code, to convert DataTable to excel file as csv:

var lines = new List<string>();

string[] columnNames = dataTable.Columns
	.Cast<DataColumn>()
	.Select(column => column.ColumnName)
	.ToArray();

var header = string.Join(",", columnNames.Select(name => $"\"{name}\""));
lines.Add(header);

var valueLines = dataTable.AsEnumerable()
	.Select(row => string.Join(",", row.ItemArray.Select(val => $"\"{val}\"")));

lines.AddRange(valueLines);

File.WriteAllLines("excel.csv", lines);

This will write a new file excel.csv into the current working directory which is generally either where the .exe is or where you launch it from.

An elegant option is writing an extension method (see below) for the DataTable class of .net framework.

This extention method can be called as follows:

using System;
using System.Collections.Generic;
using System.Linq;
using Excel = Microsoft.Office.Interop.Excel;
using System.Data;
using System.Data.OleDb;

DataTable dt;
// fill table data in dt here 
...

// export DataTable to excel
// save excel file without ever making it visible if filepath is given
// don't save excel file, just make it visible if no filepath is given
dt.ExportToExcel(ExcelFilePath);

Extension method for DataTable class:

public static class My_DataTable_Extensions
{

	// Export DataTable into an excel file with field names in the header line
	// - Save excel file without ever making it visible if filepath is given
	// - Don't save excel file, just make it visible if no filepath is given
    public static void ExportToExcel(this DataTable tbl, string excelFilePath = null) {
        try {
            if (tbl == null || tbl.Columns.Count == 0)
                throw new Exception("ExportToExcel: Null or empty input table!\n");

            // load excel, and create a new workbook
            var excelApp = new Excel.Application();
            excelApp.Workbooks.Add();

            // single worksheet
            Excel._Worksheet workSheet = excelApp.ActiveSheet;

            // column headings
            for (var i = 0; i < tbl.Columns.Count; i++) {
                workSheet.Cells[1, i + 1] = tbl.Columns[i].ColumnName;
            }

            // rows
            for (var i = 0; i < tbl.Rows.Count; i++) {
                // to do: format datetime values before printing
                for (var j = 0; j < tbl.Columns.Count; j++) {
                    workSheet.Cells[i + 2, j + 1] = tbl.Rows[i][j];
                }
            }

            // check file path
            if (!string.IsNullOrEmpty(excelFilePath)) {
                try {
                    workSheet.SaveAs(excelFilePath);
                    excelApp.Quit();
                    MessageBox.Show("Excel file saved!");
                }
                catch (Exception ex) {
                    throw new Exception("ExportToExcel: Excel file could not be saved! Check filepath.\n"
                                        + ex.Message);
                }
            } else { // no file path is given
                excelApp.Visible = true;
            }
        }
        catch (Exception ex) {
            throw new Exception("ExportToExcel: \n" + ex.Message);
        }
    }
}

Solution based on tuncalik (thanks for idea) article, but in case of big tables is working much more faster (and is a little less clear).

public static class My_DataTable_Extensions
{
    /// <summary>
    /// Export DataTable to Excel file
    /// </summary>
    /// <param name="DataTable">Source DataTable</param>
    /// <param name="ExcelFilePath">Path to result file name</param>
    public static void ExportToExcel(this System.Data.DataTable DataTable, string ExcelFilePath = null)
    {
        try
        {
            int ColumnsCount;

            if (DataTable == null || (ColumnsCount = DataTable.Columns.Count) == 0)
                throw new Exception("ExportToExcel: Null or empty input table!\n");

            // load excel, and create a new workbook
            Microsoft.Office.Interop.Excel.Application Excel = new Microsoft.Office.Interop.Excel.Application();
            Excel.Workbooks.Add();

            // single worksheet
            Microsoft.Office.Interop.Excel._Worksheet Worksheet = Excel.ActiveSheet;

            object[] Header = new object[ColumnsCount];

            // column headings               
            for (int i = 0; i < ColumnsCount; i++)
                Header[i] = DataTable.Columns[i].ColumnName;

            Microsoft.Office.Interop.Excel.Range HeaderRange = Worksheet.get_Range((Microsoft.Office.Interop.Excel.Range)(Worksheet.Cells[1, 1]), (Microsoft.Office.Interop.Excel.Range)(Worksheet.Cells[1, ColumnsCount]));
            HeaderRange.Value = Header;
            HeaderRange.Interior.Color = System.Drawing.ColorTranslator.ToOle(System.Drawing.Color.LightGray);
            HeaderRange.Font.Bold = true;

            // DataCells
            int RowsCount = DataTable.Rows.Count;
            object[,] Cells = new object[RowsCount, ColumnsCount];

            for (int j = 0; j < RowsCount; j++)
                for (int i = 0; i < ColumnsCount; i++)
                    Cells[j, i] = DataTable.Rows[j][i];

            Worksheet.get_Range((Microsoft.Office.Interop.Excel.Range)(Worksheet.Cells[2, 1]), (Microsoft.Office.Interop.Excel.Range)(Worksheet.Cells[RowsCount + 1, ColumnsCount])).Value = Cells;

            // check fielpath
            if (ExcelFilePath != null && ExcelFilePath != "")
            {
                try
                {
                    Worksheet.SaveAs(ExcelFilePath);
                    Excel.Quit();
                    System.Windows.MessageBox.Show("Excel file saved!");
                }
                catch (Exception ex)
                {
                    throw new Exception("ExportToExcel: Excel file could not be saved! Check filepath.\n"
                        + ex.Message);
                }
            }
            else    // no filepath is given
            {
                Excel.Visible = true;
            }
        }
        catch (Exception ex)
        {
            throw new Exception("ExportToExcel: \n" + ex.Message);
        }
    }
}

Try this function pass the datatable and file path where you want to export

public void CreateCSVFile(ref DataTable dt, string strFilePath)
{            
    try
    {
        // Create the CSV file to which grid data will be exported.
        StreamWriter sw = new StreamWriter(strFilePath, false);
        // First we will write the headers.
        //DataTable dt = m_dsProducts.Tables[0];
        int iColCount = dt.Columns.Count;
        for (int i = 0; i < iColCount; i++)
        {
            sw.Write(dt.Columns[i]);
            if (i < iColCount - 1)
            {
                sw.Write(",");
            }
        }
        sw.Write(sw.NewLine);

        // Now write all the rows.

        foreach (DataRow dr in dt.Rows)
        {
            for (int i = 0; i < iColCount; i++)
            {
                if (!Convert.IsDBNull(dr[i]))
                {
                    sw.Write(dr[i].ToString());
                }
                if (i < iColCount - 1)
                {
                    sw.Write(",");
                }
            }

            sw.Write(sw.NewLine);
        }
        sw.Close();
    }
    catch (Exception ex)
    {
        throw ex;
    }
}

Using DocumentFormat.OpenXml nuget package, I have created a singleton class which handles export to excel from either a DataTable or DataSet. Tables will be represented as separate sheets in a workbook.

The main class

public sealed class OfficeOpenXML
    {
        private static Lazy<OfficeOpenXML> _instance = new Lazy<OfficeOpenXML>(() => new OfficeOpenXML());
        private OfficeOpenXML()
        {

        }
        public static OfficeOpenXML GetInstance()
        {
            return _instance.Value;
        }

        public MemoryStream GetExcelStream(DataSet ds, bool firstRowAsHeader = false)
        {
            if (ds == null || ds.Tables.Count == 0)
            {
                return null;
            }

            MemoryStream stream = new MemoryStream();
            using (var excel = SpreadsheetDocument.Create(stream, SpreadsheetDocumentType.Workbook))
            {
                //create doc and workbook
                WorkbookPart workbookPart = excel.AddWorkbookPart();
                Workbook workbook = new Workbook();
                Sheets sheets = new Sheets();
                //loop all tables in the dataset
                for (int iTable = 0; iTable < ds.Tables.Count; iTable++)
                {
                    var table = ds.Tables[iTable];
                    //create sheet part
                    WorksheetPart worksheetPart = workbookPart.AddNewPart<WorksheetPart>();
                    Worksheet worksheet = new Worksheet();
                    SheetData data = new SheetData();
                    List<Row> allRows = new List<Row>();

                    //setting header of the sheet
                    Row headerRow = new Row() { RowIndex = 1 };
                    for (int iColumn = 0; iColumn < table.Columns.Count; iColumn++)
                    {
                        var col = table.Columns[iColumn];
                        //if first row of table is not the header then set columns of table as header of sheet
                        if (!firstRowAsHeader)
                        {
                            headerRow.Append(new Cell
                            {
                                DataType = CellValues.String,
                                CellValue = new CellValue(col.ColumnName)
                            });
                        }
                        else
                        {
                            headerRow.Append(new Cell
                            {
                                DataType = CellValues.String,
                                CellValue = new CellValue(Convert.ToString(table.Rows[0][col]))
                            });
                        }
                    }
                    allRows.Add(headerRow);

                    //setting other data rows
                    if (table.Rows != null && table.Rows.Count != 0)
                    {
                        for (int iRow = firstRowAsHeader ? 1 : 0; iRow < table.Rows.Count; iRow++)
                        {
                            var row = table.Rows[iRow];
                            Row valueRow = new Row { RowIndex = (uint)(iRow + (firstRowAsHeader ? 1 : 2)) };

                            for (int iColumn = 0; iColumn < table.Columns.Count; iColumn++)
                            {
                                var col = table.Columns[iColumn];
                                valueRow.Append(new Cell
                                {
                                    DataType = Format(col.DataType),
                                    CellValue = new CellValue(Convert.ToString(row[col]))
                                });
                            }
                            allRows.Add(valueRow);
                        }
                    }

                    //add rows to the data
                    data.Append(allRows);
                    worksheet.Append(data);
                    worksheetPart.Worksheet = worksheet;
                    worksheetPart.Worksheet.Save();

                    //add worksheet to main sheets
                    sheets.Append(new Sheet
                    {
                        Name = string.IsNullOrWhiteSpace(table.TableName) ? "Sheet" + (iTable + 1) : table.TableName,
                        Id = workbookPart.GetIdOfPart(worksheetPart),
                        SheetId = (uint)iTable + 1
                    });
                }//single table processing ends here

                //add created sheets to workbook
                workbook.Append(sheets);
                excel.WorkbookPart.Workbook = workbook;
                excel.WorkbookPart.Workbook.Save();


                excel.Close();
            }
            stream.Seek(0, SeekOrigin.Begin);  
            stream.Capacity = (int)stream.Length;  
            return stream;


        }
        public MemoryStream GetExcelStream(DataTable dt, bool firstRowAsHeader = false)
        {
            DataSet ds = new DataSet();
            ds.Tables.Add(dt);
            return GetExcelStream(ds, firstRowAsHeader);
        }



        #region Excel Helpers

        CellValues Format(Type t)
        {

            switch (t.ToString())
            {

                case "System.String":
                    return CellValues.String;
                case "System.DateTime":
                    return CellValues.Date;
                case "System.Boolean":
                    return CellValues.Boolean;
                case "System.Int16":
                    return CellValues.Number;
                case "System.Int32":
                    return CellValues.Number;
                case "System.Int64":
                    return CellValues.Number;
                case "System.UInt16":
                    return CellValues.Number;
                case "System.UInt32":
                    return CellValues.Number;
                case "System.UInt64":
                    return CellValues.Number;
                case "System.Decimal":
                    return CellValues.Number;
                case "System.Double":
                    return CellValues.Number;
                case "System.Single":
                    return CellValues.Number;
                default:
                    return CellValues.String;
            }
        }
        #endregion
    }

Save to a file

        var excelApp = OfficeOpenXML.GetInstance();
        var dt = GetDataTableFromDB();
        using (var stream = excelApp.GetExcelStream(dt, false))//use true to hide datatable columns from excel
        {

            using (FileStream fs = new FileStream(@"C:\Users\Public\myexcel.xlsx", FileMode.Create))
            {
                stream.CopyTo(fs);
                fs.Flush();
            }
        }

Download in an MVC application

public ActionResult DownloadReport()
{
    var ds = GetDataSetFromDB();
    var excelApp = OfficeOpenXML.GetInstance();
    var file = excelApp.GetExcelStream(ds, false);
    return File(file, "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet", Guid.NewGuid().ToString() + ".xlsx");
}

The best and easiest way

private void exportToExcel(DataTable dt)
    {
       
        /*Set up work book, work sheets, and excel application*/
        Microsoft.Office.Interop.Excel.Application oexcel = new Microsoft.Office.Interop.Excel.Application();
        try
        {
            string path = AppDomain.CurrentDomain.BaseDirectory;
            object misValue = System.Reflection.Missing.Value;
            Microsoft.Office.Interop.Excel.Workbook obook = oexcel.Workbooks.Add(misValue);
            Microsoft.Office.Interop.Excel.Worksheet osheet = new Microsoft.Office.Interop.Excel.Worksheet();

         
          //  obook.Worksheets.Add(misValue);

            osheet = (Microsoft.Office.Interop.Excel.Worksheet)obook.Sheets["Sheet1"];
            int colIndex = 0;
            int rowIndex = 1;

            foreach (DataColumn dc in dt.Columns)
            {
                colIndex++;
                osheet.Cells[1, colIndex] = dc.ColumnName;
            }
            foreach (DataRow dr in dt.Rows)
            {
                rowIndex++;
                colIndex = 0;

                foreach (DataColumn dc in dt.Columns)
                {
                    colIndex++;
                    osheet.Cells[rowIndex, colIndex] = dr[dc.ColumnName];
                }
            }

            osheet.Columns.AutoFit();
            string filepath = "C:\\Temp\\Book1";

            //Release and terminate excel
           
            obook.SaveAs(filepath);
            obook.Close();
            oexcel.Quit();
            releaseObject(osheet);

            releaseObject(obook);

            releaseObject(oexcel);
            GC.Collect();
        }
        catch (Exception ex)
        {
            oexcel.Quit();
            log.AddToErrorLog(ex, this.Name);
        }
    }

You can use my SwiftExcel library. It is especially good when performance and low memory usage come in place as it writes data directly to the file:

using (var ew = new ExcelWriter("C:\\temp\\test.xlsx"))
{
    for (var row = 1; row <= 100; row++)
    {
        for (var col = 1; col <= 10; col++)
        {
            ew.Write($"row:{row}-col:{col}", col, row);
        }
    }
}

Nuget command to install:

Install-Package SwiftExcel

> Just Make use of the CloseMXL.Excel Library. It's easy and pretty fast too.

Class

private DataTable getAllList()
        {
            string constr = ConfigurationManager.ConnectionStrings["RConnection"].ConnectionString;
            using (SqlConnection con = new SqlConnection(constr))
            {
                using (SqlCommand cmd = new SqlCommand("SELECT EmpId, gender, EmpName, pOnHold FROM Employee  WHERE EmpId= '"+ AnyVariable + "' ORDER BY EmpName"))
                {
                    using (SqlDataAdapter da = new SqlDataAdapter())
                    {
                        DataTable dt = new DataTable();
                        cmd.CommandType = CommandType.Text;
                        cmd.Connection = con;
                        da.SelectCommand = cmd;
                        da.Fill(dt);
                        dt.Columns[0].ColumnName = "Employee Id";
                        dt.Columns[1].ColumnName = "Gender";
                        dt.Columns[2].ColumnName = "Employee Name";
                        dt.Columns[3].ColumnName = "On Hold";
                       
                        return dt;
                    }
                }
            }
        }

Then another method which get the Dataset

public DataSet getDataSetExportToExcel()
        {
            DataSet ds = new DataSet();
            DataTable dtEmp = new DataTable("CLOT List");
            dtEmp = getAllList();
             ds.Tables.Add(dtEmp);
             ds.Tables[0].TableName = "Employee"; //If you which to use Mutliple Tabs
             return ds;
          }

Now you Button Click Event

protected void btn_Export_Click(object sender, EventArgs e)
        {
            DataSet ds = getDataSetExportToExcel();

            using (XLWorkbook wb = new XLWorkbook())
            {
                wb.Worksheets.Add(ds);
                wb.Style.Alignment.Horizontal = XLAlignmentHorizontalValues.Center;
                wb.Style.Font.Bold = true;

                Response.Clear();
                Response.Buffer = true;
                Response.Charset = "";
                Response.ContentType = "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet";
                Response.AddHeader("content-disposition", "attachment;filename=EmployeeonHoldList.xlsx");

                using (MemoryStream MyMemoryStream = new MemoryStream())
                {
                    wb.SaveAs(MyMemoryStream);
                    MyMemoryStream.WriteTo(Response.OutputStream);

                    Response.Flush();
                    Response.End();
                }
            }
        }

I wanted to add this answer because I spent a great deal of time looking for a fast, reliable method to do this and no complete examples of using OpenXMLWriter for this purpose existed anywhere that I could find.

First, COM/Interop (which many of the other answers use) is OK for this purpose, but it suffers from some sensitivities. I've used it for decades and it's mostly stable, but when implementing a data warehouse front-end for hundreds of users, I found it to be subject to too many issues depending on the machine and what the user did, so I switched to OpenXML. OpenXML DOM is fairly good for this purpose, but it's slower than using OpenXMLWriter. When you get into large datasets (100K+) with lots of columns, DOM is much slower than OpenXMLWriter, so I use the latter. The method below writes 420K+ rows with 30+ fields in less than 30 seconds.

I hope the comments are sufficient to guide anyone through what it's doing. It is simplified, in that it writes all values to the file as strings, but you can implement logic to write various datatypes (and use various cell formats) based on the content of your data. You can also adapt this for use on a DataGridView (instead of a DataTable) by changing just a few things (namely the loops through columns/rows).

A reference to DocumentFormat.OpenXML (d/l with the OpenXML SDK) and WindowsBase is required.

Imports DocumentFormat.OpenXml
Imports DocumentFormat.OpenXml.Spreadsheet
Imports DocumentFormat.OpenXml.Packaging

Public Sub ExportToExcelXML(ByRef dt As DataTable, filename As String)
    Dim wbp As WorkbookPart, wsp As WorksheetPart
    'If this DataTable has more rows in it than can fit in Excel, throw an exception
    If dt.Rows.Count > 1048575 Then Throw New Exception("The DataTable is too large to export to Excel.")
    'Delete any previous file of the same name that may exist.
    File.Delete(filename)
    'Create an OpenXML SpreadsheetDocument...
    Using xls = SpreadsheetDocument.Create(filename, SpreadsheetDocumentType.Workbook)
        'Add a WorkbookPart to the Spreadsheet Doc, then add a WorksheetPart to the WorkbookPart.
        wbp = xls.AddWorkbookPart()
        wsp = wbp.AddNewPart(Of WorksheetPart)
        'Now we need to add the "StyleSheet" to the WorkbookPart (that we just added above). This will allow us to apply formatting to our Cells.
        'Add the WbStylesPart and the StyleSheet.
        Dim stp As WorkbookStylesPart = wbp.AddNewPart(Of WorkbookStylesPart)
        Dim ss As New Stylesheet
        'Create the only two Fonts we're going to use (Regular and Bold).
        Dim fBold As New Font
        fBold.Append(New Bold)
        Dim fnts As New Fonts
        fnts.Append(New Font) 'This creates the default (unmodified, regular) Font. It's added first, so its index is 0.
        fnts.Append(fBold) 'This creates the Bold font. It's added second, so its index is 1.
        'Create the default Fill/Border settings (these have to be here, even though I don't set any custom fills/borders).
        Dim flls As New Fills
        Dim brdrs As New Borders
        flls.Append(New Fill)
        brdrs.Append(New Border)
        'Now I have to add formats (NumberFormat and CellFormat). First, you create a NumberFormat. This is basically the pattern of 
        '   the format (i.e. "@" for Text). For now, I only need a Text format, but I can add more patterns if needed.
        '   I give the format an ID of 164, since 163 is where the built-in Excel formats end.
        Dim nbrfmts As New NumberingFormats
        nbrfmts.Append(New NumberingFormat With {.NumberFormatId = 164, .FormatCode = "@"})
        'Create the first two CellFormats: Default, which will have an index of 0 and "Header" (Bold/Centered) with an index of 1.
        Dim cellfmts As New CellFormats()
        cellfmts.Append(New CellFormat() With {.FontId = 0, .NumberFormatId = 164, .FillId = 0, .BorderId = 0})
        cellfmts.Append(New CellFormat() With {.FontId = 1, .NumberFormatId = 164,
            .Alignment = New Alignment() With {.WrapText = True, .Horizontal = HorizontalAlignmentValues.Center}})
        'Add all of the Fonts/Fills/Borders/etc to the StyleSheet and add it all to the WorkbookStylesPart.
        ss.Append(fnts)
        ss.Append(flls)
        ss.Append(brdrs)
        ss.Append(cellfmts)
        ss.NumberingFormats = nbrfmts
        stp.Stylesheet = ss
        stp.Stylesheet.Save()
        'Now create an OpenXMLWriter using the WorksheetPart to write the cells to the worksheet.
        Using oxw As OpenXmlWriter = OpenXmlWriter.Create(wsp)
            'Write the start element for the Worksheet and the Columns...
            oxw.WriteStartElement(New Worksheet)
            oxw.WriteStartElement(New Columns())
            'Now I'm going to loop through the columns in the DataTable...
            For c As Integer = 0 To dt.Columns.Count - 1
                'Now we'll get the width for the column. To do this, we loop through all of the rows and measure the width of the text 
                '   using the default Excel Font (currently Font: Calibri Size: 11) and return the largest width (in pixels) to use below.
                '   Why not do this loop below (when I loop through the rows to write the Cells)? Because you can't. You have to
                '   write the Column XML first before writing the SheetData/Row/Cell XML (I confirmed this by trying it), so there's
                '   no way (that I'm aware of) to avoid looping through all of the rows twice if you want to AutoFit.
                'Setup vars we'll use for getting the column widths (below).
                Dim g = System.Drawing.Graphics.FromHwnd(IntPtr.Zero)
                Dim fnt = New System.Drawing.Font("Calibri", 11)
                Dim wid As Double = 0
                'Get the width of the header (because if this is wider than the widest value, we'll use the header text's width).
                '   I found that adding 2 pixels to the width was necessary to get the column as wide as Excel would make it.
                Dim tmp As Double = g.MeasureString(dt.Columns(c).ColumnName, New System.Drawing.Font(fnt, System.Drawing.FontStyle.Bold)).Width + 2
                'Loop through the rows in the dt and get the width of the value in that row/col. If it's wider than the widest
                '   width we've encountered thus far, use the new wider width as our basis.
                For Each row As DataRow In dt.Rows
                    If tmp > wid Then wid = tmp
                    tmp = g.MeasureString(row(c).ToString, fnt).Width
                Next
                'Set the column attributes and write it to the file. The Width is set using a formula that converts from pixels to Excel's column width values.
                Dim oxa As New List(Of OpenXmlAttribute) From {New OpenXmlAttribute("min", Nothing, c + 1), New OpenXmlAttribute("max", Nothing, c + 1),
                    New OpenXmlAttribute("width", Nothing, System.Math.Round((wid - 12 + 5) / 7D + 1, 2))}
                oxw.WriteStartElement(New Column(), oxa)
                oxw.WriteEndElement()
            Next
            'CLose out the Columns collection.
            oxw.WriteEndElement()
            'Write the start element for the SheetData...
            oxw.WriteStartElement(New SheetData)
            'Write the start element for the Header row.
            oxw.WriteStartElement(New Row)
            'Loop through the Columns in the dt.
            For Each col As DataColumn In dt.Columns
                'Write a cell for this column's Header. All Header cells are written with a DataType of String ("str"). 
                '   I ALSO apply the "Header" CellFormat (StyleIndex 1) to all of the Header Cells. This makes them Bold and Centered.
                WriteCell(oxw, col.ColumnName, "str", 1)
            Next
            'Close out the Header row.
            oxw.WriteEndElement()
            'Loop through all of the rows in the dt...
            For Each row As DataRow In dt.Rows
                'Write a StartElement for this row...
                oxw.WriteStartElement(New Row)
                'Loop through all of the columns in the dt...
                For c As Integer = 0 To dt.Columns.Count - 1
                    'Write a value in this row/column to the Excel file. I use the datatype of "String" and the default CellFormat/StyleIndex.
                    WriteCell(oxw, row(c).ToString, "str", 0)
                Next
                'Close out this row.
                oxw.WriteEndElement()
            Next
            'Close out the Worksheet and SheetData elements...
            oxw.WriteEndElement()
            oxw.WriteEndElement()
        End Using
        'Now we're going to create an OpenXMLWriter using the WorkbookPart (that we created above)...
        Using oxw As OpenXmlWriter = OpenXmlWriter.Create(wbp)
            'Add starting elements for the Workbook and Sheets collection.
            oxw.WriteStartElement(New Workbook())
            oxw.WriteStartElement(New Sheets())
            'Add the Sheet (name the Sheet after the file name minus the extension).
            oxw.WriteElement(New Sheet() With {.Name = Path.GetFileNameWithoutExtension(filename), .SheetId = 1, .Id = xls.WorkbookPart.GetIdOfPart(wsp)})
            'Write End elements for the Workbook/Sheets
            oxw.WriteEndElement()
            oxw.WriteEndElement()
        End Using
    End Using

End Sub

'This Sub is used to write a value to a Cell using OpenXMLWriter.
Private Sub WriteCell(ByRef oxw As OpenXmlWriter, value As String, datatype As String, style As UInt32Value)
    Dim oxa As New List(Of OpenXmlAttribute) From {New OpenXmlAttribute("t", Nothing, datatype), New OpenXmlAttribute("s", Nothing, style)}
    oxw.WriteStartElement(New Cell(), oxa)
    If value <> Nothing Then oxw.WriteElement(New CellValue(value))
    oxw.WriteEndElement()
End Sub

Excel Interop:

> This method prevents Dates getting flipped from dd-mm-yyyy to > mm-dd-yyyy

public bool DataTableToExcelFile(DataTable dt, string targetFile)
{
	const bool dontSave = false;
	bool success = true;

    //Exit if there is no rows to export
	if (dt.Rows.Count == 0) return false;

	object misValue = System.Reflection.Missing.Value;
	List<int> dateColIndex = new List<int>();
    Excel.Application excelApp = new Excel.Application();
	Excel.Workbook excelWorkBook = excelApp.Workbooks.Add(misValue);
	Excel.Worksheet excelWorkSheet = excelWorkBook.Sheets("sheet1");

    //Iterate through the DataTable and populate the Excel work sheet
	try {
		for (int i = -1; i <= dt.Rows.Count - 1; i++) {
			for (int j = 0; j <= dt.Columns.Count - 1; j++) {
				if (i < 0) {
					//Take special care with Date columns
					if (dt.Columns(j).DataType is typeof(DateTime)) {
						excelWorkSheet.Cells(1, j + 1).EntireColumn.NumberFormat = "d-MMM-yyyy;@";
						dateColIndex.Add(j);
					} 
                    //else if ... Feel free to add more Formats

                    else {
						//Otherwise Format the column as text
						excelWorkSheet.Cells(1, j + 1).EntireColumn.NumberFormat = "@";
					}
					excelWorkSheet.Cells(1, j + 1) = dt.Columns(j).Caption;
				} 
                else if (dateColIndex.IndexOf(j) > -1) {
					excelWorkSheet.Cells(i + 2, j + 1) = Convert.ToDateTime(dt.Rows(i).ItemArray(j)).ToString("d-MMM-yyyy");
				} 
                else {
					excelWorkSheet.Cells(i + 2, j + 1) = dt.Rows(i).ItemArray(j).ToString();
				}
			}
		}

		//Add Autofilters to the Excel work sheet  
 		excelWorkSheet.Cells.AutoFilter(1, Type.Missing, Excel.XlAutoFilterOperator.xlAnd, Type.Missing, true);
		//Autofit columns for neatness
		excelWorkSheet.Columns.AutoFit();
		if (File.Exists(exportFile)) File.Delete(exportFile);
		excelWorkSheet.SaveAs(exportFile);
	} catch {
		success = false;
	} finally {
		//Do this irrespective of whether there was an exception or not. 
		excelWorkBook.Close(dontSave);
		excelApp.Quit();
		releaseObject(excelWorkSheet);
		releaseObject(excelWorkBook);
		releaseObject(excelApp);
	}
	return success;
}

> If you dont care about Dates being flipped, then use see link that shows how to populate all the cells in the Excel spreadsheet in one line of code:

https://stackoverflow.com/questions/356371/excel-interop-efficiency-and-performance

CSV:

public string DataTableToCSV(DataTable dt, bool includeHeader, string rowFilter, string sortFilter, bool useCommaDelimiter = false, bool truncateTimesFromDates = false)
{
	dt.DefaultView.RowFilter = rowFilter;
	dt.DefaultView.Sort = sortFilter;
	DataView dv = dt.DefaultView;
	string csv = DataTableToCSV(dv.ToTable, includeHeader, useCommaDelimiter, truncateTimesFromDates);
	//reset the Filtering
	dt.DefaultView.RowFilter = string.Empty;
	return csv;
}

public string DataTableToCsv(DataTable dt, bool includeHeader, bool useCommaDelimiter = false, bool truncateTimesFromDates = false)
{
	StringBuilder sb = new StringBuilder();
	string delimter = Constants.vbTab;
	if (useCommaDelimiter)
		delimter = ",";

	if (includeHeader) {
		foreach (DataColumn dc in dt.Columns) {
			sb.AppendFormat("{0}" + Constants.vbTab, dc.ColumnName);
		}

		//remove the last Tab
		sb.Remove(sb.ToString.Length - 1, 1);
		sb.Append(Environment.NewLine);
	}

	foreach (DataRow dr in dt.Rows) {
		foreach (DataColumn dc in dt.Columns) {
			if (Information.IsDate(dr(dc.ColumnName).ToString()) & dr(dc.ColumnName).ToString().Contains(".") == false & truncateTimesFromDates) {
				sb.AppendFormat("{0}" + delimter, Convert.ToDateTime(dr(dc.ColumnName).ToString()).Date.ToShortDateString());
			} else {
				sb.AppendFormat("{0}" + delimter, CheckDBNull(dr(dc.ColumnName).ToString().Replace(",", "")));
			}
		}
		//remove the last Tab
		sb.Remove(sb.ToString.Length - 1, 1);
		sb.Append(Environment.NewLine);
	}
	return sb.ToString;
}

public enum enumObjectType
{
	StrType = 0,
	IntType = 1,
	DblType = 2
}

public object CheckDBNull(object obj, enumObjectType ObjectType = enumObjectType.StrType)
{
	object objReturn = null;
	objReturn = obj;
	if (ObjectType == enumObjectType.StrType & Information.IsDBNull(obj)) {
		objReturn = "";
	} else if (ObjectType == enumObjectType.IntType & Information.IsDBNull(obj)) {
		objReturn = 0;
	} else if (ObjectType == enumObjectType.DblType & Information.IsDBNull(obj)) {
		objReturn = 0.0;
	}
	return objReturn;
}

With the EPPlus NuGet package, it's very easy.

public class TestObject
{
	public int Col1 { get; set; }
	public int Col2 { get; set; }
	public string Col3 { get; set; }
	public DateTime Col4 { get; set; }
}

[TestMethod]
public void LoadFromCollection_MemberList_Test()
{
	//https://stackoverflow.com/questions/32587834/epplus-loadfromcollection-text-converted-to-number/32590626#32590626

	var TestObjectList = new List<TestObject>();
	for (var i = 0; i < 10; i++)
		TestObjectList.Add(new TestObject {Col1 = i, Col2 = i*10, Col3 = (i*10) + "E4"});

	//Create a test file
	var fi = new FileInfo(@"c:\temp\LoadFromCollection_MemberList_Test.xlsx");
	if (fi.Exists)
		fi.Delete();

	using (var pck = new ExcelPackage(fi))
	{
		//Do NOT include Col1
		var mi = typeof (TestObject)
			.GetProperties()
			.Where(pi => pi.Name != "Col1")
			.Select(pi => (MemberInfo)pi)
			.ToArray();

		var worksheet = pck.Workbook.Worksheets.Add("Sheet1");
		worksheet.Cells.LoadFromCollection(
            TestObjectList
            , true
            , TableStyles.Dark1
            , BindingFlags.Public| BindingFlags.Instance
            , mi);

		pck.Save();
	}
}

Notice that Col1 is NOT in the output:

In regards tuncalik's answer, which is great, especially if you want to have a little play with the code :) but it is putting my dates into Excel in American format i.e. 2nd March 2014 in the UK is 02/03/2014 but in the USA its 03/02/2014 with month 1st, then day of week after. I need to have it in UK format, any ideas please?

I have checked it is stored in UK format in my DataTable and also my Excel is set to UK but for some reason when it makes the Excel document it thinks its USA (is this because Microsoft are a USA company :)

I'll try experimenting with culture codes but not sure where to put that yet. Tried but this had no effect.

p.s.

I did have to change one line to get it to work by adding a 'cast' as below

// single worksheet
Excel._Worksheet workSheet = (Excel._Worksheet)excelApp.ActiveSheet;

Update: I have achieved UK formatting of the dates by converting to LongDateTime format, its only a work around though.

DateTime startDate = Convert.ToDateTime(myList[0].ToString());
string strStartDate = startDate.ToLongDateString();
DateTime endDate = Convert.ToDateTime(myList[myListTotalRows].ToString());
string strEndDate = endDate.ToLongDateString();    

cheers.

You can use EasyXLS that is a library for exporting Excel files.

Check this code:

DataSet ds = new DataSet();
ds.Tables.Add(dataTable);

ExcelDocument xls = new ExcelDocument();
xls.easy_WriteXLSFile_FromDataSet("datatable.xls", ds, 
           new ExcelAutoFormat(Styles.AUTOFORMAT_EASYXLS1), "DataTable");

See also this sample about how to export datatable to excel in C#.

Old thread - but thought i would throw my code in here. I wrote a little function to write a data table to a new excel sheet at a specified path (location). Also you will need to add a reference to microsoft excel 14.0 library.

I pulled from this thread on writing anything to excel - https://stackoverflow.com/questions/23041021/how-to-write-some-data-to-excel-file-xlsx

i used that to extrapolate how to write a datatable

*note in catch statements i have an errorhandler static class reference (you can ignore those)

 using excel = Microsoft.Office.Interop.Excel;
 using System.IO;
 using System.Data;
 using System.Runtime.InteropServices;

 //class and namespace wrapper is not shown in this example 

 private void WriteToExcel(System.Data.DataTable dt, string location)
    {
        //instantiate excel objects (application, workbook, worksheets)
        excel.Application XlObj = new excel.Application();
        XlObj.Visible = false;
        excel._Workbook WbObj = (excel.Workbook)(XlObj.Workbooks.Add(""));
        excel._Worksheet WsObj = (excel.Worksheet)WbObj.ActiveSheet;
      
        //run through datatable and assign cells to values of datatable
        try
        {
            int row = 1; int col = 1;
            foreach (DataColumn column in dt.Columns)
            {
                //adding columns
                WsObj.Cells[row, col] = column.ColumnName;
                col++;
            }
            //reset column and row variables
            col = 1;
            row++;
            for (int i = 0; i < dt.Rows.Count; i++)
            {
                //adding data
                foreach (var cell in dt.Rows[i].ItemArray)
                {
                    WsObj.Cells[row, col] = cell;
                    col++;
                }
                col = 1;
                row++;
            }
            WbObj.SaveAs(location);
        }
        catch (COMException x)
        {                
            ErrorHandler.Handle(x);
        }
        catch (Exception ex)
        {               
            ErrorHandler.Handle(ex);
        }
        finally
        {
            WbObj.Close();                
        }
    }

One way of doing it would be also with ACE OLEDB Provider (see also connection strings for Excel). Of course you'd have to have the provider installed and registered. You should have it, if you have Excel installed, but this is something you have to consider when deploying the app.

This is the example of calling the helper method from ExportHelper: ExportHelper.CreateXlsFromDataTable(myDataTable, @"C:\tmp\export.xls");

The helper for exporting to Excel file using ACE OLEDB:

public class ExportHelper
{
	private const string ExcelOleDbConnectionStringTemplate = "Provider=Microsoft.ACE.OLEDB.12.0;Data Source={0};Extended Properties=\"Excel 8.0;HDR=YES\";";

	/// <summary>
	/// Creates the Excel file from items in DataTable and writes them to specified output file.
	/// </summary>
	public static void CreateXlsFromDataTable(DataTable dataTable, string fullFilePath)
	{
		string createTableWithHeaderScript = GenerateCreateTableCommand(dataTable);

		using (var conn = new OleDbConnection(String.Format(ExcelOleDbConnectionStringTemplate, fullFilePath)))
		{
			if (conn.State != ConnectionState.Open)
			{
				conn.Open();
			}

			OleDbCommand cmd = new OleDbCommand(createTableWithHeaderScript, conn);
			cmd.ExecuteNonQuery();

			foreach (DataRow dataExportRow in dataTable.Rows)
			{
				AddNewRow(conn, dataExportRow);
			}
		}
	}

	private static void AddNewRow(OleDbConnection conn, DataRow dataRow)
	{
		string insertCmd = GenerateInsertRowCommand(dataRow);

		using (OleDbCommand cmd = new OleDbCommand(insertCmd, conn))
		{
			AddParametersWithValue(cmd, dataRow);
			cmd.ExecuteNonQuery();
		}
	}

	/// <summary>
	/// Generates the insert row command.
	/// </summary>
	private static string GenerateInsertRowCommand(DataRow dataRow)
	{
		var stringBuilder = new StringBuilder();
		var columns = dataRow.Table.Columns.Cast<DataColumn>().ToList();
		var columnNamesCommaSeparated = string.Join(",", columns.Select(x => x.Caption));
		var questionmarkCommaSeparated = string.Join(",", columns.Select(x => "?"));

		stringBuilder.AppendFormat("INSERT INTO [{0}] (", dataRow.Table.TableName);
		stringBuilder.Append(columnNamesCommaSeparated);
		stringBuilder.Append(") VALUES(");
		stringBuilder.Append(questionmarkCommaSeparated);
		stringBuilder.Append(")");
		return stringBuilder.ToString();
	}

	/// <summary>
	/// Adds the parameters with value.
	/// </summary>
	private static void AddParametersWithValue(OleDbCommand cmd, DataRow dataRow)
	{
		var paramNumber = 1;

		for (int i = 0; i <= dataRow.Table.Columns.Count - 1; i++)
		{
			if (!ReferenceEquals(dataRow.Table.Columns[i].DataType, typeof(int)) && !ReferenceEquals(dataRow.Table.Columns[i].DataType, typeof(decimal)))
			{
				cmd.Parameters.AddWithValue("@p" + paramNumber, dataRow[i].ToString().Replace("'", "''"));
			}
			else
			{
				object value = GetParameterValue(dataRow[i]);
				OleDbParameter parameter = cmd.Parameters.AddWithValue("@p" + paramNumber, value);
				if (value is decimal)
				{
					parameter.OleDbType = OleDbType.Currency;
				}
			}

			paramNumber = paramNumber + 1;
		}
	}

	/// <summary>
	/// Gets the formatted value for the OleDbParameter.
	/// </summary>
	private static object GetParameterValue(object value)
	{
		if (value is string)
		{
			return value.ToString().Replace("'", "''");
		}
		return value;
	}

	private static string GenerateCreateTableCommand(DataTable tableDefination)
	{
		StringBuilder stringBuilder = new StringBuilder();
		bool firstcol = true;

		stringBuilder.AppendFormat("CREATE TABLE [{0}] (", tableDefination.TableName);

		foreach (DataColumn tableColumn in tableDefination.Columns)
		{
			if (!firstcol)
			{
				stringBuilder.Append(", ");
			}
			firstcol = false;

			string columnDataType = "CHAR(255)";

			switch (tableColumn.DataType.Name)
			{
				case "String":
					columnDataType = "CHAR(255)";
					break;
				case "Int32":
					columnDataType = "INTEGER";
					break;
				case "Decimal":
					// Use currency instead of decimal because of bug described at 
					// http://social.msdn.microsoft.com/Forums/vstudio/en-US/5d6248a5-ef00-4f46-be9d-853207656bcc/localization-trouble-with-oledbparameter-and-decimal?forum=csharpgeneral
					columnDataType = "CURRENCY";
					break;
			}

			stringBuilder.AppendFormat("{0} {1}", tableColumn.ColumnName, columnDataType);
		}
		stringBuilder.Append(")");

		return stringBuilder.ToString();
	}
}

use the following class

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Data;
using excel = Microsoft.Office.Interop.Excel;
using EL = ExcelLibrary.SpreadSheet;
using System.Drawing;
using System.Collections;
using System.Runtime.InteropServices;
using System.Windows.Forms;


namespace _basic
{
public class ExcelProcesser
{
    public void WriteToExcel(System.Data.DataTable dt)
    {
        excel.Application XlObj = new excel.Application();
        XlObj.Visible = false;
        excel._Workbook WbObj = (excel.Workbook)(XlObj.Workbooks.Add(""));
        excel._Worksheet WsObj = (excel.Worksheet)WbObj.ActiveSheet;
        object misValue = System.Reflection.Missing.Value;

        
        try
        {
            int row = 1; int col = 1;
            foreach (DataColumn column in dt.Columns)
            {
                //adding columns
                WsObj.Cells[row, col] = column.ColumnName;
                col++;
            }
            //reset column and row variables
            col = 1;
            row++;
            for (int i = 0; i < dt.Rows.Count; i++)
            {
                //adding data
                foreach (var cell in dt.Rows[i].ItemArray)
                {
                    WsObj.Cells[row, col] = cell;
                    col++;
                }
                col = 1;
                row++;
            }
            WbObj.SaveAs(fileFullName, excel.XlFileFormat.xlWorkbookNormal, misValue, misValue, misValue, misValue, excel.XlSaveAsAccessMode.xlExclusive, misValue, misValue, misValue, misValue, misValue);
        }
        catch (Exception ex)
        {
            MessageBox.Show(ex.Message);
        }
        finally
        {
            WbObj.Close(true, misValue, misValue);
        }
    }
}

}

> This solution is basically pushing List<Object> data to Excel, It uses DataTable to achieve this, I implemented an extension method, so basically there are two things > needed. > 1. An Extension Method.

public static class ReportHelper
{
    public static string ToExcel<T>(this IList<T> data)
    {
        PropertyDescriptorCollection properties =
            TypeDescriptor.GetProperties(typeof(T));
        DataTable table = new DataTable();
        foreach (PropertyDescriptor prop in properties)
        {
            //table.Columns.Add(prop.Name, Nullable.GetUnderlyingType(prop.PropertyType) ?? prop.PropertyType);
            if (prop.Attributes[typeof(FGMS.Entity.Extensions.ReportHeaderAttribute)] != null)
            {
                table.Columns.Add(GetColumnHeader(prop), Nullable.GetUnderlyingType(prop.PropertyType) ?? prop.PropertyType);
            }
        }

        //So it seems like when there is only one row of data the headers do not appear
        //so adding a dummy blank row which fixed the issues
        //Add a blank Row - Issue # 1471
        DataRow blankRow = table.NewRow();
        table.Rows.Add(blankRow);

        foreach (T item in data)
        {
            DataRow row = table.NewRow();
            foreach (PropertyDescriptor prop in properties)
                //row[prop.Name] = prop.GetValue(item) ?? DBNull.Value;
                if (prop.Attributes[typeof(FGMS.Entity.Extensions.ReportHeaderAttribute)] != null)
                {
                    row[GetColumnHeader(prop)] = prop.GetValue(item) ?? DBNull.Value;
                }
            table.Rows.Add(row);
        }
        table.TableName = "Results";
        var filePath = System.IO.Path.GetTempPath() + "\\" + System.Guid.NewGuid().ToString() + ".xls";
        table.WriteXml(filePath);

        return filePath;
    }

    private static string GetColumnHeader(PropertyDescriptor prop)
    {
        return ((FGMS.Entity.Extensions.ReportHeaderAttribute)(prop.Attributes[typeof(FGMS.Entity.Extensions.ReportHeaderAttribute)])).ReportHeaderText;
    }       
}

> 2. Decorate your DTO classes with the Attribute [ReportHeaderAttribute("Column Name")]

public class UserDTO
    {
        public int Id { get; set; }
        public int SourceId { get; set; }
        public string SourceName { get; set; }

        [ReportHeaderAttribute("User Type")]
        public string UsereType { get; set; }

        [ReportHeaderAttribute("Address")]
        public string Address{ get; set; }

        [ReportHeaderAttribute("Age")]
        public int Age{ get; set; }

        public bool IsActive { get; set; }

        [ReportHeaderAttribute("Active")]
        public string IsActiveString
        {
            get
            {
                return IsActive ? "Yes" : "No";
            }
        }}

Everything that needs to be a column in the Excel has to be decorated with [ReportHeaderAttribute("Column Name")]

Then Simply

Var userList = Service.GetUsers() //Returns List of UserDTO;
var excelFilePath = userList.ToExcel();

HttpResponseMessage result = new HttpResponseMessage(HttpStatusCode.OK);
            var stream = new FileStream(excelFilePath, FileMode.Open);
            result.Content = new StreamContent(stream);
            result.Content.Headers.ContentType =
                new MediaTypeHeaderValue("application/vnd.ms-excel");
            result.Content.Headers.ContentDisposition = new ContentDispositionHeaderValue("attachment") { FileName = "UserList.xls" };

            return result;

To export data to Excel, you can use the ClosedXML.Report library (https://github.com/ClosedXML/ClosedXML.Report). Believe me, this is a wonderful library and easy for her to use. The library does not need Excel Interop. ClosedXML.Report generates an Excel file based on a template that you can create in Excel using any formatting. For example:

    var template = new XLTemplate(@".\Templates\report.xlsx");

    using (var db = new DbDemos())
    {
        var cust = db.customers.LoadWith(c => c.Orders).First();
        template.AddVariable(cust);
        template.Generate();
    }

    template.SaveAs(outputFile);

Private tmr As System.Windows.Forms.Timer

Private Sub TestExcel() Handles Button1.Click

	'// Initial data: SQL Server table with 6 columns and 293000 rows.


	'// Data table holding all data
	Dim dt As New DataTable("F161")

	'// Create connection
	Dim conn As New SqlConnection("Server=MYSERVER;Database=Test;Trusted_Connection=Yes;")
	Dim fAdapter As New SqlDataAdapter With
	{
		.SelectCommand = New SqlCommand($"SELECT * FROM dbo.MyTable", conn)
	}

	'// Fill DataTable
	fAdapter.Fill(dt)

	'// Create Excel application
	Dim xlApp As New Excel.Application With {.Visible = True}

	'// Temporarily disable screen updating
	xlApp.ScreenUpdating = False

	'// Create brand new workbook
	Dim xlBook As Excel.Workbook = xlApp.Workbooks.Add()
	Dim xlSheet As Excel.Worksheet = DirectCast(xlBook.Sheets(1), Excel.Worksheet)

	'// Get number of rows
	Dim rows_count = dt.Rows.Count
	'// Get number of columns
	Dim cols_count = dt.Columns.Count

	'// Here 's the core idea: after receiving data
	'// you need to create an array and transfer it to sheet.
	'// Why array?
	'// Because it's the fastest way to transfer data to Excel's sheet.
	'// So, we have two tasks:
	'// 1) Create array
	'// 2) Transfer array to sheet

	'// =========================================================
	'// TASK 1: Create array
	'// =========================================================
	'// In order to create array, we need to know that
	'// Excel's Range object expects 2-D array whose lower bounds
	'// of both dimensions start from 1.
	'// This means you can't use C# array.
	'// You need to manually create such array.
	'// Since we already calculated number of rows and columns,
	'// we can use these numbers in creating array.
	Dim arr = Array.CreateInstance(GetType(Object), {rows_count, cols_count}, {1, 1})

	'// Fill array
	For r = 0 To rows_count - 1
		For c = 0 To cols_count - 1
			arr(r + 1, c + 1) = dt.Rows(r)(c)
		Next
	Next

	'// =========================================================
	'// TASK 2: Transfer array to sheet
	'// =========================================================
	'// Now we need to transfer array to sheet.
	'// So, how transfer array to sheet fast?
	'// 
	'// THE FASTEST WAY TO TRANSFER DATA TO SHEET IS TO ASSIGN ARRAY TO RANGE.
	'// We could, of course, hard-code values, but Resize property
	'// makes this work a breeze:
	xlSheet.Range("A1").Resize.Resize(rows_count, cols_count).Value = arr

	'// If we decide to dump data by iterating over array,
	'// it will take LOTS of time.
	'// For r = 1 To rows_count
	'//     For c = 1 To cols_count
	'//         xlSheet.Cells(r, c) = arr(r, c)
	'//     Next
	'// Next

	'// Here are time results:
	'// 1) Assigning array to Range: 3 seconds
	'// 2) Iterating over array: 45 minutes

	'// Turn updating on
	xlApp.ScreenUpdating = True
	xlApp = Nothing
	xlBook = Nothing
	xlSheet = Nothing

	'// Here we have another problem:
	'// creating array took lots of memory (about 150 MB).
	'// Using 'GC.Collect()', by unknown reason, doesn't help here.
	'// However, if you run GC.Collect() AFTER this procedure is finished
	'// (say, by pressing another button and calling another procedure),
	'// then the memory is cleaned up.
	'// I was wondering how to avoid creating some extra button to just release memory,
	'// so I came up with the idea to use timer to call GC.
	'// After 2 seconds GC collects all generations.
	'// Do not forget to dispose timer since we need it only once.

	tmr = New Timer()
	AddHandler tmr.Tick,
		Sub()
			GC.Collect()
			GC.WaitForPendingFinalizers()
			GC.WaitForFullGCComplete()
			tmr.Dispose()
		End Sub
	tmr.Interval = TimeSpan.FromSeconds(2).TotalMilliseconds()
	tmr.Start()

End Sub

Purely sample code (in case it might help someone with some ideas), based on Tomasz Wiśniewski answer from here: https://stackoverflow.com/a/21079709/2717521

MainWindow ExportButton:

public int counter;

public void SaveToExcel(object sender, RoutedEventArgs e)
{
    counter = 1;
    CountChecker();
}

public void CountChecker()
{
    string filename = GlobalStrings.building_house_address;
    string path = Environment.GetFolderPath(Environment.SpecialFolder.Desktop) + "\\";


    if (CurrentID != 0)
    {
        if (!File.Exists(path + filename + ".xlsx"))
        {
            DataGridParts.Export(path + filename);
            MessageBoxEx.Show(this, "Shranjeno na namizje");
        }
        else
        {
            if (!File.Exists(path + "\\" + filename + " (" + (counter) + ")" + ".xlsx"))
            {
                DataGridParts.Export(path + filename + " (" + (counter) + ")");
                MessageBoxEx.Show(this, "Shranjeno na namizje");
            }
            else
            {
                counter++;
                CountChecker();
            }
        }
    }
    else
    {
        MessageBoxEx.Show(this, "Izbran ni noben naslov!");
    }
}

ExportToExcel Class:

using Microsoft.Office.Interop.Excel;
using System.Runtime.InteropServices;

namespace CBUGIS
{
    public static class ExportToExcel
    {
        /// <summary>
        /// Export DataTable to Excel file
        /// </summary>
        /// <param name="DataTable">Source DataTable</param>
        /// <param name="ExcelFilePath">Path to result file name</param>
        public static void Export(this System.Data.DataTable DataTable, string ExcelFilePath = null)
        {
            int ColumnsCount;
            int RowShift = 5;

            ColumnsCount = DataTable.Columns.Count;

            // load excel, and create a new workbook
            Application Excel = new Application();
            Excel.Workbooks.Add();

            // single worksheet
            _Worksheet Worksheet = Excel.ActiveSheet;
            Excel.Sheets[1].Name = "CBUGIS";

            Worksheet.Columns.NumberFormat = "@";
            Worksheet.Columns.HorizontalAlignment = XlHAlign.xlHAlignLeft;

            object[,] Title = new object[3, 1]; // Array Size

            if (GlobalStrings.building_alterantive_addresses.Length == 0)
            {
                if (GlobalStrings.building_postcode.Length != 0)
                {
                    Title[0, 0] = "NASLOV: " + GlobalStrings.building_house_street + " " + GlobalStrings.building_house_number + GlobalStrings.building_house_id + ", " + GlobalStrings.building_postcode + " " + GlobalStrings.building_area;
                    Title[1, 0] = "K.O.: " + GlobalStrings.building_cadastral_community + ", ŠT.STAVBE: " + GlobalStrings.building_building_number + ", ŠT.PARCELE: " + GlobalStrings.building_plot_number;
                }
                else
                {
                    Title[0, 0] = "NASLOV: " + GlobalStrings.building_house_street + " " + GlobalStrings.building_house_number + GlobalStrings.building_house_id;
                    Title[1, 0] = "K.O.: " + GlobalStrings.building_cadastral_community + ", ŠT.STAVBE: " + GlobalStrings.building_building_number + ", " + GlobalStrings.building_plot_number;
                }
            }
            else
            {
                if (GlobalStrings.building_postcode.Length != 0)
                {
                    Title[0, 0] = "NASLOV: " + GlobalStrings.building_house_street + " " + GlobalStrings.building_house_number + GlobalStrings.building_house_id + ", " + GlobalStrings.building_postcode + " " + GlobalStrings.building_area;
                    Title[1, 0] = "K.O.: " + GlobalStrings.building_cadastral_community + ", ŠT.STAVBE: " + GlobalStrings.building_building_number + ", ŠT.PARCELE: " + GlobalStrings.building_plot_number;
                    Title[2, 0] = "GLEJ TUDI: " + GlobalStrings.building_alterantive_addresses;
                }
                else
                {
                    Title[0, 0] = "NASLOV: " + GlobalStrings.building_house_street + " " + GlobalStrings.building_house_number + GlobalStrings.building_house_id;
                    Title[1, 0] = "K.O.: " + GlobalStrings.building_cadastral_community + ", ŠT.STAVBE: " + GlobalStrings.building_building_number + ", ŠT.PARCELE: " + GlobalStrings.building_plot_number;
                    Title[2, 0] = "GLEJ TUDI: " + GlobalStrings.building_alterantive_addresses;
                }
            }

            Range TitleRange = Worksheet.get_Range((Range)(Worksheet.Cells[3, 1]), (Range)(Worksheet.Cells[1, 1]));
            TitleRange.Value = Title;
            TitleRange.Font.Bold = true;
            TitleRange.Font.Size = 10;


            object[] Header = new object[11]; // Number of Columns

            Header[0] = "DEL";
            Header[1] = "DELEŽ";
            Header[2] = "CRP";
            Header[3] = "LASTNIK";
            Header[4] = "NASLOV";
            Header[5] = "P.Š";
            Header[6] = "OBMOČJE";
            Header[7] = "DRŽAVA";
            Header[8] = "EMŠO/MAT. ŠT.";
            Header[9] = "OPIS";
            Header[10] = "OPOMBA";

            Range HeaderRange = Worksheet.get_Range((Range)(Worksheet.Cells[RowShift, 2]), (Range)(Worksheet.Cells[RowShift, 12]));
            HeaderRange.Value = Header;
            HeaderRange.Font.Bold = true;
            HeaderRange.Interior.Color = System.Drawing.ColorTranslator.ToOle(System.Drawing.Color.LightGray);

            // DataCells
            int RowsCount = DataTable.Rows.Count;

            object[,] Cells = new object[RowsCount, ColumnsCount];

            for (int j = 0; j < RowsCount; j++)
                for (int i = 0; i < ColumnsCount - 1; i++)
                    if (i > 1)
                    {
                        Cells[j, i - 2] = DataTable.Rows[j][i];
                    }

            Range CellRange = Worksheet.get_Range((Range)(Worksheet.Cells[RowShift +1, 2]), (Range)(Worksheet.Cells[RowShift + RowsCount, 12]));
            CellRange.Value = Cells;
            CellRange.Borders.LineStyle = XlLineStyle.xlContinuous;

            Worksheet.Columns.NumberFormat = "@";
            Worksheet.Columns[1].ColumnWidth = 0.1;
            for (int b = 1; b < 12; b++)
            {
                if (b > 1)
                {
                    Worksheet.Columns[b].AutoFit();
                }
            }

            Worksheet.PageSetup.Orientation = XlPageOrientation.xlLandscape;
            Worksheet.PageSetup.TopMargin = 0.5;
            Worksheet.PageSetup.BottomMargin = 0.5;
            Worksheet.PageSetup.RightMargin = 0.5;
            Worksheet.PageSetup.LeftMargin = 0.5;

            // check fielpath
            if (ExcelFilePath != null && ExcelFilePath != "")
            {
                Worksheet.SaveAs(ExcelFilePath);
                Excel.Quit();
                Marshal.FinalReleaseComObject(Worksheet);
                Marshal.FinalReleaseComObject(TitleRange);
                Marshal.FinalReleaseComObject(HeaderRange);
                Marshal.FinalReleaseComObject(CellRange);
                Marshal.FinalReleaseComObject(Excel);
            }
            else

            // no filepath is given
            {
                Excel.Visible = true;
            }      
        }
    }
}

Some smart answers on here (Tomasz Wiśniewski, cuongle). They seem to fall into two categories, either you need a wrapper/plug-in library or the eventual writing of the data needs to loop writing cell by cell, line by line. With the latter, performance will be slow, and perhaps the first is not so convenient for what should be such a simple task as exporting a DataTable to Excel. Here's how I do it, utilising a copy/paste which is much faster (3 milliseconds for 800 rows when I tested), but we have to cheat slightly. What you have to do is create a hidden Form with a DataGridView inside on the fly. Then when you dump your DataTable in their as the DataSource, you can call the "GetClipboardContent" method of the DataGridView class, and finally simply place it in a selected Range/Cell on the Excel sheet. Example code:

private bool FncCopyDataTableToWorksheet(DataTable dt, ExcelUsing.Worksheet destWsh)
    {
        Form frm = new Form();
        frm.Size = new Size(0, 0);             
        DataGridView dgv = new DataGridView();
        DataObject obj;
        ExcelUsing.Range rng;
        int j = 0;

        frm.Controls.Add(dgv);

        bool result = false;

        try
        {
            dgv.DataSource = dt;
            dgv.RowHeadersVisible = false;
            frm.Show();
            dgv.SelectAll();
            obj = dgv.GetClipboardContent();
            frm.Hide();
            dgv.ClearSelection();

            if (obj != null)
            {
                Clipboard.SetDataObject(obj);

                //Write Headers
                foreach (DataGridViewColumn dgvc in dgv.Columns)
                {
                    if (dgvc.Visible == true)
                    {
                        rng = (ExcelUsing.Range)destWsh.Cells[1, 1 + j];
                        rng.Value2 = dgvc.Name.ToString();
                        j++;
                    }
                }

                rng = (ExcelUsing.Range)destWsh.Cells[2, 1];
                rng.Select();
                destWsh.PasteSpecial(rng, _Missing, _Missing, _Missing, _Missing, _Missing, true);

                result = true;

            }
            else { result = false; }
        }
        catch (Exception ex)
        {
            //Handle
        }

        rng = null;
        obj = null;
        dgv = null;
        frm = null;

        return result;
    }

The modest way to export excel is using Microsoft CloseXml pakage. I wrote a simple function for exporting my query result as excel sheet (I wrote it for but can be used in windows application with minor changes):

using ClosedXML.Excel;
...
public ActionResult ToExcel(List<Dictionary<string, string>> data, Dictionary<string, string> columnMap, string fileName, string sheetName)
{
	var dtDataBuffer = new System.Data.DataTable("buffer");

	foreach (string col in columnMap.Values)
	{
		dtDataBuffer.Columns.Add(col, typeof(string));
	}



	foreach (var row in data)
	{
		List<string> rowData = new List<string> { };


		foreach (string col in columnMap.Keys)
		{
			rowData.Add(row[col]);
		}

		dtDataBuffer.Rows.Add(rowData.ToArray());
	}




	var memoryStream = new MemoryStream();

	using (var workbook = new XLWorkbook())
	{
		var worksheet = workbook.Worksheets.Add(dtDataBuffer, sheetName);
		worksheet.Rows().Style.Alignment.Horizontal = XLAlignmentHorizontalValues.Center;
		worksheet.Rows().Style.Alignment.Vertical = XLAlignmentVerticalValues.Center;
		worksheet.Columns().AdjustToContents();
		workbook.SaveAs(memoryStream);
	}

	return File(memoryStream.ToArray(), "application/vnd.ms-excel", fileName);
}

And this is a usage example (in practice I use my own class to run query and generate data. You can find it here for Oracle and SQL Server):

public ActionResult myReportExport(){
	var data=List<Dictionary<string, string>>(){
		{{"Column1_Index","Column1_Value"},{"Column2_Index","Column2_Value"},...}
		...
	};
	
	
	return ToExcel(data, new Dictionary<string, string> {
			{ "Column1_Index", "Column1 Title" } ,
			{ "Column2_Index", "Column2 Title" } ,
			...
		},
		"myFileName.xlsx",
		"my sheet name"
	);
}