How to refresh DataSource of a ListBox

Form has one Combobox and one ListBox. When the "Add" button is clicked, I want to add the selected item from the ComboBox to the ListBox.

public partial class MyForm:Form
{
    List<MyData> data = new List<MyData>();
    private void ShowData()
    {
       listBox1.DataSource = data;
       listBox1.DisplayMember = "Name";
       listBox1.ValueMember = "Id";
    }

    private void buttonAddData_Click(object sender, EventArgs e)
    {
       var selection = (MyData)comboBox1.SelectedItem;
       data.Add(selection);
       ShowData();
    }
}

With this example, the selected item is replaced with the new selection inside ListBox. I need to add the item to the list.

What is wrong with my code?

listbox1.DataSource property looks for value changes but by assigning the same list all the time the value won't really change.

You can use a BindingList<T>, instead of your List<T>, to automatically recognize new items added. Your ShowData() method must be called once at startup.

public partial class MyForm:Form
{
    public MyForm(){
        InitializeComponent();
        ShowData();
    }

    BindingList<MyData> data = new BindingList<MyData>();

    private void ShowData()
    {
       listBox1.DataSource = data;
       listBox1.DisplayMember = "Name";
       listBox1.ValueMember = "Id";
    }

    private void buttonAddData_Click(object sender, EventArgs e)
    {
       var selection = (MyData)comboBox1.SelectedItem;
       data.Add(selection);
    }
}

I would suggest to use BindingSource as it would properly update connected controls.

public partial class MyForm : Form
{
    List<MyData> data = new List<MyData>();
    BindingSource bs = new BindingSource();

    public MyForm()
    {
        IntializeComponents();
        bs.DataSource = data;

       listBox1.DisplayMember = "Name";
       listBox1.ValueMember = "Id";
       listBox1.DataSource = bs;
    }

    private void buttonAddData_Click(object sender, EventArgs e)
    {
       var selection = (MyData)comboBox1.SelectedItem;
       data.Add(selection);
       
       bs.ResetBindings(false);
    }
}

Changing controls data source on fly produces strange result sometime.

The listbox didn't detect that you have changed the DataSource. It will only refresh when Datasource has changed, so set DataSource to null first:

listBox1.DataSource = null;
listBox1.DataSource = data;

You could also clear the items then set the DataSource again:

listBox1.Items.Clear();
listBox1.DataSource = data;

Alternatively and probably the most correct way to implement this is to use the provided ObservableCollection<T>. It is designed with the sole purpose of implementing INotifyCollectionChanged.

public partial class MyForm : Form
{
    ObservableCollection<MyData> data = new ObservableCollection<MyData>();

    public MyForm()
    {
        listBox1.DataSource = data;
        listBox1.DisplayMember = "Name";
        listBox1.ValueMember = "Id";
    }

    private void buttonAddData_Click(object sender, EventArgs e)
    {
       var selection = (MyData)comboBox1.SelectedItem;
       data.Add(selection);
    }
}

Because ObservableCollection<T> implements INotifyCollectionChanged the DataSource binding will automatically update the ListBox whenever your data changes.

Call ShowData() when the form initializes to populate your listbox on initialization

 public Department()
        {
            InitializeComponent();
            ShowData();
        }

ShowData() Method, where BindingSource, DisplayMember and ValueMember are set

private void ShowData()
            {
                using (var uow = new UnitOfWork(new SellContext()))
                {
                    listBox1.DataSource = uow.Departments.GetAll().ToList();
                    listBox1.DisplayMember = "DepartmentName";
                    listBox1.ValueMember = "DepartmentId"; 
                    //listBox1.Invalidate();       
                }
            }

In the implementation below when a department is deleted from database the listbox refreshes with the current collection

private void button1_Click(object sender, EventArgs e)
    {
        try {
            using (var uow = new UnitOfWork(new SellContext()))
            {
                int count = uow.Departments.FindDepartmentByName(txtDeptName.Text.ToString());
                if (count>0)
                {
                    MessageBox.Show("This Department Already Exists", "SellRight", MessageBoxButtons.OK, MessageBoxIcon.Hand);
                }

                else
                {
                    department dept = new department();
                    dept.DepartmentName = txtDeptName.Text.ToString();
                    uow.Departments.Create(dept);
                    if (uow.Complete() > 0)
                    {           
                        MessageBox.Show("New Department Created", "SellRight", MessageBoxButtons.OK, MessageBoxIcon.Information);
                        txtDeptName.Text = "";
                        ShowData();      
                    }
                    else
                    {
                        MessageBox.Show("Unable to add Department", "SellRight", MessageBoxButtons.OK, MessageBoxIcon.Error);
                        txtDeptName.Text = "";
                        ShowData();
                    }
                }
            }                              
        }
        catch(Exception ex)
        {
            ex.ToString();
        }
    }

Maybe this solution has not the better performance, but after many tries and a few hours it was what it worked for me:

This line was executed on the form constructor:

listBox1.DataSource = myData;

This lines were executed after the information was modified:
listBox1.DataSource = new List<Movil>();
listBox1.DataSource = myData;

Hope it helps!

refreshing also works via

listbox.ItemsSource = null;
listbox.ItemsSource = data;