int value under 10 convert to string two digit number

string strI;   

for (int i = 1; i < 100; i++)
    strI = i.ToString();

in here, if i = 1 then ToString yields "1"

But I want to get "01" or "001"

It looks quite easy, but there's only article about

datetime.ToString("yyyy-MM-dd")`

i.ToString("00")

or

i.ToString("000")

depending on what you want

Look at the MSDN article on custom numeric format strings for more options: http://msdn.microsoft.com/en-us/library/0c899ak8(VS.71).aspx

I can't believe nobody suggested this:

int i = 9;
i.ToString("D2"); // Will give you the string "09"

or

i.ToString("D8"); // Will give you the string "00000009"

If you want hexadecimal:

byte b = 255;
b.ToString("X2"); // Will give you the string "FF"

You can even use just "C" to display as currency if you locale currency symbol. See here: https://docs.microsoft.com/en-us/dotnet/api/system.int32.tostring?view=netframework-4.7.2#System_Int32_ToString_System_String_

The accepted answer is good and fast:

i.ToString("00")

or

i.ToString("000")

If you need more complexity, String.Format is worth a try:

var str1 = "";
var str2 = "";
for (int i = 1; i < 100; i++)
{
    str1 = String.Format("{0:00}", i);
    str2 = String.Format("{0:000}", i);
}

For the i = 10 case:

> str1: "10"
> str2: "010"

I use this, for example, to clear the text on particular Label Controls on my form by name:

private void EmptyLabelArray()
{
    var fmt = "Label_Row{0:00}_Col{0:00}";
    for (var rowIndex = 0; rowIndex < 100; rowIndex++)
    {
        for (var colIndex = 0; colIndex < 100; colIndex++)
        {
            var lblName = String.Format(fmt, rowIndex, colIndex);
            foreach (var ctrl in this.Controls)
            {
                var lbl = ctrl as Label;
                if ((lbl != null) && (lbl.Name == lblName))
                {
                    lbl.Text = null;
                }
            }
        }
    }
}

ToString can take a format. try:

i.ToString("000");

This blog post is a great little cheat-sheet to keep handy when trying to format strings to a variety of formats.

link to trojan removed

Edit

The link was removed because Google temporarily warned that the site (or related site) may have been spreading malicious software. It is now off the list an no longer reported as problematic. Google "SteveX String Formatting" you'll find the search result and you can visit it at your discretion.

I will start my answer saying that most of previous answers were perfectly good answers at the time of writing them. So, thank you to them who wrote them.

Now, you can also use String Interpolation for same solution.

Edit: Adding this explanation after receiving a perfectively valid constructive comment from Heretic Monkey. I have preferred to use .ToString whenever I had need to convert an integer to string and not add the result to any other string. And, I have preferred to use interpolation whenever I had need to combine string(s) and an integer, like in below examples.

String Interpolation

i.ToString("00")
01

i.ToString("000")
001

i.ToString("0000")
0001

$"Prefix_{i:00}"
Prefix_01

$"Prefix_{i:000}"
Prefix_001

$"Prefix_{i:0000}_Suffix"
Prefix_0001_Suffix

You can also do it this way

private static string GetPaddingSequence(int padding)
{
      StringBuilder SB = new StringBuilder();
      for (int i = 0; i < padding; i++)
      {
           SB.Append("0");
      }

      return SB.ToString();
  }

public static string FormatNumber(int number, int padding)
{
    return number.ToString(GetPaddingSequence(padding));
}

Finally call the function FormatNumber

string x = FormatNumber(1,2);

Output will be 01 which is based on your padding parameter. Increasing it will increase the number of 0s