int value under 10 convert to string two digit number

Question

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");