In C#, in a numerical textbox, how do I prevent a decimal point being placed in as the first digit only?

Question

Currently I have this code for preventing character entering, and more than one decimal place,
but how to prevent the first character being a decimal?

    private void textBoxNoLetters_KeyPress(object sender, KeyPressEventArgs e)
    {
        if (!char.IsDigit(e.KeyChar) && e.KeyChar != (char)Keys.Back && e.KeyChar != '.')
        {
            e.Handled = true;
        }

        else if ((e.KeyChar == '.') && ((sender as TextBox).Text.IndexOf('.') > -1))
        {
            e.Handled = true;
        }
    }

Answer 1

Below is your code, modified to handle a decimal separator typed fist, additionally the decimal separator character of your system is obtained to assist in localizing your application.

  char decimalChar = Convert.ToChar(CultureInfo.CurrentCulture.NumberFormat.NumberDecimalSeparator);

  if (!char.IsDigit(e.KeyChar) && e.KeyChar != (char)Keys.Back && e.KeyChar != decimalChar)
  {
    e.Handled = true;
  }
  else if ((e.KeyChar == decimalChar) && ((sender as TextBox).Text.IndexOf(decimalChar) > -1))
  {
    e.Handled = true;
  }
  else if ((e.KeyChar == decimalChar) && ((sender as TextBox).Text.Length == 0))
  {
    e.Handled = true;
  }