C# – Get number of character selected in a multiline textbox in asp.net

Two options:

1. Use a NumericUpDown instead. NumericUpDown does the filtering for you, which is nice. Of course it also gives your users the ability to hit the up and down arrows on the keyboard to increment and decrement the current value.

2. Handle the appropriate keyboard events to prevent anything but numeric input. I've had success with this two event handlers on a standard TextBox:

   private void textBox1_KeyPress(object sender, KeyPressEventArgs e)
   {
       if (!char.IsControl(e.KeyChar) && !char.IsDigit(e.KeyChar) &&
           (e.KeyChar != '.'))
       {
               e.Handled = true;
       }
   
       // only allow one decimal point
       if ((e.KeyChar == '.') && ((sender as TextBox).Text.IndexOf('.') > -1))
       {
           e.Handled = true;
       }
   }
   

You can remove the check for '.' (and the subsequent check for more than one '.') if your TextBox shouldn't allow decimal places. You could also add a check for '-' if your TextBox should allow negative values.

If you want to limit the user for number of digit, use: textBox1.MaxLength = 2; // this will allow the user to enter only 2 digits

The problem is your third requirement:

The extended TextBox may have different fonts and font sizes. 

What this means is that you will have to figure out how tall each line is by which characters in the line have the largest font size, and to know that, you will have to also know which characters are in each line. It gets worse, because (surprisingly) different fonts can have different heights given the same point size. That all might be workable, if it weren't for the fact that each character in a proportionally-spaced font is a different width.

And if you intend to support superscripts and subscripts...