You can use a Regular Expression:

  RegEx re = new RegEx("^[-'a-zA-Z]*$");
  if (re.IsMatch(stringToTest)) ....//string is correct

Another possible approach:

Create a Custom User Control TextBox and override OnKeyPress to
handle ASCII characters.

protected override void OnKeyPress(System.Windows.Forms.KeyPressEventArgs e)
       {
           // KeyChar property of the event Gets or Sets the
           // character corresponding to the key pressed
           // and returns the ASCII character that is composed.
           // Handled property of the event Gets or Sets a value
           // indicating whether the System.Windows.Forms.Control
           //      .KeyPress event was handled
           // and returns true if the event is handled; otherwise,
           // false

           int i = (int)e.KeyChar;    //cast KeyChar property to integer

           // Capital letters, small case letters, dash (or minus),
           // and apostrophe keys are not handled (handled returns
           // false) so are passed to the textBox and displayed.
           // All other KeyPresses are handled (handled returns true)
           // so are not passed to the textBox and are not displayed.

           if (i >= 65 && i <= 90 || i >= 96 && i <= 122 || i == 39)
           {
               e.Handled = false;
               return;
           }
               e.Handled = true;
       }

-Tim Sprout

Be carefull what you check for. "letters" is language dependent.

If you test with (i >= 65 && i <= 90 || i >= 96 && i <= 122 || i == 39)
or regex ranges [a-zA-Z] or other such English-centric ideas, you will
affect other languages using characters outside these ranges.

And that is almost every language out there, with very few exceptions :-)