What i got you want to prevent the user to enter any numbers inside the textbox , am i right ?
If yes
You didn't mention you want to do this on windows or web application
for windows application you can inherit the texbox class to a new one for example called CustomTextbox
override the the keydown event (to prevent it while the user is entering )
or override the textchanged (to validate the text after the user enter the text ) , it's up to you to choose the write event. In the first case Ex:
public class CustomTextbox : TextBox
{
protected override void OnKeyDown(KeyEventArgs e)
{
base.OnKeyDown(e);
}
}
Inside the e you will find too much data to validate the new character entered by the user and you can escape it if it's not valid
this will help you later on to make this textbox multi function control throw adding some new properties like the validation type (numeric or characters only)
So hope that i could help , happy programming