Allow only alphanumeric in textbox.

Question

I have a textbox and it need the user not allow to enter any specialcharecters He can enter 1. A-Z
2. a-z
3. 0-9
4. Space.

How can I make the KeyDown event to do this?

Answer 1

Use a regex to filter out the other characters. Or use Char.IsDigit, IsXXX methods to filter out unwanted characters. Lots of ways to do this.

Update: If you must use KeyDown then it seems that you need to also handle KeyPressed and set obEventArgs.Handled = true to disallow the characters. See the example on the KeyDown MSDN Page

Update: Now that you specify it's WPF. The below code will allow only a-z and A-Z characters to be entered into the textbox. Extend as needed...

private void _txtPath_KeyDown(object sender, KeyEventArgs e)
      {
         if ((e.Key < Key.A) || (e.Key > Key.Z))
            e.Handled = true;
      }

This will break if you copy-paste stuff into the text-box. Validate the entire text once the user leaves the control or when he clicks OK/Submit as MusicGenesis says.

Answer 2

Handling the KeyDown or KeyPress events is one way to do this, but programmers usually forget that a user can still copy-and-paste invalid text into the textbox.

A somewhat better way is to handle the TextChanged event, and strip out any offending characters there. This is a bit more complicated, as you have to keep track of the caret position and re-set it to the appropriate spot after changing the box's Text property.

Depending on your application's needs, I would just let the user type in whatever they want, and then flag the textbox (turn the text red or something) when the user tries to submit.

Answer 3

and your regExp could look like [0-9a-zA-Z]* to allow only English alphanumeric chracters

Answer 4

I know that winForms have available a MaskedTextBox control, which lets you specify exactly this sort of thing. I don't know WPF, so I dunno if that's available there, but if it is, do that. Its MUCH easier than all this stuff with keypresses and events, and more robust too.

Answer 5

I think it's worth considering doing the filtering on the TextBox's TextChanged event. You can create an operation that gets rid of any non-valid characters from your text string. This is a bit more messy than blocking the KeyDown event.

But, I think this is the way to go because you are not blocking WPF's built-in KeyDown/Up event handling mechanisms, so copy/paste still works. You would be working at a higher level of abstractions so I think it will be easier to figure out what is going on.

Answer 6

I can not vote up yet, but I support the GWLlosa answer.