Ensure textbook does not allow alphabetical key presses..

Question

I seem to have a very simple problem, but don't understand why it seems to not work atm.

I have to the keypress event attached this function.

    void length_textbox_numeric_check(object sender, KeyPressEventArgs e)
    {
        if ((char.IsNumber(e.KeyChar)) && (char.IsControl(e.KeyChar)) )
        {
            e.Handled = true;
        }
        else
        {
            e.Handled = false;
        }
    }

it accept both character, number and spaces and so on?... but according the code should it only accept numbers and backspace? so why is it also accepting letters?

setting KeyPressEventArgs.Handled as true is saying that the character is handled and does not need to be processed anymore (i.e. its not going to get put into the textbox) — Blake Thingstad, Feb 15 '18 at 15:19
Your expression will always be false - the character cannot be a number AND a control character at the same time. So it will always be setting e.Handled to false - which allows all cahracters to be accepte. — PaulF, Feb 15 '18 at 15:20
ok... I am not sure I understand the logic behind it... I guess that's the way the cookie crumbles.. — Lke, Feb 15 '18 at 15:23
I think you understand the Handled now that I see other answers about the if condition, so I deleted my answer about it, let me know if this is a source of confusion. — Blake Thingstad, Feb 15 '18 at 15:26
I guess what confuses me is the wording.. I guess the semantics could have been a bit clearer.. but hey.. its ok.. — Lke, Feb 15 '18 at 15:38
You are using it correctly at least. If you do not want the character to be entered into the text box then set e.Handled = true, by default it is set to false (so you actually don't need the else). — Blake Thingstad, Feb 15 '18 at 15:42

Lke · Answer 1 · 2018-02-15T21:53:29.757

1

Based on @BlakeThingstad comment. This fixed it.

   void length_textbox_numeric_check(object sender, KeyPressEventArgs e)
    {
        if (!(char.IsNumber(e.KeyChar)) && !(char.IsControl(e.KeyChar)) )
        {
            e.Handled = true;    // Handled states whether it should handled
                                 // normally (true) or differently (false) 
        }
        else
        {
            e.Handled = false;
        }
    }

edited Feb 15 '18 at 21:53

answered Feb 15 '18 at 15:24

Lke

11
3

1

To be accurate, setting the Handled property to true indicates that the key press has been handled by the event handler & so should not be processed further. Setting it to false indicates it has not been dealt with in the event, so should be passed on to the default event handler. – PaulF Feb 15 '18 at 15:39
The [documentation](https://msdn.microsoft.com/en-us/library/system.windows.forms.keypresseventargs.handled(v=vs.110).aspx) states this well, "If the event is not handled, it will be sent to the operating system for default processing. Set Handled to true to cancel the KeyPress event." – Blake Thingstad Feb 15 '18 at 15:43

score 0 · Answer 2 · answered Feb 15 '18 at 15:20

0

You want to suppress the KeyPress. Don't know which event you listened. I would go for KeyDown:

    private void textBox1_KeyDown(object sender, KeyEventArgs e)
    {
        if (!char.IsNumber((char)e.KeyValue) && !char.IsControl((char)e.KeyValue))
            e.SuppressKeyPress = true;
    }

answered Feb 15 '18 at 15:20

kara

3,205
4
20
34

I went for KeyPress... Why do you suggest KeyDown? This is a just a dummy project - but what cases would it make sense? – Lke Feb 15 '18 at 15:28
Normally you want to know when the user starts to do something in your textbox. Not if he started pressing it a minute ago and changed focus or something. https://stackoverflow.com/questions/5871383/difference-between-the-keydown-event-keypress-event-and-keyup-event-in-visual-s – kara Feb 15 '18 at 15:31
@Lke: check out the documentation for both. It may not be an issue for your project but you will see the KeyPress event does not allow you see [some characters](https://msdn.microsoft.com/en-us/library/system.windows.forms.keypresseventargs.keychar(v=vs.110).aspx). other events, such as KeyDown, let you see all characters & modifiers (CTRL/ALT etc). – PaulF Feb 15 '18 at 15:35

Ensure textbook does not allow alphabetical key presses..

2 Answers2