How to only allow numbers (while typing) in a Numeric UpDown?

Question

I'm trying to come up with a better approach for my other problem: Avalonia: Get Numeric UpDown to parse the text on lost focus
In short: I'd like a TextBox that ignores non-numeric characters while typing. If I am using numeric updowns like this:

<NumericUpDown Value="{Binding Samples}" ParsingNumberStyle="Integer" ShowButtonSpinner="False"/>

The user can type in non-numeric characters, which makes the box reset to it's former value, when I click outside of it. However pressing Return does not work unless the text is a proper integer (see link above). I guess I could use

OnMyValueChanging(int oldValue, int new Value)
{
  //validate
}

However the value is an integer, so I can't verify it while typing. I could use a regular TextBox, but then I'd have to write the OnValueChanging for every occurence of my numeric inputs. Maybe I can use Styles for that? But I don't know how to point to a project-wide function, that can be shared across multiple view models.

You can do this in XAML itself, given that avalonia allows you setup code behind xaml files, this answer should help you: stackoverflow.com/questions/1268552/… — Karan Shishoo
– Karan Shishoo, Commented Jan 30 at 11:01

Frederik Steinmetz · Accepted Answer · 2025-02-06 14:32:49Z

@Karan Shishoo thank you for the link,there are a lot of answers some incompatible with Avalonia, but looks like I figured most of it out:

<NumericUpDown KeyDown="HandleNonNumericInput">

And in the code behind

private void HandleNonNumericInput(object? sender, KeyEventArgs? e)
{
    string? letter = e.KeySymbol;
    bool rejectKey;
    if (string.IsNullOrEmpty(letter))
    {
        rejectKey = true;
    }
    else if (e.Key == Key.Enter || e.Key == Key.Return || e.Key == Key.Escape)
    {
        TextBox? tb = e.Source as TextBox;
        TopLevel? tl = TopLevel.GetTopLevel(this);
        tl!.Focus();
        rejectKey = true;
    }
    else
    {
        rejectKey = !char.IsNumber(letter[0]);
    }
    Debug.WriteLine($"Key: {e.Key}, Symbol: <{letter}>, rejected: {rejectKey}");
    e.Handled = rejectKey;char.IsNumber(e.KeySymbol[0]);
}

I may have forgotten to check for something, if I realize, I'll update. One big problem remains, but that's probably for another question: If at any time the input string is "" there is an exception below the TextBox that doesn't go away.

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