Number Only Behavior for WPF

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Introduction

I have struggled to try and create a good behavior to allow users only to enter number keys. Of course, most of my problem was I did not know the right approach. Seemed that the event to use was the KeyPress event, but it is not because there is no easy way to distinguish if the user has entered a digit or not because the event arguments return only the key, and not the resulting character. The only good way that I know of is to attach to the TextInput event. In this event, I use LINQ to check each if any of the characters of the string are not a digit, and if they are not, set the Handled equal to true. I have seen a lot of implementations that use a Regular Expression to check if all the characters are digits, but I am sure that this is a lot heavier than just checking that all are digits.

I also use the DataObject.AddPastingHandler to add an event handler that will ensure that pasted values are text and only contain digits.

The Code

There is one DependencyProperty for this behavior:

public static readonly DependencyProperty IsEnabledProperty =
  DependencyProperty.RegisterAttached("IsEnabled", typeof(bool),
  typeof(NumberOnlyBehaviour), new UIPropertyMetadata(false, OnValueChanged));

public static bool GetIsEnabled(Control o) { return (bool)o.GetValue(IsEnabledProperty); }

public static void SetIsEnabled(Control o, bool value) { o.SetValue(IsEnabledProperty, value); }

As usual for my properties on behaviours, I try to use something familiar, and a bool IsEnabled seemed to be an obvious name for the property. To use it just need to set IsEnabled to true.

The event handler for this DespendencyProperty is OnValueChanged. The code is as follows:

private static void OnValueChanged(DependencyObject dependencyObject,
  DependencyPropertyChangedEventArgs e)
{
 var uiElement = dependencyObject as Control;
 if (uiElement == null) return;
 if (e.NewValue is bool && (bool)e.NewValue)
 {
  uiElement.PreviewTextInput += OnTextInput;
  uiElement.PreviewKeyDown += OnPreviewKeyDown;
  DataObject.AddPastingHandler(uiElement, OnPaste);
 }

 else
 {
  uiElement.PreviewTextInput -= OnTextInput;
  uiElement.PreviewKeyDown -= OnPreviewKeyDown;
  DataObject.RemovePastingHandler(uiElement, OnPaste);
 }
}

Basically this event handler subscribes to the two events that will filter out any user input that is not a number, and also add a handler that will deal with pasting of text when the behavior is enabled by setting the IsEnabled equal to true, and otherwise removes the handlers.

The two keyboard event handlers are:

private static void OnTextInput(object sender, TextCompositionEventArgs e)
{
 if (e.Text.Any(c => !char.IsDigit(c))) { e.Handled = true; }
}

private static void OnPreviewKeyDown(object sender, KeyEventArgs e)
{
 if (e.Key == Key.Space)e.Handled = true;
}

The OnPreviewKeyDown event handler is required because the PreviewTextInput event does not fire when the space key is pressed, so also have include an event handler for the PreviewKeyDown. All other key presses cause a change in the TextInput, so the checking if the text contains a non-digit value. Unfortunately it is very difficult to know the effect of the key press on the KeyDown or KeyUp events because the KeyEventArgs only provide the enumeration of the Key and not the effect, so handling only one KeyPress event is very hard to implement.

And finally the paste handler:

private static void OnPaste(object sender, DataObjectPastingEventArgs e)
{
 if (e.DataObject.GetDataPresent(DataFormats.Text))
 {
  var text = Convert.ToString(e.DataObject.GetData(DataFormats.Text)).Trim();
  if (text.Any(c => !char.IsDigit(c))) { e.CancelCommand(); }
 }
 else
 {
  e.CancelCommand();
 }
}

How to use this behaviour

To use this behavior is very easy. On the control you want to allow only numbers, you would add this behavior as shown in bold:

<TextBox Style="{StaticResource EditFormTextBoxNumericStyle}"

	behaviours:NumberOnlyBehaviour.IsEnabled="True"

	Text="{Binding Sequence,
		UpdateSourceTrigger=PropertyChanged,
		ValidatesOnDataErrors=True}" />

History

• 1^st October, 2015: Initial version
• 3 March 2016 added updated source code to handle the space key as recommended by George Swan.