#948 – Complete Example of Limiting TextBox Input

If you want to limit text allowed as input to a TextBox, a full strategy for checking text being input should include handling the PreviewKeyDown and PreviewTextInput events, as well as implementing a pasting handler.  Below is a full example that limits text input to alphabetic characters only.

        <TextBox Name="txtMyText" Margin="5" Height="100"
                 TextWrapping="Wrap"
                 AcceptsReturn="True"
                 VerticalScrollBarVisibility="Auto"
                 PreviewTextInput="TextBox_PreviewTextInput"
                 PreviewKeyDown="TextBox_PreviewKeyDown"/>

 

        public MainWindow()
        {
            this.InitializeComponent();
            DataObject.AddPastingHandler(txtMyText, PasteHandler);
        }

        private bool IsAlphabetic(string s)
        {
            Regex r = new Regex(@"^[a-zA-Z]+$");

            return r.IsMatch(s);
        }

        private void TextBox_PreviewTextInput(object sender, TextCompositionEventArgs e)
        {
            // Prohibit non-alphabetic
            if (!IsAlphabetic(e.Text))
                e.Handled = true;
        }

        private void TextBox_PreviewKeyDown(object sender, KeyEventArgs e)
        {
            // Prohibit space
            if (e.Key == Key.Space)
                e.Handled = true;
        }

        private void PasteHandler(object sender, DataObjectPastingEventArgs e)
        {
            TextBox tb = sender as TextBox;
            bool textOK = false;

            if (e.DataObject.GetDataPresent(typeof(string)))
            {
                // Allow pasting only alphabetic
                string pasteText = e.DataObject.GetData(typeof(string)) as string;
                if (IsAlphabetic(pasteText))
                    textOK = true;
            }

            if (!textOK)
                e.CancelCommand();
        }
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#945 – A Strategy for Limiting Allowed Text in a TextBox

You can limit the text that a user enters into a TextBox by handling the PreviewTextInput event and setting the TextCompositionEventArgs.Handled property to true for characters that you do not want to allow as input.

The PreviewTextInput event will not give you access to every possible keystroke that you might want to use in limiting input.  It’s not fired, for example, when the user presses the spacebar.

You often will also want to handle the PreviewKeyDown event to block keystrokes that don’t trigger PreviewTextInput.

Finally, you may want to intercept Paste events on a TextBox, in order to filter out text that you don’t want a user to paste into the TextBox.

A full strategy for limiting user-entered text might then include:

  • Handling PreviewTextInput and blocking undesirable text
  • Handling PreviewKeyDown and blocking undesirable keystrokes
  • Handling paste operations and blocking undesirable text

#638 – PreviewTextInput Is Not Fired In Many Cases

You’d normally use the PreviewTextInput event to filter data being entered into a text-based control like the TextBox.  But the PreviewTextInput event is fired for text that is being added to the control, but not fired for certain keypresses that could also result in changes to the text.

Key presses that do not result in the PreviewTextInput event being fired include:

  • Spacebar
  • Backspace
  • Home/End/Delete/Insert keys
  • Arrow keys
  • Control key combinations, including Ctrl+V

This means that if you were only validating text input in PreviewTextInput, a user could use Ctrl+V to do a paste operation into a TextBox and you wouldn’t get a chance to validate the text being added.

All of these key press do result in the PreviewKeyDown event being fired.  This means that to do complete validation of all changes to a TextBox, you’ll likely want to do some validation in PreviewTextInput and some additional validation in PreviewKeyDown.

#637 – Block Repeating Keys by Setting Handled to true

In a keypress event handler, the IsRepeat property indicates whether the current keypress is a result of a character that is repeating because the user is holding the key down.  If you want to prohibit the use of repeating keys in a control, you can check this property and set the Handled property to true if IsRepeat is true.

Let’s say that we want to prohibit repeating characters in a TextBox.  We can do this by adding logic to a handler for the PreviewKeyDown event.  We don’t do this in the KeyDown event, since the TextBox will swallow this event for certain keys.

We define the handler in XAML.

<TextBox HorizontalAlignment="Center" Width="150" Margin="10"
         PreviewKeyDown="TextBox_PreviewKeyDown" />

We then set Handled based on the value of IsRepeat.

        private void TextBox_PreviewKeyDown(object sender, KeyEventArgs e)
        {
            e.Handled = e.IsRepeat;
        }

#620 – Why Are There So Many KeyPress Events?

When you press a single key within a WPF application, there are potentially a number of different keypress events that can fire.  But why are there so many different events that can possibly fire?

There are several reasons why there isn’t just a single keypress event.

  • Your application will only see the events for which you define a handler.  There is a large set of possible events so that you can choose which ones make sense for your application to handle.
  • Preview events exist so that a parent control can intercept keypress events for its children or descendants.  The tunneling allows you to decide at what level you want to preview the event.
  • The actual keypress event bubbles up the logical tree so that you can decide at what level to actually respond to the event.  In this way, you can handle multiple events in a single place.

#619 – Event Sequence for the Key Up/Down Events

There are four basic events related to a key being pressed or released that a GUI element can fire in WPF.  An event fires when you press a key down (KeyDown) and a different event fires when you release the key (KeyUp).  When these events fire for an element, they fire first for the element and then work back up the logical tree, firing for each ancestor element.  These are bubbling events.

There are also the PreviewKeyDown and PreviewKeyUp events, which fire before the KeyDown and KeyUp events, but fire from the top of the logical tree down to the control where the event originated.  (Tunneling events).

For a Window containing a StackPanel that contains a TextBox, the event sequence when a user presses a key while the TextBox has focus is:

  • Window_PreviewKeyDown
  • StackPanel_PreviewKeyDown
  • TextBox_PreviewKeyDown
  • TextBox_KeyDown
  • StackPanel_KeyDown
  • Window_KeyDown
  • Window_PreviewKeyUp
  • StackPanel_PreviewKeyUp
  • TextBox_PreviewKeyUp
  • TextBox_KeyUp
  • StackPanel_KeyUp
  • Window_KeyUp

 

#588 – If You Handle PreviewKeyDown Event, KeyDown Won’t Fire

The PreviewKeyDown and KeyDown events in WPF are paired routed events.  When a user presses a key in a control, the PreviewKeyDown event fires first, as a tunneling event.  When the event has propagated down the logical tree to the control where the key press originated, the KeyDown event fires.  KeyDown propagates up the logical tree, since it is defined as a bubbling event.

If you handle the PreviewKeyDown event as it is propagating down the tree and you mark the event as handled (setting KeyEventArgs.Handled to true), the PreviewKeyDown event will not continue propagating down the tree.

But if you mark PreviewKeyDown as handled, the corresponding KeyDown event will not fire at all.  This works because the two events share the same instance of a KeyEventArgs object, so when PreviewKeyDown marks the event as handled, KeyDown also treats the event as handled.