#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.