Question

I’d like to implement a custom command to capture a Backspace key gesture inside of a textbox, but I don’t know how. I wrote a test program in order to understand what’s going on, but the behaviour of the program is rather confusing. Basically, I just need to be able to handle the Backspace key gesture via wpf commands while keyboard focus is in the textbox, and without disrupting the normal behaviour of the Backspace key within the textbox. Here’s the xaml for the main window and the corresponding code-behind, too (note that I created a second command for the Enter key, just to compare its behaviour to that of the Backspace key):

<Window x:Class="WpfApplication1.Window1"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Title="Window1" Height="300" Width="300">
    <Grid>
        <TextBox Margin="44,54,44,128"
                 Name="textBox1" />
    </Grid>
</Window>

And here’s the corresponding code-behind:

using System.Windows;
using System.Windows.Input;

namespace WpfApplication1
{
    /// <summary>
    /// Interaction logic for EntryListView.xaml
    /// </summary>
    public partial class Window1 : Window
    {
        public static RoutedCommand EnterCommand = new RoutedCommand();
        public static RoutedCommand BackspaceCommand = new RoutedCommand();

        public Window1()
        {
            InitializeComponent();

            CommandBinding cb1 = new CommandBinding(EnterCommand, EnterExecuted, EnterCanExecute);
            CommandBinding cb2 = new CommandBinding(BackspaceCommand, BackspaceExecuted, BackspaceCanExecute);
            this.CommandBindings.Add(cb1);
            this.CommandBindings.Add(cb2);

            KeyGesture kg1 = new KeyGesture(Key.Enter);
            KeyGesture kg2 = new KeyGesture(Key.Back);
            InputBinding ib1 = new InputBinding(EnterCommand, kg1);
            InputBinding ib2 = new InputBinding(BackspaceCommand, kg2);
            this.InputBindings.Add(ib1);
            this.InputBindings.Add(ib2);
        }

        #region Command Handlers
        private void EnterCanExecute(object sender, CanExecuteRoutedEventArgs e)
        {
            MessageBox.Show("Inside EnterCanExecute Method.");
            e.CanExecute = true;
        }

        private void EnterExecuted(object sender, ExecutedRoutedEventArgs e)
        {
            MessageBox.Show("Inside EnterExecuted Method.");
            e.Handled = true;
        }

        private void BackspaceCanExecute(object sender, CanExecuteRoutedEventArgs e)
        {
            MessageBox.Show("Inside BackspaceCanExecute Method.");
            e.Handled = true;
        }

        private void BackspaceExecuted(object sender, ExecutedRoutedEventArgs e)
        {
            MessageBox.Show("Inside BackspaceExecuted Method.");
            e.Handled = true;
        }
        #endregion Command Handlers
    }
}

Any help would be very much appreciated. Thanks!

Andrew

Answer 1

Try adding the input bindings and command bindings to the textblock instead of the main window.

In XAML:

<TextBox x:Name ="tb1"/>

In Code

tb1.CommandBindings.Add(cb2);

….

tb1.InputBindings.Add(ib2);

I’m not sure why, but when you hit the backspace, the textblock prevents the keydown event from bubbling up to the window. (You can test this by adding a handler to the KeyDown event on the main window. When you press Enter, the handler fires, but when you press backspace, the event handler does not). Since RoutedCommands are based on RoutedEvents, as described in this post by Josh Smith, it means that the command on the window never fires.