Question

In my application, I want to let users customize keyboard shortcuts, just like it’s done in Visual Studio’s keyboard options. The user can focus a blank text box and then type any shortcut he wants to assign to a command.

The closest I’ve come to make it work is by subscribing to the TextBox.PreviewKeyDown event, setting it as handled to prevent actual text input in the text box. I then ignore the KeyDown events associated with modifier keys (is there a cleaner way to determine if a Key is a modifier key?).

// Code-behind
private void ShortcutTextBox_PreviewKeyDown(object sender, KeyEventArgs e)
{
    // The text box grabs all input
    e.Handled = true;

    if (e.Key == Key.LeftCtrl || 
        e.Key == Key.RightCtrl || 
        e.Key == Key.LeftAlt ||
        e.Key == Key.RightAlt || 
        e.Key == Key.LeftShift ||
        e.Key == Key.RightShift)
        return;

    string shortcutText = "";
    if ((Keyboard.Modifiers & ModifierKeys.Control) == ModifierKeys.Control)
        shortcutText += "Ctrl+";
    if ((Keyboard.Modifiers & ModifierKeys.Shift) == ModifierKeys.Shift)
        shortcutText += "Shift+";
    if ((Keyboard.Modifiers & ModifierKeys.Alt) == ModifierKeys.Alt)
        shortcutText += "Alt+";
    _ShortcutTextBox.Text = shortcutText + e.Key.ToString();

}

The above works for any shortcut starting with Ctrl and Ctrl+Shift, but fails for any Alt shortcuts. The e.Key is always set to Key.System when I press a shortcut containing Alt.

How can I record Alt shortcuts from the user? Is there a better, more robust way to record shortcuts form the user?

Answer 1

The trick is to use the SystemKey property if the Key property is set to Key.System:

private void ShortcutTextBox_PreviewKeyDown(object sender, KeyEventArgs e)
{
    // The text box grabs all input.
    e.Handled = true;

    // Fetch the actual shortcut key.
    Key key = (e.Key == Key.System ? e.SystemKey : e.Key);

    // Ignore modifier keys.
    if (key == Key.LeftShift || key == Key.RightShift
        || key == Key.LeftCtrl || key == Key.RightCtrl
        || key == Key.LeftAlt || key == Key.RightAlt
        || key == Key.LWin || key == Key.RWin) {
        return;
    }

    // Build the shortcut key name.
    StringBuilder shortcutText = new StringBuilder();
    if ((Keyboard.Modifiers & ModifierKeys.Control) != 0) {
        shortcutText.Append("Ctrl+");
    }
    if ((Keyboard.Modifiers & ModifierKeys.Shift) != 0) {
        shortcutText.Append("Shift+");
    }
    if ((Keyboard.Modifiers & ModifierKeys.Alt) != 0) {
        shortcutText.Append("Alt+");
    }
    shortcutText.Append(key.ToString());

    // Update the text box.
    _ShortcutTextBox.Text = shortcutText.ToString();
}

I added the left and right Windows keys to the modifier list, because they sometimes appeared in the shortcut key name when a complex (Ctrl+Shift+Alt) key combination was typed from a Terminal Server session. They’re never present in Keyboard.Modifiers, though, since they’re reserved for global shortcuts, so I don’t handle them there.

I also used a StringBuilder to avoid creating too many string instances.

This solution works with any key combination, except Shift+Alt (the Alt modifier is not seen in that case). That might be an artifact of my Terminal Server environment, though, so your mileage may vary.

Finally, I added a _File menu to the window to see what would happen, and the Alt+F shortcut key is effectively trapped by the text box before it reaches the menu, which seems to be what you want.