kirupa.com - Using Keyboard Input in WPF

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Using Keyboard Input in WPF - Page 3
by kirupa | 7 March 2007

In the previous page you learned how to detect individual key presses. Another important thing you would want to do is detect combination key presses, and you just saw the code for doing that. Let's pick up from there and look at the implementation for Ctrl + S in greater detail:

// Ctrl + S
if ((Keyboard.Modifiers == ModifierKeys.Control) && (e.Key == Key.S))
{
}

To check if the Ctrl key has been pressed, I cannot access the Key enum and access the Ctrl key like I would any other key from the keyboard. This is the approach you used earlier to detect when the K key was pressed.

The key approach doesn't work for another reason because your commonly used Alt, Ctrl, Shift, and Windows keys can't be accessed from the Key enum at all. Instead, those four keys can only be accessed using the ModifierKeys enum and checking whether Keyboard.Modifiers is equal to that key. That's what is done in the first condition of the if statement for the Ctrl key:

Keyboard.Modifiers == ModifierKeys.Control

Learning to use ModifierKeys and setting it equal to Keyboard.Modifiers is really the tricky part of implementing key combinations. For the remaining keys, you can access them directly by checking if the key passed in by your event argument e is equal to the key on the keyboard you are checking. That can be seen in the second part of the if statement which takes care of your normal (non-modifier) key S:

(Keyboard.Modifiers == ModifierKeys.Control) && (e.Key == Key.S)

One final thing to note is that you use ampersands to see if your modifier key is pressed along with your key. Think of key combinations as checking whether two inequalities are equal. In the above code, I check if the Control (Ctrl) modifier key has been pressed and whether the second key that has been pressed is the S key. When both conditions are true, then your application recognizes your key combination.

Use Key Combinations with KeyUp instead of KeyDown
If you noticed, you added the above code to the event handler mapping KeyUp. That wasn't done arbitrarily, for there is a real good reason why KeyUp was used instead of KeyDown. When you are using key combinations, for example Ctrl + S, remember that you are actually pressing the keyboard buttons Ctrl and S. If you map those events to KeyDown you will find that not only is your application interpreting the Ctrl + S, it is sending those keys for display on the screen. You probably do not want that.

The result is something similar to what you see in the following image. Notice that after pressing Ctrl + S, the s character also displays in the textbox:

[ notice that the key combination also results in the pressed letter being shown ]

You can avoid having the above problem by having your key combination code located in an event handler linked to KeyUp.

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