Working Principle of Keyboard Input
Windows Forms handles keyboard input by raising keyboard events in response to Windows messages. Most Windows Forms applications handle keyboard input exclusively by processing keyboard events. However, to implement more advanced keyboard input scenarios (such as intercepting keystrokes before they reach a control), it is essential to understand how keyboard messages work. This topic describes the types of keystroke data that Windows Forms can recognize and outlines how keyboard messages are delivered. For information about keyboard events, see Using Keyboard Events.
Windows Forms identifies keyboard input as virtual key codes represented by the bitwise Keys enumeration. The Keys enumeration allows you to combine a series of keystrokes to generate a single value, which corresponds to the values accompanied by the WM_KEYDOWN and WM_SYSKEYDOWN Windows messages. Most physical key operations can be detected by handling the KeyDown or KeyUp events.
Character keys are a subset of the Keys enumeration, corresponding to values accompanied by the WM_CHAR and WM_SYSCHAR Windows messages. If a keystroke combination produces a character, you can detect it by handling the KeyPress event. Alternatively, you can use the Keyboard object exposed by the Visual Basic programming interface to identify and send pressed keys. For more information, see Accessing the Keyboard.
As listed above, three keyboard-related events can occur on a control. The following is the general order in which these events occur:
- The user presses the "a" key. The key is preprocessed and dispatched, and a
KeyDown event occurs.
- The user holds down the "a" key. The key is preprocessed and dispatched, and a
KeyPress event occurs.
- This event occurs multiple times if the user holds down the key.
- The user releases the "a" key. The key is preprocessed and dispatched, and a
KeyUp event occurs.
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