How Keyboard Input Works
Windows Forms processes keyboard input by raising keyboard events in response to Windows messages. Most Windows Forms applications process keyboard input exclusively by handling the keyboard events. However, you need to understand how keyboard messages work so you can implement more advanced keyboard-input scenarios, such as intercepting keys before they reach a control. This topic describes the types of key data that Windows Forms recognizes and provides an overview of how keyboard messages are routed. For information about keyboard events, see Using Keyboard Events.
Types of Keys
Windows Forms identifies keyboard input as virtual-key codes that are represented by the bitwise Keys enumeration. With the Keys enumeration, you can combine a series of pressed keys to result in a single value. These values correspond to the values that accompany the WM_KEYDOWN and WM_SYSKEYDOWN Windows messages. You can detect most physical key presses by handling the KeyDown or KeyUp events. Character keys are a subset of the Keys enumeration and correspond to the values that accompany the WM_CHAR and WM_SYSCHAR Windows messages. If the combination of pressed keys results in a character, you can detect the character by handling the KeyPress event. Alternatively, you can use Keyboard, exposed by Visual Basic programming interface, to discover which keys were pressed and send keys. For more information, see Accessing the Keyboard.
Order of Keyboard Events
As listed previously, there are 3 keyboard related events that can occur on a control. The following sequence shows the general order of the events:
The user pushes the "a" key, the key is preprocessed, dispatched, and a KeyDown event occurs.
The user holds the "a" key, the key is preprocessed, dispatched, and a KeyPress event occurs.
This event occurs multiple times as the user holds a key.
The user releases the "a" key, the key is preprocessed, dispatched and a KeyUp event occurs.
Like other messages, keyboard messages are processed in the WndProc method of a form or control. However, before keyboard messages are processed, the PreProcessMessage method calls one or more methods that can be overridden to handle special character keys and physical keys. You can override these methods to detect and filter certain keys before the messages are processed by the control. The following table shows the action that is being performed and the related method that occurs, in the order that the method occurs.
Preprocessing for a KeyDown event
|Check for a command key such as an accelerator or menu shortcut.||ProcessCmdKey||This method processes a command key, which takes precedence over regular keys. If this method returns
|Check for a special key that requires preprocessing or a normal character key that should raise a KeyDown event and be dispatched to a control.||IsInputKey||If the method returns
|Check for a navigation key (ESC, TAB, Return, or arrow keys).||ProcessDialogKey||This method processes a physical key that employs special functionality within the control, such as switching focus between the control and its parent. If the immediate control does not handle the key, the ProcessDialogKey is called on the parent control and so on to the topmost control in the hierarchy. If this method returns
Preprocessing for a KeyPress Event
|Check to see the key is a normal character that should be processed by the control||IsInputChar||If the character is a normal character, this method returns
|Check to see if the character is a mnemonic (such as &OK on a button)||ProcessDialogChar||This method, similar to ProcessDialogKey, will be called up the control hierarchy. If the control is a container control, it checks for mnemonics by calling ProcessMnemonic on itself and its child controls. If ProcessDialogChar returns
Processing Keyboard Messages
After keyboard messages reach the WndProc method of a form or control, they are processed by a set of methods that can be overridden. Each of these methods returns a Boolean value specifying whether the keyboard message has been processed and consumed by the control. If one of the methods returns
true, then the message is considered handled, and it is not passed to the control's base or parent for further processing. Otherwise, the message stays in the message queue and may be processed in another method in the control's base or parent. The following table presents the methods that process keyboard messages.
|ProcessKeyMessage||This method processes all keyboard messages that are received by the WndProc method of the control.|
|ProcessKeyPreview||This method sends the keyboard message to the control's parent. If ProcessKeyPreview returns
|ProcessKeyEventArgs||This method raises the KeyDown, KeyPress, and KeyUp events, as appropriate.|
Overriding Keyboard Methods
There are many methods available for overriding when a keyboard message is preprocessed and processed; however, some methods are much better choices than others. Following table shows tasks you might want to accomplish and the best way to override the keyboard methods. For more information on overriding methods, see Overriding properties and methods in derived classes.
|Intercept a navigation key and raise a KeyDown event. For example you want TAB and Return to be handled in a text box.||Override IsInputKey. Note: Alternatively, you can handle the PreviewKeyDown event and set IsInputKey of the PreviewKeyDownEventArgs to
|Perform special input or navigation handling on a control. For example, you want the use of arrow keys in your list control to change the selected item.||Override ProcessDialogKey|
|Intercept a navigation key and raise a KeyPress event. For example in a spin-box control you want multiple arrow key presses to accelerate progression through the items.||Override IsInputChar.|
|Perform special input or navigation handling during a KeyPress event. For example, in a list control holding down the "r" key skips between items that begin with the letter r.||Override ProcessDialogChar|
|Perform custom mnemonic handling; for example, you want to handle mnemonics on owner-drawn buttons contained in a toolbar.||Override ProcessMnemonic.|