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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:

  1. The user pushes the "a" key, the key is preprocessed, dispatched, and a KeyDown event occurs.

  2. 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.

  3. The user releases the "a" key, the key is preprocessed, dispatched and a KeyUp event occurs.

Preprocessing Keys

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

Action Related method Notes

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 true, the key message is not dispatched and a key event does not occur. If it returns false, IsInputKey is called.

Check to see is a special key that requires preprocessing or if it is a normal character which should raise a KeyDown event and be dispatched to a control.

IsInputKey

If the method returns true, it means the control is a regular character and a KeyDown event is raised. If false, ProcessDialogKey is called.

NoteNote

To ensure a control gets a key or combination of keys, you can handle the PreviewKeyDown event and set IsInputKey of the PreviewKeyDownEventArgs to true for the key or keys you want.

Check to see if the key is a navigational 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 true, preprocessing is complete and a key event is not generated. If it returns false, a KeyDown event occurs.

Preprocessing for a KeyPress Event

Action Related method Notes

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 true, the KeyPress event is raised and no further preprocessing occurs. Otherwise ProcessDialogChar will be called.

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 true, a KeyPress event does not occur.

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.

Method Notes

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 true, no key event is generated, otherwise ProcessKeyEventArgs is called.

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.

Task Method

Intercept a navigational key and raise a KeyDown event. For example you want TAB and Return to be handled in a text box

Override IsInputKey.

Perform special input or navigational 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 a 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 navigational 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.

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