Overview of using the keyboard (Windows Forms .NET)

Artikel
04/19/2024

In Windows Forms, user input is sent to applications in the form of Windows messages. A series of overridable methods process these messages at the application, form, and control level. When these methods receive keyboard messages, they raise events that can be handled to get information about the keyboard input. In many cases, Windows Forms applications will be able to process all user input simply by handling these events. In other cases, an application may need to override one of the methods that process messages in order to intercept a particular message before it is received by the application, form, or control.

Keyboard events

All Windows Forms controls inherit a set of events related to mouse and keyboard input. For example, a control can handle the KeyPress event to determine the character code of a key that was pressed. For more information, see Using keyboard events.

Methods that process user input messages

Forms and controls have access to the IMessageFilter interface and a set of overridable methods that process Windows messages at different points in the message queue. These methods all have a Message parameter, which encapsulates the low-level details of Windows messages. You can implement or override these methods to examine the message and then either consume the message or pass it on to the next consumer in the message queue. The following table presents the methods that process all Windows messages in Windows Forms.

Method	Notes
PreFilterMessage	This method intercepts queued (also known as posted) Windows messages at the application level.
PreProcessMessage	This method intercepts Windows messages at the form and control level before they have been processed.
WndProc	This method processes Windows messages at the form and control level.
DefWndProc	This method performs the default processing of Windows messages at the form and control level. This provides the minimal functionality of a window.
OnNotifyMessage	This method intercepts messages at the form and control level, after they have been processed. The EnableNotifyMessage style bit must be set for this method to be called.

Keyboard and mouse messages are also processed by an additional set of overridable methods that are specific to those types of messages. For more information, see the Preprocessing keys section. .

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.

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.

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 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 `true`, it means the control is a regular character and a KeyDown event is raised. If `false`, ProcessDialogKey is called. Note: 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 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 `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. For more information on overriding methods, see Inheritance (C# Programming Guide) or Inheritance (Visual Basic)

Task	Method
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 `true` for the key or keys you want.
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.

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