INFO: Changing the Mouse Cursor for a Window (31747)

SUMMARY

There are two different methods that an application can use to change the mouse cursor in a window:

The application can use the SetCursor function, which will change the mouse cursor immediately.
The application can use the SetClassWord function to change the mouse cursor for all windows of a particular window class. This method affects the mouse cursor only while it resides within the client area of a window of that class.

This article provides additional details regarding these two methods.

MORE INFORMATION

A number of factors should be considered during the design of an application that changes the mouse cursor. The major consideration is that Windows sends the WM_SETCURSOR message any time the mouse cursor moves on the screen. Normally, Windows sends the message to the window "under" the mouse cursor. However, if a window sets the mouse capture, using the SetCapture function, that window receives all mouse messages, without regard to the position of the mouse cursor.

When an application calls SetCursor, the mouse cursor changes to reflect the cursor specified in the call. The cursor retains that shape until SetCursor is called again, either explicitly by the application, by the DefWindowProc function, or by another application.

Because Windows is a nonpreemptive multitasking environment, no other application will gain control of the processor until the application that has the processor releases it. If the application calls one of a number of Windows functions, it can potentially lose control of the processor. For a list of Windows functions that can cause control of the processor to pass between applications, search on the following words in the Microsoft Knowledge Base:

prod(winsdk) and nonpreemptive and multitasking

When the DefWindowProc or DefDlgProc function processes a WM_SETCURSOR message, it calls SetCursor to change the cursor to the default cursor for the application's window.

The application can prevent the cursor from changing by processing the WM_SETCURSOR message. A typical application that processes WM_SETCURSOR will have a global variable for the handle to the current cursor. When the application receives a WM_SETCURSOR message, it checks the global variable. If the variable is NULL, the application passes the WM_SETCURSOR message to DefWindowProc. Otherwise, the application calls SetCursor with the value in the global variable. To return the cursor to the window default cursor, set the global variable to NULL.

When Windows sends a WM_SETCURSOR message, it places the hit-test area code into the low-order word of the lParam parameter. The application can use the hit-test area code to determine what particular portion of the window is "under" the mouse cursor. For more information on the hit-test area codes, see the documentation for the WM_NCHITTEST message in the "Microsoft Windows Software Development Kit Reference Volume 1."