DefMDIChildProcA function (winuser.h)

Article
02/09/2023

Provides default processing for any window message that the window procedure of a multiple-document interface (MDI) child window does not process. A window message not processed by the window procedure must be passed to the DefMDIChildProc function, not to the DefWindowProc function.

Syntax

LRESULT LRESULT DefMDIChildProcA(
  [in] HWND   hWnd,
  [in] UINT   uMsg,
  [in] WPARAM wParam,
  [in] LPARAM lParam
);

Parameters

[in] hWnd

Type: HWND

A handle to the MDI child window.

[in] uMsg

Type: UINT

The message to be processed.

[in] wParam

Type: WPARAM

Additional message-specific information.

[in] lParam

Type: LPARAM

Additional message-specific information.

Return value

Type: LRESULT

The return value specifies the result of the message processing and depends on the message.

Remarks

The DefMDIChildProc function assumes that the parent window of the MDI child window identified by the hWnd parameter was created with the MDICLIENT class.

When an application's window procedure does not handle a message, it typically passes the message to the DefWindowProc function to process the message. MDI applications use the DefFrameProc and DefMDIChildProc functions instead of DefWindowProc to provide default message processing. All messages that an application would usually pass to DefWindowProc (such as nonclient messages and the WM_SETTEXT message) should be passed to DefMDIChildProc instead. In addition, DefMDIChildProc also handles the following messages.

Message	Response
WM_CHILDACTIVATE	Performs activation processing when MDI child windows are sized, moved, or displayed. This message must be passed.
WM_GETMINMAXINFO	Calculates the size of a maximized MDI child window, based on the current size of the MDI client window.
WM_MENUCHAR	Passes the message to the MDI frame window.
WM_MOVE	Recalculates MDI client scroll bars if they are present.
WM_SETFOCUS	Activates the child window if it is not the active MDI child window.
WM_SIZE	Performs operations necessary for changing the size of a window, especially for maximizing or restoring an MDI child window. Failing to pass this message to the DefMDIChildProc function produces highly undesirable results.
WM_SYSCOMMAND	Handles window menu commands: SC_NEXTWINDOW, SC_PREVWINDOW, SC_MOVE, SC_SIZE, and SC_MAXIMIZE.

Note

The winuser.h header defines DefMDIChildProc as an alias which automatically selects the ANSI or Unicode version of this function based on the definition of the UNICODE preprocessor constant. Mixing usage of the encoding-neutral alias with code that not encoding-neutral can lead to mismatches that result in compilation or runtime errors. For more information, see Conventions for Function Prototypes.

Requirements

Requirement	Value
Minimum supported client	Windows 2000 Professional [desktop apps only]
Minimum supported server	Windows 2000 Server [desktop apps only]
Target Platform	Windows
Header	winuser.h (include Windows.h)
Library	User32.lib
DLL	User32.dll