DefWindowProc

The DefWindowProc function calls the default window procedure to provide default processing for any window messages that an application does not process. This function ensures standard behavior for windows that your program creates.

Syntax

LRESULT DefWindowProc(
    HWND   hWnd,
    UINT   Msg,
    WPARAM wParam,
    LPARAM lParam
);

Parameters

Parameter	Type	Description
`hWnd`	`HWND`	Handle to the window receiving the message.
`Msg`	`UINT`	Message identifier.
`wParam`	`WPARAM`	Additional message information. The content of this parameter depends on the value of `Msg`.
`lParam`	`LPARAM`	Additional message information. The content of this parameter depends on the value of `Msg`.

Return Value

The return value is a LRESULT that indicates the result of message processing. Its meaning depends on the message specified by Msg.

Remarks

Every window procedure should call DefWindowProc for any messages it does not explicitly handle.
If you process a message and do not call DefWindowProc, you must ensure you return an appropriate value for that message.
Commonly used messages that require default processing include WM_NCCREATE, WM_NCHITTEST, and WM_GETMINMAXINFO.
The function is exported from User32.dll.

Example

#include <windows.h>

LRESULT CALLBACK MainWndProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam)
{
    switch (message)
    {
        case WM_CREATE:
            // Initialization code here
            return 0;

        case WM_PAINT:
            // Paint handling here
            return 0;

        case WM_DESTROY:
            PostQuitMessage(0);
            return 0;

        default:
            // Let the default window procedure handle everything else
            return DefWindowProc(hWnd, message, wParam, lParam);
    }
}

Microsoft Docs – Windows API