WM_CLOSE message

Sent as a signal that a window or an application should terminate.

A window receives this message through its WindowProc function.

#define WM_CLOSE                        0x0010

wParam

This parameter is not used.

lParam

This parameter is not used.

Type: LRESULT

If an application processes this message, it should return zero.

LRESULT CALLBACK WindowProc(
    __in HWND hWindow,
    __in UINT uMsg,
    __in WPARAM wParam,
    __in LPARAM lParam)
{
    switch (uMsg)
    {
    case WM_CLOSE:
        DestroyWindow(hWindow);
        break;
    case WM_DESTROY:
        PostQuitMessage(0);
        break;
    default:
        return DefWindowProc(hWindow, uMsg, wParam, lParam);
    }

    return 0;
}

Example from Windows Classic Samples on GitHub.

An application can prompt the user for confirmation, prior to destroying a window, by processing the WM_CLOSE message and calling the DestroyWindow function only if the user confirms the choice.

By default, the DefWindowProc function calls the DestroyWindow function to destroy the window.

Reference

DefWindowProc

DestroyWindow

Conceptual

Windows