Win32以递归方式调用SendMessage

Question

如下面的代码所示，对于自定义消息 WM_TESTMSG 处理程序，它调用 SendMessage 来发送相同的消息。我认为将会有一个 Stack Overflow异常并且 MessageBeep 将不会被调用，因为SendMessage将直接调用WindowProc。但是当我测试代码时，它运行良好！ 为什么没有例外？谢谢！

#include <Windows.h>

#define WM_TESTMSG (WM_USER + 0x100)
LRESULT CALLBACK MainWndProc(IN HWND hWnd, IN UINT uMsg, IN WPARAM wParam, IN LPARAM lParam)
{
    LRESULT ret = 0;
    switch (uMsg)
    {
        case WM_CREATE:
        {
            PostMessage(hWnd, WM_TESTMSG, 0, 0);
            break;
        }

        case WM_TESTMSG:
        {
            SendMessage(hWnd, WM_TESTMSG, 0, 0);
            MessageBeep(MB_ICONERROR);  // I think the program can't run to here
            break;
        }

        case WM_CLOSE:
        {
            DestroyWindow(hWnd);
            break;
        }
        case WM_DESTROY:
        {
            PostQuitMessage(0);
            break;
        }

        default:
        {
            ret = ::DefWindowProc(hWnd, uMsg, wParam, lParam);
            break;
        }
    }

    return ret;
}



BOOL RegisterWndClass(IN HINSTANCE hInstance)
{
    WNDCLASSEX wndClassEx;

    ZeroMemory(&wndClassEx, sizeof(wndClassEx));
    wndClassEx.cbSize = sizeof (wndClassEx);
    wndClassEx.style = CS_HREDRAW | CS_VREDRAW;
    wndClassEx.lpfnWndProc = MainWndProc;
    wndClassEx.cbClsExtra = 0;
    wndClassEx.cbWndExtra = 0;
    wndClassEx.hInstance = hInstance;
    wndClassEx.hIcon = LoadIcon(NULL, MAKEINTRESOURCE(IDI_HAND));
    wndClassEx.hCursor = LoadCursor(NULL, MAKEINTRESOURCE(IDC_ARROW));
    wndClassEx.hbrBackground = (HBRUSH)GetStockObject(DKGRAY_BRUSH);
    wndClassEx.lpszMenuName = NULL;
    wndClassEx.lpszClassName = TEXT("MyMainWnd");
    wndClassEx.hIconSm = LoadIcon(NULL, MAKEINTRESOURCE(IDI_ERROR));

    return ::RegisterClassEx(&wndClassEx);
}


int WINAPI WinMain(IN HINSTANCE hInstance, IN HINSTANCE hPrevInstance, LPSTR lpCmdLine, int nCmdShow)
{
    RegisterWndClass(hInstance);

    HWND hMainWnd = CreateWindowEx(0, L"MyMainWnd", L"MainWnd", WS_OVERLAPPEDWINDOW | WS_VISIBLE, CW_USEDEFAULT, CW_USEDEFAULT, CW_USEDEFAULT, CW_USEDEFAULT, NULL, 0, hInstance, 0);

    ShowWindow(hMainWnd, SW_SHOWNORMAL);
    UpdateWindow(hMainWnd);

    MSG msg;
    while (GetMessage(&msg, NULL, 0, 0))
    {
        TranslateMessage(&msg);
        DispatchMessage(&msg);
    }

    return msg.wParam;
}