用WINAPI创建的窗口不是绘图对象。有什么问题吗？

#include <windows.h>

LRESULT CALLBACK WindowProc(HWND hwnd, UINT uMsg, WPARAM wParam, LPARAM lParam)
{
    switch (uMsg)
    {
        case WM_PAINT:
        {
            PAINTSTRUCT ps;
            HDC hdc = BeginPaint(hwnd, &ps);
            
            // 创建红色画刷
            HBRUSH hBrush = CreateSolidBrush(RGB(255, 0, 0));
            
            // 选择画刷
            SelectObject(hdc, hBrush);
            
            // 绘制矩形
            Rectangle(hdc, 100, 100, 200, 200);
            
            // 释放画刷
            DeleteObject(hBrush);
            
            EndPaint(hwnd, &ps);
            return 0;
        }
        
        case WM_DESTROY:
            PostQuitMessage(0);
            return 0;
    }
    
    return DefWindowProc(hwnd, uMsg, wParam, lParam);
}

int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine, int nCmdShow)
{
    // 注册窗口类
    WNDCLASS wc = {};
    wc.lpfnWndProc = WindowProc;
    wc.hInstance = hInstance;
    wc.lpszClassName = "MyWindowClass";
    
    RegisterClass(&wc);
    
    // 创建窗口
    HWND hwnd = CreateWindowEx(0, "MyWindowClass", "My Window", WS_OVERLAPPEDWINDOW, CW_USEDEFAULT, CW_USEDEFAULT, 500, 500, NULL, NULL, hInstance, NULL);
    
    if (hwnd == NULL)
        return 0;
    
    // 显示窗口
    ShowWindow(hwnd, nCmdShow);
    
    // 消息循环
    MSG msg = {};
    while (GetMessage(&msg, NULL, 0, 0))
    {
        TranslateMessage(&msg);
        DispatchMessage(&msg);
    }
    
    return 0;
}