求助 关于window api 窗口回调函数问题

[只负责写Bug]

// namespace
class D3D
{
public:
	D3D();
	~D3D();
	LRESULT CALLBACK WndProc(HWND hwnd, UINT message, WPARAM wParam, LPARAM lParam);

private:

};

LRESULT CALLBACK D3D::WndProc(HWND hwnd, UINT message, WPARAM wParam, LPARAM lParam)
{
	switch (message)
	{
		case WM_DESTROY:
			 PostQuitMessage(0);
			 return 0;
	}
	return DefWindowProc(hwnd, message, wParam, lParam);
}

wndclass.lpfnWndProc = (WNDPROC) D3D::WndProc;  //这行报错

报错说类型转换无效，但是我把类改成命名空间就没有问题，求大佬指点下声明原因，谢谢