WNDCLASS wc;
wc.lpfnWndProc = WndProc;

error LNK2001: неразрешенный внешний символ ""long __stdcall WndProc(struct HWND__ *,unsigned int,unsigned int,long)" (?WndProc@@YGJPAUHWND__@@IIJ@Z)"

#include <windows.h>

LRESULT __stdcall WndProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam);

int __stdcall WinMain (HINSTANCE hInstance,
                       HINSTANCE hPrevInstance,
                       LPSTR lpCmdLine, int nCmdShow)
{
	WNDCLASS wc; // WNDCLASS - структура!
	wc.style = CS_OWNDC;
	wc.lpfnWndProc = WndProc;
	wc.cbClsExtra = 0;
	wc.cbWndExtra = 0;
	wc.hInstance = hInstance;
	wc.hIcon = LoadIcon(NULL, IDI_APPLICATION);
	wc.hCursor = LoadCursor(NULL, IDC_ARROW);
	wc.hbrBackground = (HBRUSH)(6);
	wc.lpszMenuName = NULL;
	wc.lpszClassName = L"class";
	
	RegisterClass(&wc);

	HWND hWnd = CreateWindow(L"class",L"заготовка программы",
                         WS_OVERLAPPEDWINDOW,
                         150,100,500,400,
                         NULL,NULL,hInstance,NULL);

	ShowWindow(hWnd, nCmdShow);
	UpdateWindow(hWnd);
	MSG msg;

	while (true)
	{
	  if (PeekMessage(&msg,hWnd,0,0,PM_REMOVE))
	  {
		if (msg.message == WM_QUIT)
		  break;

		TranslateMessage(&msg);
		DispatchMessage(&msg);
	  }
	}
}

LRESULT __stdcall WndProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam)
{
 return 0;
}