c - 如果在注册窗口类时已经提供了 hInstance，为什么 CreateWindow() 将 hInstance 作为参数？

Question

注册窗口类时需要填写WNDCLASS结构，它有一个 hInstance领域:

typedef struct {
    UINT style;
    WNDPROC lpfnWndProc;
    int cbClsExtra;
    int cbWndExtra;
    HINSTANCE hInstance;
    HICON hIcon;
    HCURSOR hCursor;
    HBRUSH hbrBackground;
    LPCTSTR lpszMenuName;
    LPCTSTR lpszClassName;
} WNDCLASS

然后，使用 CreateWindow() 创建窗口它接受这些参数:

HWND CreateWindow(          
    LPCTSTR lpClassName,
    LPCTSTR lpWindowName,
    DWORD dwStyle,
    int x,
    int y,
    int nWidth,
    int nHeight,
    HWND hWndParent,
    HMENU hMenu,
    HINSTANCE hInstance,
    LPVOID lpParam
);

---

• 如果hInstance提供了 WNDCLASS注册类(class)时，为什么需要提供 hInstance至 CreateWindow() ?

• 是 hInstance提供给WNDCLASS应该与 hInstance 相同提供给CreateWindow() ?

• 如果WNDCLASS hInstance和 CreateWindow() hInstance不同，它们是什么意思？

---

这是一个演示类注册/窗口创建的示例程序:

#include <windows.h>

LPSTR szClassName = "MyClass";
HINSTANCE hInstance;
LRESULT CALLBACK MyWndProc(HWND, UINT, WPARAM, LPARAM);

int WINAPI WinMain(HINSTANCE hInst, HINSTANCE hPrevInstance, LPSTR szCmdLine, int iCmdShow)
{
   WNDCLASS wnd;
   MSG msg;
   HWND hwnd;

   hInstance = hInst;

   wnd.style = CS_HREDRAW | CS_VREDRAW; //we will explain this later
   wnd.lpfnWndProc = MyWndProc;
   wnd.cbClsExtra = 0;
   wnd.cbWndExtra = 0;
   wnd.hInstance = hInstance;
   wnd.hIcon = LoadIcon(NULL, IDI_APPLICATION); //default icon
   wnd.hCursor = LoadCursor(NULL, IDC_ARROW);   //default arrow mouse cursor
   wnd.hbrBackground = (HBRUSH)(COLOR_BACKGROUND+1);
   wnd.lpszMenuName = NULL;                     //no menu
   wnd.lpszClassName = szClassName;

   if(!RegisterClass(&wnd))                     //register the WNDCLASS
   {
       MessageBox(NULL, "This Program Requires Windows NT", 
                        "Error", MB_OK);
       return 0;
   }

   hwnd = CreateWindow(szClassName,
                       "Window Title",
                       WS_OVERLAPPEDWINDOW, //basic window style
                       CW_USEDEFAULT,
                       CW_USEDEFAULT,       //set starting point to default value
                       CW_USEDEFAULT,
                       CW_USEDEFAULT,       //set all the dimensions to default value
                       NULL,                //no parent window
                       NULL,                //no menu
                       hInstance,
                       NULL);               //no parameters to pass
   ShowWindow(hwnd, iCmdShow);              //display the window on the screen
   UpdateWindow(hwnd);             //make sure the window is updated correctly

   while(GetMessage(&msg, NULL, 0, 0))      //message loop
   {
       TranslateMessage(&msg);
       DispatchMessage(&msg);
   }
   return msg.wParam;
}

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

Answer 1

注册窗口类时，它可以是全局的(设置了 CS_GLOBALCLASS 样式)，也可以是注册它的模块的本地类。对于全局类，类名必须是唯一的，但对于模块本地类，类名可能会发生冲突——没有什么可以阻止两个 DLL 注册同名的窗口类，并且一个可执行文件可以加载这两个 DLL。

因此，您需要告诉 CreateWindow 函数您要使用指定类的哪个模块实例来创建窗口。