CreateWindowEx函数失败，但GetLastError（）返回ERROR_SUCCESS

Question

I am trying to create a simple window with C/C++ using the native Windows message queue system (without .NET). I followed the MSDN tutorial and wrote some basic code that creates an empty window:

void main()
    {
    HINSTANCE hinst;
    HWND hwndMain;
    WNDCLASSEX wnd;
    MSG msg;

    hinst = GetModuleHandle( NULL );
    memset( &wnd, 0, sizeof( wnd ) );
    wnd.cbSize = sizeof( wnd );
    wnd.lpszClassName = "MainWClass";
    wnd.lpfnWndProc = MainWProc;
    wnd.hInstance = hinst;
    int result = RegisterClassEx( &wnd );
    if( !result )
    {
        printf("RegisterClassEx error: %d\r\n", GetLastError() );
    }

    hwndMain = CreateWindowEx
        (
        0, //extended styles
        wnd.lpszClassName, //class name
        "Main Window", //window name
        WS_OVERLAPPEDWINDOW | WS_HSCROLL | WS_VSCROLL | WS_MINIMIZEBOX, //style tags
        CW_USEDEFAULT, //horizontal position
        CW_USEDEFAULT, //vertical position
        CW_USEDEFAULT, //width
        CW_USEDEFAULT, //height
        (HWND) NULL, //parent window
        (HMENU) NULL, //class menu
        (HINSTANCE) wnd.hInstance, //some HINSTANCE pointer
        NULL //Create Window Data?
        );

    if( !hwndMain )
    {
        printf("Oh shi- %d\n", GetLastError() );
    }
    ShowWindow( hwndMain, SW_SHOWDEFAULT );
    UpdateWindow( hwndMain );
}

When I run/debug the program, CreateWindowEx returns 0 which means it failed. This triggers the error message "Oh shi- [error code]". The most confusing part is that the error message prints to console:

Oh shi- 0

The error code returned by GetLastError() is 0, which is ERROR_SUCCESS!

I am at a total loss; what is happening? I am so confuse...

PS I am using Visual C++ Express 2010 on Windows 7 32-bit. I have written a Windows Procedure elsewhere but it simply returns 0 for all cases. If, however, anyone wants to see it, I will be happy to show it.

I have changed the Project Default character set of my Visual C++ project to "Not Set". I should not need to prefix L to my things.

Edit: added wnd.hInstance = hinst;

Edit: removed the unnecessary (WNDPROC) cast

Edit: added error checking for RegisterClassEx

Answer 1

wnd.lpfnWndProc = (WNDPROC) MainWProc;

We can't see the real reason you need to use the cast but it is very fishy. Windows returns 0 from GetLastError() if it didn't see anything going wrong. Which can happen if the window procedure is broken. Like this one:

LRESULT CALLBACK MainWProc(HWND hWnd, UINT uMsg, WPARAM wParam, LPARAM lParam)
{
    return 0;
}

Windows sends the WM_NCCREATE message to ask for the window to be created. If that message doesn't get processed then there will be no window. And no error. Fix:

LRESULT CALLBACK MainWProc(HWND hWnd, UINT uMsg, WPARAM wParam, LPARAM lParam)
{
   return DefWindowProc(hWnd, uMsg, wParam, lParam);
}

Tweak as necessary to customize the window. Just make sure that DefWindowProc() gets called for every message that you don't want to handle yourself. And keep Petzold close at hand to avoid the simple mistakes. And lose the cast.

Answer 2

All modern versions of Windows use Unicode internally, and by default, Visual Studio projects #define _UNICODE / UNICODE , which causes your application to link to the Unicode versions of the Windows headers.

When you're compiling an application as Unicode, however, the character (and thus "string") types are different. Instead of char , they're now wchar_t . That means that you have to explicitly declare your string literals as long strings by prefixing them with an L .

Alternatively, the Windows headers hide all of this behind macros, but it's no longer necessary because Windows has been Unicode for a long time and that's very unlikely to change.

Beyond that, you're missing several things in your initialization of the WNDCLASSEX structure, like the hInstance member. These things all have to be set perfectly, or things will fail. As well, the RegisterClass(Ex) and CreateWindow(Ex) functions must be passed the exact same string values corresponding to the name of the window class, otherwise they will assume you're talking about two different things. Typos are not forgiven!

I highly recommend that you use the Visual Studio wizards to create a blank (but working!) project template.

The correct boilerplate code goes something like this:

#include <windows.h>
#include <tchar.h>

// Define these here to minimize typos, or preferably, load them from a
// resource file at the top of the main function
#define MYCLASSNAME    TEXT("MainWndClass")
#define MYWINDOWNAME   TEXT("Main Window")

// Global variable to keep track of your hInstance
HINSTANCE g_hInstance;

LRESULT CALLBACK WndProc(HWND hWnd, UINT uMsg, WPARAM wParam, LPARAM lParam)
{
   // If you don't process any of the messages yourself, you
   // must pass them to DefWindowProc for default handling.
   return DefWindowProc(hWnd, uMsg, wParam, lParam);
}

int APIENTRY _tWinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance,
                       LPTSTR lpCmdLine, int nCmdShow)
{     
    // Save the instance handle in a global variable.
    g_hInstance = hInstance;

    // Register your window class.
    // (A full-featured app will probably want to set additional members.)
    WNDCLASSEX wcex = {0};
    wcex.cbSize = sizeof(wcex);
    wcex.lpfnWndProc = WndProc;
    wcex.hInstance = hInstance;
    wcex.lpszClassName = MYCLASSNAME;
    if (!RegisterClassEx(&wcex))
    {
        MessageBox(NULL, TEXT("Call to RegisterClassEx failed!"), NULL, MB_OK);
        return 1;
    }

    // Create your main window.
    HWND hwndMain = CreateWindowEx(0, MYCLASSNAME, MYWINDOWNAME, WS_OVERLAPPEDWINDOW,
                                   CW_USEDEFAULT, 0, CW_USEDEFAULT, 0, NULL, NULL,
                                   hInstance, NULL);
    if (!hwndMain)
    {
        MessageBox(NULL, TEXT("Call to CreateWindowEx failed!"), NULL, MB_OK);
        return 1;
    }

    // Show your main window.
    ShowWindow(hwndMain, nCmdShow);
    UpdateWindow(hwndMain);

    // Run the main message loop.
    BOOL bRetVal;
    MSG msg;
    while ((bRetVal = GetMessage(&msg, NULL, 0, 0)) != 0)
    {
        if (bRetVal == -1)
        {
            MessageBox(NULL, TEXT("Error encountered in message loop!"), NULL, MB_OK);
            return 1;
        }
        else
        {
            TranslateMessage(&msg);
            DispatchMessage(&msg);
        }
    }

    return (int) msg.wParam;
}

Answer 3

In my case i had to handle WNC_NCCREATE manually. DefWindowProc returned zero for WNC_NCCREATE, i fixed that with:

LRESULT CALLBACK MainWProc(HWND hWnd, UINT uMsg, WPARAM wParam, LPARAM lParam)
{
    if (uMsg == WNC_NCCREATE) 
        return true;
    return DefWindowProc(hWnd, uMsg, wParam, lParam);
}

Answer 4

I had the same problem.

In my case its just that i used visual sutdio without admin rights. And i discovered that in this case i cant debug my application. In debug mode without admin right, CreateWindowEx return null with error code result to 0 like you. But if you go to your build directory you can use your app (not in debug mode). So if this the case fro you too, just start visula studio with admin right and its done.

I think there is almost a way to use visual studio debug mode with admin right without start visual stuido with admin password each time. i dont know how to do that, but i think it may be possible.