PrintWindow 有什么问题? [英] What is wrong with PrintWindow?

问题描述

下面的代码有什么问题?为什么 PrintWindow 返回 0?

What is wrong with the following code? Why does PrintWindow return 0?

HWND hwnd = GetDesktopWindow();
CHK(hwnd);

HDC hdc = GetWindowDC(hwnd);
CHK(hdc);

if (hdc)
{
    HDC hdcMem = CreateCompatibleDC(hdc);
    CHK(hdcMem);

    if (hdcMem)
    {
        RECT rc;
        CHK(GetWindowRect(hwnd, &rc));

        HBITMAP hbitmap = CreateCompatibleBitmap(hdc, rc.right-rc.left, rc.bottom-rc.top);
        CHK(hbitmap);   

        if (hbitmap)
        {
            SelectObject(hdcMem, hbitmap);

            CHK(PrintWindow(hwnd, hdcMem, 0)); //HERE return 0

            DeleteObject(hbitmap);
        }

        DeleteObject(hdcMem);
    }

    ReleaseDC(hwnd, hdc);
}

推荐答案

PrintWindow 是一个相当精简的操作.它真正做的是将 WM_PRINT 消息发布到相关窗口的队列中，在这种情况下是桌面，并希望该窗口能够正确响应 WM_PRINT全部(参见此处和此处).

PrintWindow is a fairly thin operation. What it really does is post a WM_PRINTmessage to the queue for the window in question, in this case the desktop, and hopes that that window will respond to WM_PRINT correctly if at all (see here and here).

我重现了你的行为，但我也不能 100% 确定它失败的原因.也许您无法在您的进程不拥有的 HWND 上调用 PrintWindow，或者桌面不响应 WM_PRINT 消息.

I repro'd your behavior but I'm not 100% sure why it's failing either. Perhaps you cannot call PrintWindow on an HWND that your process does not own, or perhaps the desktop does not respond to WM_PRINT messages.

上面的第二个链接包含关于使用 BitBlt 代替:

The second link above includes a comment about using BitBlt instead:

尝试获取句柄 (HWND)桌面窗口 - 并使用 BitBlt捕获所有内容.提个醒 -你只会捕捉到什么是可见的屏幕.

Try getting a handle (HWND) to the desktop window - and use BitBlt to capture all the contents. Mind you - you'll only capture what is visible on the screen.

也许这会有所帮助.

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