c++ - 为什么我得不到虚拟打印机的上下文？

Question

Windows 8.1 x64

我已经安装了PdfCreater虚拟打印机。它是我电脑上的默认打印机(而且是一台)。

现在我想获取它的上下文:

HDC GetPrinterDC(void)
{
    PRINTER_INFO_5 pinfo5[3];
    ZeroMemory(pinfo5, sizeof(pinfo5));
    DWORD dwNeeded, dwReturned;
    if (EnumPrinters(PRINTER_ENUM_DEFAULT, NULL, 5,
        (LPBYTE)pinfo5, sizeof(pinfo5), 
        &dwNeeded, /* I get 0 */
        &dwReturned /* I get 0 */)) {
        // And I am here...
        return CreateDC(NULL, pinfo5[0].pPrinterName, NULL, NULL); // NULL
    }
    return 0;
}

但我得到 NULL。为什么会这样？

Answer 1

不要对第一个 EnumPrinters 的结果进行错误检查(Petzold 也不这样做)

另请注意，一台打印机所需的内存量可能为数百字节。但是 PRINTER_INFO_5 pinfo5[1]; 分配的可能比这少。因此，您必须分配第一个 EnumPrinters 调用告诉您的内容。

HDC printerDC = NULL;
DWORD flags = PRINTER_ENUM_LOCAL;// | PRINTER_ENUM_CONNECTIONS;
DWORD memsize, printer_count;

//figure out how much memory we need
EnumPrinters(flags, NULL, 5, NULL, 0, &memsize, &printer_count);
if (memsize < 1) return;

//allocate memory
BYTE* bytes = new BYTE[memsize];

if (EnumPrinters(flags, NULL, 5, bytes, memsize, &memsize, &printer_count))
{
    if (printer_count > 0)
    {
        PRINTER_INFO_5* printerInfo = (PRINTER_INFO_5*)bytes;
        printerDC = CreateDC(NULL, printerInfo->pPrinterName, NULL, NULL);

        //optional, list printer names
        for (UINT i = 0; i < printer_count; i++)
        {
            OutputDebugString(printerInfo->pPrinterName);
            OutputDebugString(L"\n");
            printerInfo++;
        }
    }
}
delete[] bytes;

如果您想访问默认打印机，另一种选择是使用 PRINTDLG(实际上不显示任何打印对话框)

PRINTDLG pdlg;
memset(&pdlg, 0, sizeof(PRINTDLG));
pdlg.lStructSize = sizeof(PRINTDLG);
// Set the flag to return printer DC, but don't show dialog
pdlg.Flags = PD_RETURNDEFAULT | PD_RETURNDC;

PrintDlg(&pdlg);
printerDC = pdlg.hDC;

另见
How To Print a Document
How To: Retrieve a Printer Device Context