WinUI3 : How to print the current UI to PDF without showing Print UI

Thank you for the response @Castorix31 . I tried using this API like this to print to PDF (For winui3 desktop application and XAML island) -

void PrintPDF(HWND pHwnd) {

    HWND hwndTarget = pHwnd;
    DOCINFO di = { sizeof(DOCINFO), "MyPDFDocument", NULL, NULL, 0 };
    HDC hdcTarget = CreateDC("WINSPOOL", "Microsoft Print to PDF", NULL, NULL);

    if (!hwndTarget && !hdcTarget) {
        return;
    }

    if (StartDoc(hdcTarget, &di) > 0) {

        StartPage(hdcTarget);

        // Perform printing operations here
        if (!PrintWindow(hwndTarget, hdcTarget, PW_CLIENTONLY)) {
            // Handle error: PrintWindow failed
            DeleteObject(hdcTarget);
            ReleaseDC(hwndTarget, hdcTarget);

            MessageBox(NULL, "Error", "Print Failed", 0);

            return;
        }
        EndPage(hdcTarget);
        EndDoc(hdcTarget);
    }

}

But this printed an empty PDF. Where did i go wrong?

I used a memory DC I can only test in C# (too long to compile in C++/WinRT), but it is the same code in C++ =>

 {
     IntPtr hPrinterDC = CreateDC("WINSPOOL", "Microsoft Print to PDF", null, IntPtr.Zero);
     if (hPrinterDC != IntPtr.Zero)
     {
         int nWidth = GetDeviceCaps(hPrinterDC, HORZRES);
         int nHeight = GetDeviceCaps(hPrinterDC, VERTRES);
         IntPtr hDC = GetDC(IntPtr.Zero);
         IntPtr hBitmap = Create32BPPDIBSection(hDC, _apw.Size.Width, _apw.Size.Height);
         IntPtr hMemoryDC = CreateCompatibleDC(hDC);
         IntPtr hBitmapOld = SelectObject(hMemoryDC, hBitmap);

         bool bRet = PrintWindow(hWndMain, hMemoryDC, PW_RENDERFULLCONTENT);

         DOCINFO di = new DOCINFO();
         di.cbSize = Marshal.SizeOf(typeof(DOCINFO));
         di.lpszDocName = "Print Window";
         di.lpszOutput = "E:\\temp\\testprint.pdf";
         if (StartDoc(hPrinterDC, di) > 0)
         {
             if (StartPage(hPrinterDC) > 0)
             {  
                 DIBSECTION ds = new DIBSECTION();
                 GetObject(hBitmap, Marshal.SizeOf(typeof(DIBSECTION)), ref ds);
                 double nRatio = (double)nWidth / (double)_apw.Size.Width;
                 nHeight = (int)(_apw.Size.Height * nRatio);
                
                 StretchDIBits(hPrinterDC, 0, 0, nWidth, nHeight, 0, 0, _apw.Size.Width, _apw.Size.Height, ds.dsBm.bmBits, ref ds.dsBmih, DIB_RGB_COLORS, SRCCOPY);               
                 
                 EndPage(hPrinterDC);
             }
             EndDoc(hPrinterDC);
         }

         SelectObject(hMemoryDC, hBitmapOld);
         DeleteDC(hMemoryDC);
         DeleteObject(hBitmap);
         ReleaseDC(IntPtr.Zero, hDC);

         DeleteDC(hPrinterDC);
     }
 }

        public IntPtr Create32BPPDIBSection(IntPtr hDC, int iWidth, int iHeight)
        {
            IntPtr hBitmap = IntPtr.Zero;
            IntPtr pBits = IntPtr.Zero;
            BITMAPINFO bmi = new BITMAPINFO();
            bmi.bmiHeader.biSize = Marshal.SizeOf(typeof(BITMAPINFOHEADER));
            bmi.bmiHeader.biWidth = iWidth;
            bmi.bmiHeader.biHeight = iHeight;
            bmi.bmiHeader.biPlanes = 1;
            bmi.bmiHeader.biBitCount = 32;
            bmi.bmiHeader.biCompression = BI_RGB;
            hBitmap = CreateDIBSection(hDC, ref bmi, DIB_RGB_COLORS, ref pBits, IntPtr.Zero, 0);
            return hBitmap;
        }

At beginning (to get window dimensions from AppWindow, GetWindowRect can also be used) :

private IntPtr hWndMain = IntPtr.Zero;
private Microsoft.UI.Windowing.AppWindow _apw;
 
hWndMain = WinRT.Interop.WindowNative.GetWindowHandle(this);
Microsoft.UI.WindowId myWndId = Microsoft.UI.Win32Interop.GetWindowIdFromWindow(hWndMain);
_apw = Microsoft.UI.Windowing.AppWindow.GetFromWindowId(myWndId);

Thank you this was very helpful. I was able to print to PDF with this

But I want to remove the title and border. I tried using PW_CLIENTONLY flag for PrintWindow, but it did not work. I got the output like this -

How to fix this?

It works for me with both flags : PW_RENDERFULLCONTENT | PW_CLIENTONLY

Yea it worked for me with PW_RENDERFULLCONTENT | PW_CLIENTONLY. But im still getting the black border