Why does EnumerateMetafile only work with Aero enabled

Question

My code enumerates a metafile:

private void Parse() {     Graphics graphics = Graphics.FromHwnd(IntPtr.Zero);     PointF pointf = new PointF();     graphics.EnumerateMetafile(_metafile, pointf, ParseCallback); }  private bool ParseCallback(EmfPlusRecordType recordType,      int flags, int dataSize, IntPtr data, PlayRecordCallback callbackData) {     // do stuff } 

My development machine is Windows 7 VirtualBox guest on Ubuntu host.

The code used to work fine. However, when I turned off Aero, the code stopped working: The ParseCallback would never be called.

Only when I turned Aero back on, ParseCallback was executed again.

Why and how can I make this code work on non-Aero-enabled machines?

Answer 1

I don't have a full answer to the "why?" question, but it does not work because you're getting the Graphics GDI+ object from the Window handle. Instead, you want to get it from a GDI DC, like this:

private void Parse() {     IntPtr hdc = GetDC(IntPtr.Zero); // get entire screen dc     Graphics graphics = Graphics.FromHdc(hdc));     PointF pointf = new PointF();     graphics.EnumerateMetafile(_metafile, pointf, ParseCallback);     ReleaseDC(IntPtr.Zero, hdc); }  [DllImport("user32.dll")] static extern IntPtr GetDC(IntPtr hWnd);  [DllImport("user32.dll")] static extern IntPtr ReleaseDC(IntPtr hwnd, IntPtr hdc); 

Note you could also use the Graphics object from the Form OnPaint(PaintEventArgs e) method, it should also work, just like in the official sample code for EnumerateMetafile method here: Graphics.EnumerateMetafile Method