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?

That's a very obscure problem. You'll surely have to make that metafile available for anybody to have a shot at diagnosing the problem. — Hans Passant, Jul 31 '14 at 12:41
The problem occurs with various EMF+ files. Here's one (a simple diagonal line): http://xltoolbox.sf.net/files/test.emf -- But you are right, the problem is obscure, and I've come to think that I'd rather implement my own metafile reader to enumerate the records. — bovender, Aug 01 '14 at 10:03
I recommend adding graphics.Dispose() to the end of your Parse method, but I have doubts on it being the main issue. — Jason Ausborn, Mar 05 '15 at 21:52
@Aron Nice! But in fact I made a deliberate decision not to enable Aero -- I dislike the Glass design. Plus, I guess I would have difficulties executing `pc.Install(GeForce.FromStore());` on a virtual machine ;-) — bovender, Mar 18 '15 at 18:16
@bovender Surely you meant `pc.Install(nVidiaFactory.BuildLatestGfxCard());` — Aron, Mar 19 '15 at 01:12
@SimonMourier desktop composition/"Aero" **cannot be disabled** on Windows 8 and the method works just fine on my 8.1 machine. — cbr, Mar 24 '15 at 15:59
@GrawCube - yep I know it cannot be disabled (some would say its always disabled, dead). The question was precisely whether it works or not. — Simon Mourier, Mar 24 '15 at 18:20

score 1 · Answer 1 · answered Mar 26 '15 at 10:21

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

Why does EnumerateMetafile only work with Aero enabled

1 Answers1