Using NETLink.MathKernel.Compute() for graphics results fails with GraphPlot[]

Question

I'm using MMA 8.0 to provide backend on a .NET application I'm writing. I am trying to use the high-level Wolfram.NETLink.MathKernel interface to export an image created with GraphPlot[].

Although the Result comes back as '-Graphics-', I find that Graphics[] is empty. Changing the Compute() to another graphic generating command returns an image in the Graphic[] as expected.

Here's my example code:

Wolfram.NETLink.IKernelLink _iKernelLink;

string cmdLine = "-linkmode launch -linkname \"C:/Program Files/Wolfram Research/Mathematica/8.0/MathKernel.exe\"";
_iKernelLink = Wolfram.NETLink.MathLinkFactory.CreateKernelLink(cmdLine);
_iKernelLink.WaitAndDiscardAnswer();

var k = new Wolfram.NETLink.MathKernel(_iKernelLink); // new MathKernel

k.AutoCloseLink = true; // close link when object is disposed
k.CaptureGraphics = true; // capture graphics output
k.CapturePrint = true; // capture print output
k.CaptureMessages = true; // capture messages
k.HandleEvents = true; // lets Compute thread handle application events
k.UseFrontEnd = false; // don't use Mathematica Notebook front-end

// this produces empty Graphic[] result
string command = @"GraphPlot[{1 -> 2, 2 -> 1, 3 -> 1, 3 -> 2, 4 -> 1, 4 -> 2, 4 -> 4}]";

// this command produces the expected Graphic[] result
//string command = @"BarChart@Range[5]";

k.Compute(command);

if (k.Graphics.Length > 0) // <-- should be an image for both commands
    return k.Graphics[0];
else
    return null;

Answer 1

I think your problem is that Graph will actually not evaluate to a Graphics but only be rendered as something like that by the FrontEnd. You could look at ToBoxes[yourgraph] to learn some details and try to extract something that can be transfered to .NET. I don't have any experience with NETLink, but I remember that at least older versions of JLink would return graphics as rastered images anyway, so a more simple approach could be to use

Rasterize[GraphPlot[{1 -> 2, 2 -> 1, 3 -> 1, 3 -> 2, 4 -> 1, 4 -> 2, 4 -> 4}]]

You might want to finetune this with the options of Rasterize to meet your needs.