It turns out, however, that the Graphviz library is fairly simple to understand. Once I got the correct DLL configuration copied to my application's directory, the rest of the P/Invoke stuff was easy. On top of it all, Graphviz includes the ability to render an image in-memory, so there's no need to write to the disk and read the result back again. Writing to the disk was the biggest performance bottleneck for my application, so the in-memory rendering feature was a huge relief.
To get started, we need a class to hold our P/Invoke methods and a few helper constants.
public class Graphviz {
public const string LIB_GVC = "gvc.dll";
public const string LIB_GRAPH = "graph.dll";
public const int SUCCESS = 0;
}
To use Graphviz as a library, we first have to create a Graphviz context, which the other methods will use. Add these to our Graphviz class.
/// <summary>
/// Creates a new Graphviz context.
/// </summary>
[DllImport(LIB_GVC)]
public static extern IntPtr gvContext();
/// <summary>
/// Releases a context's resources.
/// </summary>
[DllImport(LIB_GVC)]
public static extern int gvFreeContext(IntPtr gvc);
Now we need some methods to load and close a graph. These are found in graph.dll.
/// <summary>
/// Reads a graph from a string.
/// </summary>
[DllImport(LIB_GRAPH)]
public static extern IntPtr agmemread(string data);
/// <summary>
/// Releases the resources used by a graph.
/// </summary>
[DllImport(LIB_GRAPH)]
public static extern void agclose(IntPtr g);Once we have our graph loaded, we need to apply a layout.
/// <summary>
/// Applies a layout to a graph using the given engine.
/// </summary>
[DllImport(LIB_GVC)]
public static extern int gvLayout(IntPtr gvc, IntPtr g, string engine);
/// <summary>
/// Releases the resources used by a layout.
/// </summary>
[DllImport(LIB_GVC)]
public static extern int gvFreeLayout(IntPtr gvc, IntPtr g);Once the graph has a layout, we can render it.
/// <summary>
/// Renders a graph to a file.
/// </summary>
[DllImport(LIB_GVC)]
public static extern int gvRenderFilename(IntPtr gvc, IntPtr g,
string format, string fileName);
/// <summary>
/// Renders a graph in memory.
/// </summary>
[DllImport(LIB_GVC)]
public static extern int gvRenderData(IntPtr gvc, IntPtr g,
string format, out IntPtr result, out int length);
Now, you may be thinking that you can just copy gvc.dll and graph.dll over and all will be well. But, sadly, that's not the case. Those DLLs have a few dependencies. You'll need all of the following from your Graphviz installation's /bin directory:
Graphviz loads layout and rendering plugins dynamically using a configuration file, which must also be included alongside your application. If you installed Graphviz using the MSI, there should already be a configuration file in the /bin directory. Mine was called config6, but I'm not sure if the name varies. If you don't have a configuration file, you can generate one with dot -c.
Now that we have the necessary DLLs, we can actually do some rendering!
/// <summary>
/// Renders a DOT string to the specified image format.
/// </summary>
public static byte[] RenderDot(string dot, string format) {
// Create a Graphviz context
IntPtr gvc = gvContext();
// Load the DOT data into a graph
IntPtr g = agmemread(dot);
// Apply a layout
gvLayout(gvc, g, "dot");
IntPtr result;
int length;
// Render the graph
gvRender(gvc, g, format, out result, out length);
// Create an array to hold the image
byte[] image = new byte[length];
// Copy the image from the IntPtr
Marshal.Copy(result, image, 0, length);
// Free up the resources
gvFreeLayout(gvc, g);
agclose(g);
gvFreeContext(gvc);
return image;
}Here's the code in action, rendering a graph of the Graphviz DLL dependencies:
string dot = @"digraph Graphviz {
cdt -> msvcr80, cdt -> kernel32,
graph -> cdt, graph -> msvcr80, graph -> kernel32,
gvc -> libexpat, gvc -> ltdl, gvc -> zlib1, gvc -> cdt,
gvc -> pathplan, gvc -> graph, gvc -> msvcr80, gvc -> kernel32,
ltdl -> msvcrt, ltdl -> kernel32,
pathplan -> msvcr80, pathplan -> kernel32,
zlib1 -> msvcrt, zlib1 -> kernel32,
libexpat -> kernel32
}";
byte[] image = Graphviz.RenderDot(dot, "png");And the result:
]]>
