在MEF合成期间处理ReflectionTypeLoadException

Question

I am using a DirectoryCatalog in MEF to satisfy imports in my application. However, there are sometimes obfuscated assemblies in the directory that cause a ReflectionTypeLoadException when I try to compose the catalog.

I know I can get round it by using a separate directory or by using a search filter on the DirectoryCatalog but I want a more general way to solve the problem. Is there some way I can handle the exception and allow composition to continue? Or is there another more general solution?

Answer 1

To save others from writing their own implementation of the SafeDirectoryCatalog, here is the one I came up with based upon Wim Coenen's suggestions:

public class SafeDirectoryCatalog : ComposablePartCatalog
{
    private readonly AggregateCatalog _catalog;

    public SafeDirectoryCatalog(string directory)
    {
        var files = Directory.EnumerateFiles(directory, "*.dll", SearchOption.AllDirectories);

        _catalog = new AggregateCatalog();

        foreach (var file in files)
        {
            try
            {
                var asmCat = new AssemblyCatalog(file);

                //Force MEF to load the plugin and figure out if there are any exports
                // good assemblies will not throw the RTLE exception and can be added to the catalog
                if (asmCat.Parts.ToList().Count > 0)
                    _catalog.Catalogs.Add(asmCat);
            }
            catch (ReflectionTypeLoadException)
            {
            }
            catch (BadImageFormatException)
            {
            }
        }
    }
    public override IQueryable<ComposablePartDefinition> Parts
    {
        get { return _catalog.Parts; }
    }
}

Answer 2

DirectoryCatalog already has code to catch ReflectionTypeLoadException and ignore those assemblies. Unfortunately, as I have reported , merely creating the AssemblyCatalog will not yet trigger the exception so that code doesn't work.

The exception is actually triggered by the first call to AssemblyCatalog.Parts .

Instead of using the DirectoryCatalog from MEF, you will have to do it yourself:

• scan a directory for assemblies
• load each assembly and creates a AssemblyCatalog for it
• invoke AssemblyCatalog.Parts.ToArray() to force the exception, and catch it
• aggregate all the good catalogs with a AggregateCatalog

Answer 3

I was doing this from an API I was writing and the SafeDirectoryCatalog would not log multiple exports matching a single import from different assemblies. MEF debugging is typically done via debugger and TraceListener. I already used Log4Net and I didn't want someone to need to add another entry to the config file just to support logging. http://blogs.msdn.com/b/dsplaisted/archive/2010/07/13/how-to-debug-and-diagnose-mef-failures.aspx I came up with:

    // I don't want people to have to add configuration information to get this logging. 
    // I know this brittle, but don't judge... please. It makes consuing the api so much
    // easier.
    private static void EnsureLog4NetListener()
    {
        try
        {
            Assembly compositionAssembly = Assembly.GetAssembly(typeof (CompositionContainer));
            Type compSource = compositionAssembly.GetType("System.ComponentModel.Composition.Diagnostics.CompositionTraceSource");

            PropertyInfo canWriteErrorProp = compSource.GetProperty("CanWriteError");
            canWriteErrorProp.GetGetMethod().Invoke(null,
                                                    BindingFlags.Public | BindingFlags.NonPublic |
                                                    BindingFlags.Static, null, null,
                                                    null);

            Type traceSourceTraceWriterType =
                compositionAssembly.GetType(
                    "System.ComponentModel.Composition.Diagnostics.TraceSourceTraceWriter");

            TraceSource traceSource = (TraceSource)traceSourceTraceWriterType.GetField("Source",
                                                                          BindingFlags.Public |
                                                                          BindingFlags.NonPublic |
                                                                          BindingFlags.Static).GetValue(null);

            traceSource.Listeners.Add(new Log4NetTraceListener(logger));                
        }
        catch (Exception e)
        {
            logger.Value.Error("Cannot hook MEF compisition listener. Composition errors may be swallowed.", e);
        }
    }