Question

I’ve made up a small test app to demo the issue I’m trying to resolve. I have a plugin architecture where the plugins derive from an interface.

Here’s is the interface code:

namespace Test
{
  public interface ITestBase
  {
    int LifeTheUniverseAndEverything();
    bool Pessimist();
    bool Optimist();
  }
}

This builds to a .dll that is referenced by the main program solution and the plugin solution.

Here is the plugin code:

namespace Test
{
  public class TestPlugin : ITestBase
  {
    public int LifeTheUniverseAndEverything() { return 41; }
    public bool Pessimist() { return false; }
    public bool Optimist() { return true; }
  }
}

This produces a .dll that is dropped into a “plugins” folder off the main working directory.

Here’s the main program.

namespace Test
{
  class Program
  {
    static void Main(string[] args)
    {
      Console.WriteLine("testing...");
      string[] PluginFileLocs;
      try
      {
        string sWorkingDir = Directory.GetCurrentDirectory();
        string sPluginDir = sWorkingDir + "\\Plugins";
        PluginFileLocs = Directory.GetFiles(sPluginDir, "*.dll");
        foreach (string plugin in PluginFileLocs)
        {
          Assembly theAssembly = Assembly.ReflectionOnlyLoadFrom(plugin);
          foreach (Type type in theAssembly.GetTypes())
          {
            string aType = type.FullName;
            Console.WriteLine(aType);
          }
        }
      }
      catch (Exception e)
      {
        Console.WriteLine("Error: " + e.Message);
      }
    }
  }
}

The issue is, that if the testplugin class derives off the ITestBase interface, the theAssembly.GetTypes throws an exception: unable to load one or more of the requested types. retrieve the LoaderExceptions. Why, and how can I get my plugin to derive off an interface?

Answer 1

Based on the documentation for the Assembly.ReflectionOnlyLoadFrom, it will not automatically load dependencies http://msdn.microsoft.com/en-us/library/system.reflection.assembly.reflectiononlyloadfrom.aspx

I’m guessing that’s what’s going on here. Since your interface is defined in a separate assembly from the plugin, you must first load that assembly before trying to call GetTypes.