Question

The biggest problem I’m having so far with MEF is that when I compose parts in my plugin loader wrapper, loading completely bails when it finds an import resolution problem with one of the assemblies. Ideally, I’d like ComposeParts to exhibit some sort of “ignore and continue” behavior, because the ideal user experience would entail loading as many plugins as possible, and simply logging an event when a specific plugin fails to load. I haven’t been able to find information about this in the documentation anywhere.

If you have any other suggestions for how to solve this, I’m listening!

Answer 1

Wim’s example has the basic ideas but instead of pulling on the container directly I would suggest you do a Lazy ImportMany like such:

[Export]
public class MyApplication
{
   [ImportMany(typeof(IPlugin))]
   public IEnumerable<Lazy<IPlugin>> Plugins { get; set; }
}

Then you can initialize the plugins one by one and catch any errors from them like:

void InitializePlugins()
{
   foreach (Lazy<IPlugin> plugin in Plugins)
   {
       try
       {
          plugin.Value.Initialize();
       }
       catch (CompositionException e)
       {
          // Handle the error accordingly
       }
   }   
}

The actual plugin will not be created until you pull on .Value the first time and that is when errors will occur if the plugin has bugs in the constructor or property setters of the imports. Also note that I’m catch CompositionException which is what will come out of the .Value call if the plugin does something wrong.