Question

I’ve got an easy sample from the internet that works fine in the .NET 3.5 framework using System.ComponentModel.Composition.dll version v2.0.50727

I’ve changed the project definition and changed the target to .NET 4.0 and it works perfect.

When I replace the v2.0.50727 version of the above .dll to the latest version which is v4.0.30319 I get an error that complains during the composition of the container. The code where it break is as follows:

        private void LoadPlugins() {
            var catalog = new AssemblyCatalog(Assembly.GetExecutingAssembly());

            var container = new CompositionContainer(catalog);
                        container.ExportsChanging += new EventHandler(container_ExportsChanging);

            var batch = new CompositionBatch();
            batch.AddPart(this);
            container.Compose(batch); // throws Exception
        }

And the exception is the following:

System.ComponentModel.Composition.ChangeRejectedException was unhandled
Message=The composition remains unchanged. The changes were rejected because of the following error(s): The composition produced a single composition error. The root cause is provided below. Review the CompositionException.Errors property for more detailed information.

1) More than one export was found that matches the constraint '((exportDefinition.ContractName == "MefTutorial.IPlugin") AndAlso (exportDefinition.Metadata.ContainsKey("ExportTypeIdentity") AndAlso "MefTutorial.IPlugin".Equals(exportDefinition.Metadata.get_Item("ExportTypeIdentity"))))'.

Resulting in: Cannot set import 'MefTutorial.PluginConsumer._myPlugins (ContractName="MefTutorial.IPlugin")' on part 'MefTutorial.PluginConsumer'.
Element: MefTutorial.PluginConsumer._myPlugins (ContractName="MefTutorial.IPlugin") --> MefTutorial.PluginConsumer

What do I need to do to migrate to the .NET 4.0 concerning MEF?

Answer 1

OK, I found the problem. Apparently in the previous version the notation was as mentioned in my previous comment, but in the new .NET 4.0 version the syntax for import should be:

code>
[ImportMany(typeof(IPlugin))] 
internal List _myPlugins { get; set; } 

Notice the use of List and ImportMany instead of IList and Import.