Can't load multiple MEF parts

Question

I have a Winforms desktop application that is loading multiple MEF parts with the same Interface type.

Problem: When I try to load more than one of the same type I get the following exception:

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) No valid exports were found that match the constraint '((exportDefinition.ContractName = "BOCA.TaskPilot.Common.Extensions.IFolderViewExtension") && (exportDefinition.Metadata.ContainsKey("ExportTypeIdentity") && "BOCA.TaskPilot.Common.Extensions.IFolderViewExtension".Equals(exportDefinition.Metadata.get_Item("ExportTypeIdentity"))))', invalid exports may have been rejected.

Resulting in: Cannot set import 'TaskPilot.Windows.MainForm.FolderViewExtension (ContractName="BOCA.TaskPilot.Common.Extensions.IFolderViewExtension")' on part 'TaskPilot.Windows.MainForm'. Element: TaskPilot.Windows.MainForm.FolderViewExtension (ContractName="BOCA.TaskPilot.Common.Extensions.IFolderViewExtension") --> TaskPilot.Windows.MainForm

Here is the code to load the parts:

            AggregateCatalog catalog = new AggregateCatalog();
        catalog.Catalogs.Add(new AssemblyCatalog(Assembly.GetExecutingAssembly()));
        //string myExecName = Assembly.GetExecutingAssembly().Location;
        //string myPath = Path.GetDirectoryName(myExecName);
        catalog.Catalogs.Add(new DirectoryCatalog(@"C:\Data\TaskPilot\Development\Source\BOCA.TaskPilot.FolderView\bin\Debug"));
        catalog.Catalogs.Add(new DirectoryCatalog(@"C:\Data\TaskPilot\Development\Source\BOCA.TaskPilot.TaskView\bin\Debug"));
        // Uncomment below line and it works without exceptions raised
        //catalog.Catalogs.Add(new DirectoryCatalog(@"C:\Data\TaskPilot\Development\Source\BOCA.FileManager\bin\Debug"));

        var container = new CompositionContainer(catalog);
        container.ComposeParts(this);

Here's the code at the class for each of the MEF parts:

[Export(typeof(IFolderItemsViewExtension))
public partial class TaskTreeView : DevExpress.XtraEditors.XtraUserControl, IFolderItemsViewExtension, IPartImportsSatisfiedNotification]

Here's the Import used on the Main form:

[ImportMany(AllowRecomposition = true)]
    private IEnumerable<IFolderItemsViewExtension> TaskViewExtensions = null;

If I uncomment the last Catalog.Catalogs.Add line it throws the exception. If I run it without that it runs just fine. That line loads a different user control that implements the IFolderItemsViewExtension Interface. I've tried to just load a dummy project that all it has is the user control and that interface and I still get the same exception. No matter what I do I still get this exception.

It seems that everything runs fine as long as I'm not loading more than one of the same type of MEF part export.

This is using the latest version of 2009.22.10.0 of the System.ComponentModel.Composistion from the MEF download.

Answer 1

The error indicates that it can't find an export of type IFolderViewExtension. Note that this is different from the import of IFolderItemsViewExtension you have shown.

My guess is that the problem is not that you have multiple IFolderItemsViewExtensions, but that you have multiple IFolderViewExtensions, or there is some other contract that you have more than one of that you are using with an import that requires exactly one.

This may be caused because you have the same assembly in more than one of your directory catalogs. It is easy for this to happen if you have a reference to an assembly and copy local is set to true.