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I have been trying to use MEF on a new project and am having some difficulty getting imports to work, which I cannot explain why. I am following the samples on MSDN and elsewhere, but they are not working for me. This is using MEF 4.0 in a .NET 4 project.

I have defined a simple contract:

public interface ICommand
{
    int Execute(string[] args);
}

I implemented some parts and added some metadata:

[Export(typeof(ICommand))]
[ExportMetadata("Name", "init")]
public class InitCommand : ICommand { ... }

[Export(typeof(ICommand))]
[ExportMetadata("Category", "service")]
[ExportMetadata("Name", "start")]
public class StartServiceCommand : ICommand { ... }

I defined the following metadata interface:

public interface ICommandMetadata
{
    [DefaultValue(null)]
    string Category { get; }

    string Name { get; }
}

In my main program, I am creating a catalog-based export provider and then trying to compose the main program object:

internal class Program
{
    [ImportMany]
    private IEnumerabe<Lazy<ICommand, ICommandMetadata>> commands;

    private static int Main(string[] args)
    {
        var assembly = Assembly.GetExecutingAssembly();
        var assemblyExportProvider = new CatalogExportProvider(new AssemblyCatalog(assembly));
        var compositionContainer = new CompositionContainer(assemblyExportProvider);
        assemblyExportProvider.SourceProvider = compositionContainer;
        var batch = new CompositionBatch();
        batch.AddPart(this);
        compositionContainer.Compose(batch);

        return 0;            
    }
}

When I run the above code, this works as expected and both parts are resolved. When I go a step further and create a custom export attribute, the code stops working. Here's my custom metadata attribute and updated parts:

[AttributeUsage(AttributeTargets.Class, AllowMultiple = true)]
[MetadataAttribute]
public sealed class CommandAttribute : ExportAttribute
{
    public CommandAttribute() : base(typeof(ICommand))
    {
    }

    [DefaultValue(null)]
    public string Category { get; set; }

    public string Name { get; set; }
}

[Command(Name = "init")]
public class InitCommand : ICommand { ... }

[Command(Category = "service", Name = "start")]
public class StartServiceCommand : ICommand { ... }

By applying the custom export attribute, my commands collection in my program is an empty array. I played around with this some more, and I found that if I change the properties in the metadata interface to arrays of strings, the importing works again:

public interface ICommandMetadata
{
    [DefaultValue(null)]
    string[] Category { get; }

    string[] Name { get; }
}

Can anyone tell me if I am doing something wrong? Why does this only work if I use arrays in the metadata interface? Is there a way to make this work without using arrays for the properties?

Thanks in advance for your help.

Michael Collins
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  • Can you turn off the AllowMultiple? I suspect it's trying to consolidate the metadata from multiple possible CommandAttribute instances. – Dan Bryant Mar 19 '13 at 18:59
  • Ok. You were right. I removed AllowMultiple and it started working again. I saw it in the example code and used it without thinking. This makes sense why I needed the arrays. Thanks. – Michael Collins Mar 19 '13 at 19:05
  • And CommandAttribute should implement ICommandMetadata or not? – Marc Mar 20 '13 at 07:05
  • No. CommandAttribute does not need to implement ICommandMetadata in order for the metadata to work. – Michael Collins Mar 20 '13 at 21:51

0 Answers0