如何通过类名称获取MEF导出值？

Question

I have a window called MainWindow and a viewmodel called MainWindowViewModel .

I want to look into MEF's container to see if I can find <WindowName>ViewModel .

The code I have is:

CompositionContainer container;

var catalog = new AssemblyCatalog(typeof(App).Assembly);
container = new CompositionContainer(catalog);
container.ComposeParts(this);
container.SatisfyImportsOnce(this);

I saw the method

container.GetExports(Type, Type, String)

but it only allows me to get exports of the first Type parameter. All I have is a string name.

I wanted to do something like

allExports.FirstOrDefault(e => e.GetType().Name.StartsWith(something))

Is there a way I can get an exported value by a string name ?

Answer 1

Since allExports is IEnumerable< Lazy< T >> you can't get each exported type without creating the associated value (by calling .Value), and then inspecting value type. And this is not really a good thing to create all values. All you can get is the typeof(T) by analyzing typeof(Lazy< T >), that's all.

Metadata is the good way to go :

[MetadataAttribute]
[AttributeUsage(AttributeTargets.Class, AllowMultiple = false)]
public class ExportViewModelAttribute : ExportAttribute, IViewModelMetadata
{
    public ExportViewModelAttribute(Type declaredType)
        : base(null, typeof(IViewModel))
    {
        this.DeclaredType = declaredType;
    }

    public Type DeclaredType { get; private set; }
}

with interface as :

public interface IViewModelMetadata
{
    Type DeclaredType { get; } 
}

then you export with :

[ExportViewModel(typeof(MyViewModel))]
public class MyViewModel: BaseViewModel, IViewModel 
{
    [...]
}

And then retreive it with a where clause on metadata

IViewModel vm = container.GetExports<IViewModel, IViewModelMetadata>().Where(i => i.Metadata.DeclaredType == typeof(MyViewModel)).Select(i => i.Value).FirstOrDefault();

or with

i => i.Metadata.DeclaredType.Name == "mysearchedViewModel"