Problem lining up MEF contracts

Question

I have this class:

public interface IHandles<DOMAINEVENT> where DOMAINEVENT : IDomainEvent
{
    void Handle(DOMAINEVENT args);
}

[Export(typeof(IHandles<IDomainEvent>))]
public class NewMemberWasRegisteredHandler : IHandles<NewMemberWasRegistered>
{
    public void Handle(NewMemberWasRegistered args)
    {
        //EmailSender.Send
        Console.WriteLine("***New Member Registration Email Sent");
    }
}

which I'm trying to Export into:

public class HandlerContainer { public HandlerContainer() { Handlers = new List>(); }

    [ImportMany()]
    public List<IHandles<IDomainEvent>> Handlers { get; set; }
}

The exception I'm getting is:

1) The export 'Multi_Tenant_MEF_Solution.NewMemberWasRegisteredHandler (ContractName="Multi_Tenant_MEF_Solution.IHandles(Multi_Tenant_MEF_Solution.IDomainEvent)")' is not assignable to type 'Multi_Tenant_MEF_Solution.IHandles`1[[Multi_Tenant_MEF_Solution.IDomainEvent, Multi-Tenant-MEF-Solution, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null]]'.

I'm a MEF newb ( 2 hours in ) so this is baffling to me. All the examples I can find use the the parameterless constructors of Import and Export so I'm unclear on what my typeof() or string contracts should be.

Daniel Plaisted Daniel Plaisted · Accepted Answer · 2011-03-08T01:53:48

The problem is NewMemberWasRegisteredHandler can't be converted to IHandles<IDomainEvent>. By using MEF and overriding the contract that is exported, you are getting a runtime error, where without MEF you would have gotten a compile-time error. Here's what it would look like without MEF:

public interface IDomainEvent { }

public class NewMemberWasRegistered : IDomainEvent { }

public interface IHandles<DOMAINEVENT> where DOMAINEVENT : IDomainEvent
{
    void Handle(DOMAINEVENT args);
}

public class NewMemberWasRegisteredHandler : IHandles<NewMemberWasRegistered>
{
    public void Handle(NewMemberWasRegistered args) { }
}

public class HandlerContainer
{
    public List<IHandles<IDomainEvent>> Handlers { get; set; }

    public HandlerContainer()
    {
        Handlers = new List<IHandles<IDomainEvent>>();

        Handlers.Add(new NewMemberWasRegisteredHandler());
    }

}

You get a compilation error that you can't convert NewMemberWasRegisteredHandler to IHandles<IDomainEvent>. This is because DOMAINEVENT is not covariant in the IHandles interface. You can make it covariant by adding the out keyword, but then your Handle method wouldn't be legal.

Problem lining up MEF contracts

1 Answers