如何将派生类型的接口添加到基类型的接口字典中

Question

I am trying to create a class where I can register a bunch of different handlers for requests that the application must service. The request produces a response that will be sent back to the remote. I have an interface for anything that can handle the request. All requests and responses inherit from ProtoBase. (The protobase class handles the serialization of its state into a byte array). The interface is below.

public interface IHandler<TReq, TResp> 
    where TReq: ProtoBase
    where TResp: ProtoBase
{
    TResp Handle(TReq, object origin);
}

public class Foo
{
    private Dictionary<ReqType, IHandler<ProtoBase, ProtoBase>> handlers;

    public Foo()
    {
        handlers = new Dictionary<ReqType, IHandler<ProtoBase, ProtoBase>>();
    }

    public void RegisterHandler(IHandler<BarRequest, BarResp> fooBarHandler)
    {
        handlers[ReqType.FooBar] = fooBarHandler;
    }
    
    public void Run()
    {
        while(true)
        {
            var req = GetRequest();
            var handler = handlers[req.ReqType];
            var resp = handler.Handle(req);
            Send(resp);
        }
    }

}

The code above produces a compiler error

Cannot implicitly convert type 'IHandler<BarRequest, BarResp>' to 'IHandler<ProtoBase, ProtoBase>'. An explicit conversion exists (are you missing a cast?)

I tried playing with contravariance and covariance on the interface but it made no difference.

Answer 1

You cannot make IHandler<ProtoBase, ProtoBase> a superclass for your specific handlers, no matter how much you play around with co- and contravariance.

Why? Let's assume this were possible, then we could have the following code:

IHandler<SomeRequest, SomeResponse> specificHandler = ...;
IHandler<ProtoBase, ProtoBase> handler = specificHandler; // won't work, but let's assume it does

// Should work from a type system POV, since SomeOtherRequest also inherits from ProtoBase,
// but can't work from a logic POV, since your handler can only handle SomeRequest.
handler.Handle(new SomeOtherRequest());

Answer 2

For that cast to work, it would need to be IHandler<out TReq, out TResp> , but in terms of variance, your Handle method is only compatible with IHandler<in TReq, out TResp> .

So: you can't. You might need to make the type actually implement IHandler<ProtoBase, ProtoBase> .