C# 将对象转换为通用接口

Question

I am trying to create CQRS in my small project and I need to cast object to Interface that has generic parameter.

edit: typo fixed

NewUsersIdsMessageHandler newUserHandler = new NewUsersIdsMessageHandler();
IMessageHandler<IMessage> handler = newUserHandler; // compilation error


    public class NewUsersIdsMessage : IMessage
    {
        public NewUsersTroopRequestIdsMessage()
        {
        }
    }


    public class NewUsersIdsMessageHandler : IMessageHandler<NewUsersIdsMessage >
    {
        public NewUsersIdsMessageHandler()
        {
        }
    }

    public interface IMessageHandler<in TType>
        where TType : IMessage
    {
        Task<bool> Handle(TType message);
    }

How can I assign ?

IMessageHandler<IMessage> handler = new NewUsersIdsMessageHandler();

I know that I can assign

IMessageHandler<NewUsersIdsMessage> handler = new NewUsersIdsMessageHandler();

but that is not universal enough for me. Thanks!

Answer 1

It is impossible.

This interface

public interface IMessageHandler<out TType> where TType : IMessage
{
    bool Handle(TType Message);
}   

cannot have <out TType> specification, since TType acts as an input parameter.

While having <out TType> specification is required to be able to assign like this:

IMessageHandler<IMessage> handler = newUserHandler;

This is called covariance and contravariance .

Logically thinking: IMessageHandler<IMessage> can handle any type of message, while newUserHandle can handle only specific messages, so this assignment cannot go through static type-check at compile-time, as it might cause runtime error.

In case the ultimate goal is to be able to find the appropriate handler for given message type, you can use IoC containers, eg Autofac; Here is how it will help you:

builder
.Register(typeof(NewUsersIdsMessageHandler))
.As(typeof(IMessageHandler<NewUsersIdsMessage>))
.InstancePerDependency();

Then in code

var handler = container.Resolve<IMessageHandler<NewUserIdsMessage>>()