C# Downcasting a generic type

Question

interface IMyEvent { }

class SpecificEvent : IMyEvent {  }

interface IMySubscriber<TEvent> where TEvent : IMyEvent { }

class MySubscriber<TEvent> : IMySubscriber<TEvent> where TEvent: IMyEvent { }

class EventPublisher
{
    public void Subscribe(IMySubscriber<IMyEvent> subscriber)
    {

    }
}

I can't understand why it's impossible to execute Subscribe method with a MySubscriber<SpecificEvent> object.

new EventPublisher().Subscribe(new MySubscriber<SpecificEvent>());

It says that it can't cast MySubscriber<SpecificEvent> to IMySubscriber<IMyEvent>. Could you please explain why it happens or provide some links where I can read about it.

@Archer, can't understand what do you mean. An object is created, but it's impossible to pass it into the method. — E. Shcherbo, Apr 23 '18 at 16:41
I changed your example calling code to match what I think you meant. Please review. — Jamiec, Apr 23 '18 at 16:41
You want to make your interface contravariant I believe. `interface IMySubscriber where TEvent : IMyEvent { }` — Jonathon Chase, Apr 23 '18 at 16:42
@Lee, yes, but I think it's not necessary to write them here. — E. Shcherbo, Apr 23 '18 at 16:42
[Covariance and Contravariance in Generics](https://learn.microsoft.com/en-us/dotnet/standard/generics/covariance-and-contravariance) — Jonathon Chase, Apr 23 '18 at 16:44
It is necessary since you won't be able to make `IMySubscriber` covariant (which would make your example code compile) if any of the methods have a parameter with type `TEvent`. — Lee, Apr 23 '18 at 16:57
@Lee, yes, I see. I'm going to add the `void Handle(TEvent myEvent)` method, but it doesn't work. So I must find out another implementation these things. — E. Shcherbo, Apr 23 '18 at 17:05

score 2 · Accepted Answer · answered Apr 23 '18 at 16:43

2

For this to work you must mark the interface with an out generic parameter:

interface IMySubscriber<out TEvent> where TEvent : IMyEvent { }

http://rextester.com/FAXW57974

See: still confused about covariance and contravariance & in/out

answered Apr 23 '18 at 16:43

Jamiec

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I saw your example, but it looks like it's not working there. – E. Shcherbo Apr 23 '18 at 16:44
@E.Shcherbo well it doesnt error any more. If you take out the `out1 it will. – Jamiec Apr 23 '18 at 16:45
1

Tidy. Wasn't aware of this so thanks for today's thing I've learned. – Reinstate Monica Cellio Apr 23 '18 at 16:45
Thank you very much for your help! – E. Shcherbo Apr 23 '18 at 16:47

C# Downcasting a generic type

1 Answers1