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Why C# doesn’t recognize the Generic

Asked By: Anonymous

I think I got lost. I tried to create a method with two generic entries with constraints to them.

The compiler, however, even receiving an instance of the specified generic type, says it is not possible to cast.

Compiler invaliding the Generic Type

What is the explanation for this?

The "expression type" pointed in the compilation error message (IResponseStreamMessage) is exactly the TResponse constraint IResponseStreamMessage. What Am I not seeing? Or aware of?

Just to make sure, this is the GetResponseStreamOf method signature:

public static IResponseStreamMessage GetResponseStreamOf(IRequestStreamMessage request)

UPDATE: once I put an explicit cast, it accepts:

public static TResponse Get<TRequest, TResponse>(TRequest request, Func<System.IO.Stream> dataProviderFunction)
            where TRequest : IRequestStreamMessage
            where TResponse : IResponseStreamMessage
        {
            return (TResponse) StreamMessagesFactory.GetResponseStreamOf(request);
        }

But I still don’t get why I have too.


Solution

Answered By: Anonymous

While TResponse is IResponseStreamMessage, IResponseStreamMessage is not TResponse, say you have a class which implements that interface:

class CustomResponse : IResponseStreamMessage
{
}

and then you used that class as the stand-in type for the generic TResponse. The method sig would look like so:

public static CustomResponse Get<TRequest, CustomResponse>(TRequest request, Func<System.IO.Stream> dataProviderFunction);

The question is, what type does StreamMessagesFactory.GetResponseStreamOf actually return, more than likely not CustomResponse so, even tho it returns IResponseStreamMessage you won’t be able to cast and return it as CustomResponse.

You can however check and see if the returned response is of type TResponse and if not throw an exception, also, TRequest isn’t necessary, because what difference does it make if your request parameter is IRequestStreamMessage or TRequest, sins TRequest is IRequestStreamMessage, so your complete function would be:

public static TResponse Get<TResponse>(IRequestStreamMessage request, Func<System.IO.Stream> dataProviderFunction) where TResponse : IResponseStreamMessage
{
    var responseInterface = StreamMessagesFactory.GetResponseStreamOf(request);

    if (responseInterface is TResponse responseObject)
    {
        return responseObject;
    }
    else if (ReferenceEquals(responseInterface, null) && ReferenceEquals(default(TResponse), null))
    {
        return default(TResponse);
    }
    throw new InvalidOperationException($"Expected response of type {typeof(TResponse)}, but recieved a response of type {responseInterface?.GetType()}");          
}

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