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.
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()}");
}