Vfrdtyky

I´m trying to create an API in C# to evaluate dependent analytics by resolving its dependency tree and treating caching, and other issues in the background.

I´ll not delve into details here, bug I´m struggling with C# type inference mechanism.

for starters. suppose I have these classes:

public interface IAnalytic {}



public interface IAnalytic<TInput, TOutput> : IAnalytic

{

    TOutput Calculate(TInput input);

}



public interface IAnalyticResolver <T> where T: IAnalytic

{

    object EvaluateUntyped();

}



public interface IResolver

{

    IAnalyticResolver<TAnalyticImpl> GetResolver<TAnalyticImpl> () where TAnalyticImpl : IAnalytic;

}

in a way that if I have a class such as this:

public class ParseAnalytic : IAnalytic<string, int>

{

    public int Calculate(string input) => int.Parse(input);

}

I can call my API as:

IResolver r = //..

int result = (int)r.GetResolver<ParseAnalytic>().EvaluateUntyped();

However, I really dislike having to type cast my result at the end. So I´ve tried to create an extension method to work around this issue:

public static class ResolverExtensions 

{

    public static TOutput Evaluate<TAnalytic, TInput, TOutput>(this IAnalyticResolver<TAnalytic> resolver)

        where TAnalytic : IAnalytic<TInput, TOutput>

    {

        return (TOutput)resolver.EvaluateUntyped();

    }

}

I was expecting that by calling this method, C# would be able to infer the TOutput type because of the parameter IAnalyticResolver<TAnalytic> with the restriction where TAnalytic : IAnalytic<TInput, TOutput>

However that is not the case, when I try to use:

int i = r.GetResolver<ParseAnalytic>().Evaluate();

I get an

CS0411 The type arguments for method 'ResolverExtensions.Evaluate(IAnalyticResolver)' cannot be inferred from the usage. Try specifying the type arguments explicitly.

I think I understand why this is happening (TAnalytic could be implementing multiple IAnalytic<,> interfaces)

Is there a work around this issue ?, Or do will I have to explicitly call the extension method with the type arguments? (that would not be very user friendly)

Not sure this is exactly what you are looking for but here's how you only need to specify the type of the resolver during instantiation or in an implementation (you can't avoid specification/casts altogether):

public interface IAnalytic { }



public interface IAnalytic<TInput, TOutput> : IAnalytic

{

    TOutput Calculate(TInput input);

}



public interface IAnalyticResolver<T, TOutput> where T : IAnalytic

{

    TOutput Evaluate();

}



public interface IResolver<TOutput>

{

    IAnalyticResolver<TAnalyticImpl, TOutput> GetResolver<TAnalyticImpl>() where TAnalyticImpl : IAnalytic;

}



public class ParseAnalytic : IAnalytic<string, int>

{

    public int Calculate(string input) => int.Parse(input);

}



public class IntResolver : IResolver<int>

{

    public IAnalyticResolver<TAnalyticImpl, int> GetResolver<TAnalyticImpl>() where TAnalyticImpl : IAnalytic

    {

        throw new NotImplementedException();

    }

}



class Program

{

    static void Main(string args)

    {

        IResolver<int> r = new IntResolver();

        int result = r.GetResolver<ParseAnalytic>().Evaluate();

    }

}

Thanks everyone,
But I ended up creating a intermediary interface such as

public interface IAnalytic<TOutput> : IAnalytic { }

and making IAnalytic<,>inherit from it:

public interface IAnalytic<TInput, TOutput> : IAnalytic<TOutput>

{

    TOutput Calculate(TInput input);

}

And also changed the IAnalyticResolverInterface to work with it:

public interface IAnalyticResolver<TAnalytic, TOutput> where TAnalytic : IAnalytic<TOutput>

{

    TOutput Evaluate();

}



public interface IResolver

{

    IAnalyticResolver<TAnalyticImpl, TOutput> GetResolver<TAnalyticImpl, TOutput> () 

           where TAnalyticImpl : IAnalytic<TOutput>;

}

with this, I can call the resolver returning the correct type. However, I have to explicitly state the TOutput type.

int i = r.GetResolver<ParseAnalytic, int>().Evaluate();

this is redundant, since ParseAnalytic already is a IAnalytic<string, int>. But I can cope with that because at least int is the only acceptable type in this situation. If I try to write something like

double i = r.GetResolver<ParseAnalytic, double>().Evaluate();

the compiler will say:

CS0311 The type 'UserQuery.ParseAnalytic' cannot be used as type parameter 'TAnalyticImpl' in the generic type or method 'UserQuery.IResolver.GetResolver()'. There is no implicit reference conversion from 'UserQuery.ParseAnalytic' to 'UserQuery.IAnalytic'.

Avoiding me to make stupid mistakes.