Moving extension methods onto IResult (to enable covariance)

[redx177]

Covariance is only working with interfaces. Meaning something like this is possible:

public interface IVehicle { }
public class Car : IVehicle { }
public class Bus : IVehicle { }

IResult<IVehicle> carResult1 = Result.Success(new Car());

This on the other hand is not possible:

Result<IVehicle> carResult2 = Result.Success(new Car());

The solution seams simple, just go with IResult rather than Result.
But all the extension methods (those) are on Result, and not on IResult.

Is there a possibility to put all extension methods onto IResult rather than Result?
The "creation" methods (those) can stay on Result, but they return an IResult.

I guess when the extension methods are moved to IResult, we break compatibility...
If the extension methods are copied to IResult, the codebase gets much bigger...

[hankovich]

Yes, it's one of the C# limitations. The solution here will be to specify type explicitely like

var carResult2 = Result.Success<IVehicle>(new Car());

The problem I see with the extensions to IResult are

1. Async extensions. Tasks/ValueTasks are not delegates and interfaces, so Task<Result<int>> with not be suitable for extensions accepting Task<IResult<T>>, only to extensions accepting Task<Result<T>>. The possible solution is to parameterize extensions to accept Task<TResult<TValue>> where TResult : IResult<TValue>, but C# will not infer type parameters, so everyone will have to provide them explicitely
2. IResult is an interface, so there will be a lot of unnecessary allocations