Question

Okay. So I am currently trying to create a list of an interface in C# that takes an interface as a parameter. To make this clearer, let me give an example:

public interface IPlate<T> where T : IWaffle {}
public interface IWaffles {}

public class BelgiumWaffle : IWaffle {}
public class FalafelWaffle : IWaffle {}
public class HugePlate : IPlate<BelgiumWaffle> {}
public class SmallPlate : IPlate<FalafelWaffle> {}


// Now, I want to do the following:
var plates = new List<IPlate<IWaffle>>();
plates.add(new HugePlate());
plates.add(new SmallPlate());

The goal is to be able to serialize a list of IPlate objects into XML. I was hoping to use generics to do this but I keep getting errors telling me that there are some argument errors when trying to add (aka – the types don’t match up). Just not sure what I’m doing wrong here. It seems right to me but I must be missing something (obviously).

Update: I should mention that this is .NET v3.5

Update: Sorry! Some typos when writing the question regarding definition of Plate classes.

Answer 1

I think you should use covariance (.NET 4.0)

public interface IPlate<out T> where T : IWaffle {}

and replace IPancake with IWaffle

var plates = new List<IPlate<IPancake>>();