Constraining T to a common generic property

Question

I have the following:

public interface IEntity
{
    IEntityContent Data { get; set; }
}

public interface IEntityContent { }

public interface IThingService<T>
{
    void DoThing(T item);
}

public class BaseEntity<T> : IEntity
    where T : IEntityContent
{
   public abstract T Data { get; set; }
}

public class FooEntity : BaseEntity<FooContent>
{
    public override FooContent Data { get; set; }
}

public class FooContent : IEntityContent
{
    // Some properties
}

public class ThingService<T> : IThingService<T>
    where T : IEntity
{
    public void DoThing(T item)
    {
        Serializer.Instance.Serialize(item.Content);
    }
}

The signature of Serializer.Instance.Serialize is:

string Serialize<T>(T from)

But I get the following:

'BaseEntity<T>' does not implement interface member 'IEntity.Data'. 'BaseEntity<T>.Data' cannot implement 'IEntity.Data' because it does not have the matching return type of 'IEntityContent'

Why is this? As it stands, I am forced to create a bunch of near-identical strongly-typed implementations of IThingService - which is a shedload of duplication - just to specify different type arguments which, as far as I can see, should be generic.

Is this somehow related to a lack of covariance in BaseEntity? How can I make ThingService<T> work?

Answer 1

Why not simply have:

public interface IEntity<T>
{
    T Data { get; set; }
}

For an implementation of the interface has to match the interface (as you've not declared any contra/covariance), including the return types (hence your error message)