Interfaces covariance problem

Question

Hi,

So i have such code.

public interface IGeoDataSet<out T> : IDataSet
    where T : IGeoPrimitive<IGeoPrimitiveContent>
{
    IEnumerable<T> Items { get; } 
}

public interface IDataSet { }

public interface IGeoPrimitive<T> : IPrimitive
    where T : IGeoPrimitiveContent
{
    T Content { get; }
}

public interface IPrimitive { }

public interface IGeoPrimitiveContent { }

And such inplementation for previous interfaces.

public class TriangleDataSet : IGeoDataSet<Triangle>
{
    public IEnumerable<Triangle> Items { get; private set; }
}

public class Triangle : IGeoPrimitive<TriangleContent>
{
    public TriangleContent Content { get; private set; }
}

public class TriangleContent : IGeoPrimitiveContent { }

When I try to compile this code I've got an error:

The type '.Triangle' cannot be used as type parameter 'T' in the generic type or method '.IGeoDataSet<T>'. There is no implicit reference conversion from '.Triangle' to '.IGeoPrimitive<.IGeoPrimitiveContent>'.

I can't understand why, maybe someone knows what is the problem?

Br, Jevgenij

Answer 1

You have to declare IGeoPrimitive<T> covariant:

public interface IGeoPrimitive<out T> : IPrimitive
    where T : IGeoPrimitiveContent
{
    T Content { get; }
}

Answer 2

Are you better off declaring your interfaces as:

public interface IGeoDataSet<out T, out V> : IDataSet
    where T : IGeoPrimitive<V>
    where V : IGeoPrimitiveContent
{
    IEnumerable<T> Items { get; } 
}

and

public interface IGeoPrimitive<out T> : IPrimitive
    where T : IGeoPrimitiveContent
{
    T Content { get; }
}

Answer 3

You need for your IGeoPrimitive<T> interface to also be covariant in order to be able to use Triangle as the type parameter for a IGeoDataSet<out T> implementation:

// Note addition of 'out' keyword
public interface IGeoPrimitive<out T> : IPrimitive
    where T : IGeoPrimitiveContent
{
    T Content { get; }
}

This is because you have a where T : IGeoPrimitive<IGeoPrimitiveContent> constraint; but without IGeoPrimitive<T> being covariant, a Triangle (which implements IGeoPrimitive<TriangleContent>) does not meet this constraint.