C# 4.0: Covariance And Contravariance In Generics Made Easy

In my last post, I went through what is variance in .NET 4.0 and C# 4.0 in a rather theoretical way.

Now, I’m going to try to make it a bit more down to earth.

Given:

class Base { }

class Derived : Base { }

Such that:

Trace.Assert(typeof(Base).IsClass && typeof(Derived).IsClass && typeof(Base).IsGreaterOrEqualTo(typeof(Derived)));

  • Covariance

    interface ICovariantIn<out T> { }

    Trace.Assert(typeof(ICovariantIn<Base>).IsGreaterOrEqualTo(typeof(ICovariantIn<Derived>)));

  • Contravariance

    interface IContravariantIn<in T> { }

    Trace.Assert(typeof(IContravariantIn<Derived>).IsGreaterOrEqualTo(typeof(IContravariantIn<Base>)));

  • Invariance

    interface IInvariantIn<T> { }

    Trace.Assert(!typeof(IInvariantIn<Base>).IsGreaterOrEqualTo(typeof(IInvariantIn<Derived>))
        && !typeof(IInvariantIn<Derived>).IsGreaterOrEqualTo(typeof(IInvariantIn<Base>)));

Where:

public static class TypeExtensions
{
    public static bool IsGreaterOrEqualTo(this Type self, Type other)
    {
        return self.IsAssignableFrom(other);
    }
}

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