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 ICovariantIn<out 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);
    }
}