Covariance / Contravariance

This is a little experiment to get your head around co- and contravariance, as applied to generic interfaces in C#. Covariance allows the result of a computation to vary, while Contravariance allows the input to vary. There are three interfaces in the code sample. The first demonstrates covariance by emulating a function that only returns a result. The second demonstrates contravariance by emulating an action that only takes an input. The third demonstrates both co- and contravariance. Let's look at the code:

#region Covariance
interface IFunction<out TResult>
{
    TResult Invoke();
}

class Function<TResult> : IFunction<TResult>
{
    public TResult Invoke()
    {
        throw new NotImplementedException();
    }
}
#endregion

#region Contravariance
interface IAction<in T>
{
    void Invoke(T arg);
}

class Action<T> : IAction<T>
{
    public void Invoke(T arg)
    {
        throw new NotImplementedException();
    }
}
#endregion

#region Both
interface IFunction<in T, out TResult>
{
    TResult Invoke(T arg);
}

class Function<T, TResult> : IFunction<T, TResult>
{
    public TResult Invoke(T arg)
    {
        throw new NotImplementedException();
    }
}
#endregion

None of the methods are implemented, because a) I'm lazy and b) you don't need to see an implementation to witness the co- or contravariance first hand. The next code block shows the phenomena at work:

IFunction<object> f1 = new Function<string>();
IAction<string> a1 = new Action<object>();
IFunction<string, object> f2 = new Function<object, string>();

Notes:

1. In the case of covariance, the generic argument type of the class is more specific than the interface (covariance: we can always implicitly cast from string to object when reading from the container).
2. In the case of contravariance, the generic argument type of the interface is more specific than the class (contravariance: we can always implicitly cast from string to object when writing to the container).
3. In the mixed case, we can see covariance for the output and contravariance for the input.