IList using covariance and contravariance in c#, is this possible?

Question

Hi all

would this be possible? (I don't have vs. 2010, so I can't try it myself, sorry)

public interface IComplexList<out TOutput, in TInput> where TOutput : TInput
{
    public IEnumerator<TOutput> GetEnumerator();
    public void Add(TInput item);
}

public interface IList<T> : IComplexList<T, T>
{
}

If I get it right, you could use this to actually implement covariance and contravariance in the same interface.

Answer 1

No, you can't. In your example IList<T> is invariant. IList<T> would require to declare in/out to be covariant/contravariant. It's not possible to do that just by inheriting some interface that is covariant.

Answer 2

Well, your question is slightly confusing because of the existing IList<T> type. However, the following does compile:

public interface IComplexList<out TOutput, in TInput> where TOutput : TInput
{
    IEnumerator<TOutput> GetEnumerator();
    void Add(TInput item);
}

public interface ISimpleList<T> : IComplexList<T, T>
{
}

You can even change it to extend IEnumerable<TOutput>:

public interface IComplexList<out TOutput, in TInput>
    : IEnumerable<TOutput>
    where TOutput : TInput
{        
    void Add(TInput item);
}

public interface ISimpleList<T> : IComplexList<T, T>
{
}

The indexer is tricky, because you'd want different types involved. You could do:

TOutput Get(int index);
void Set(int index, TInput item);

and then put the indexer into ISimpleList<T> instead of course...

That doesn't let you use ISimpleList<T> variantly though, because you've basically forced TInput=TOutput.

An alternative approach is to separate out the input from the output:

public interface IReadableList<out T> : IEnumerable<T>
{
    T Get(int index);
}

public interface IWritableList<in T>
{
    void Add(T item);
    void Set(int index, T item);
}

 public interface IMyList<T> : IReadableList<T>, IWritableList<T> {}

Then you could write:

public void Foo(IWritableList<string> x) { ... }

IMyList<object> objects = new MyList<object>();
Foo(objects);

and vice versa for IReadableList. In other words, you allow variance for each side individually, but you never get variance for the two sides together.