Conversion from java to c# -Covariance and Contravariance

  public interface IStorage<T> extends Iterable<T> {
        public void copyTo( IStorage<? super T> dest);
        public void copyFrom( IStorage<? extends T> src);
    }

Above is the java code that I have to put in c#, for the moment it looks like

 interface IStorage<T>:  IEnumerable<T>
    {
     void copyTo( IStorage<? super T> dest);
     void copyFrom( IStorage<? extends T> src);}

But I have troubles to find the equivalent of when it appears in the parameter of function, I found the in/out or where approaches but it's still not clear to me.

Solution

Generic variance in C# is very different to generic variance in .NET.

You sort of want something like this:

public interface IStorage<out T> : IEnumerable<T>
{
    // This won't compile - the constraint is on the wrong argument
    void CopyTo<TDest>(IStorage<TDest> dest) where T : TDest
}

But that's invalid, as indicated.

As written, the method doesn't really make sense to me - you'd need something else in the interface which would accept values of type T, at which point IStorage could no longer be covariant in T anyway.

Given that you can't achieve exactly the same effect, I suggest you think about what you really want to achieve, and consider something like:

public interface IStorage<out T> : IEnumerable<T>
{
    void AddAll(IStorage<T> source);
}

Or even just:

public interface IStorage<out T> : IEnumerable<T>
{
    void AddAll(IEnumerable<T> source);
}

So you reverse the target of the call from the source to the destination, at which point the destination can pull values from the source, which is more in-keeping with IEnumerable<T> being a source of values.