The following example shows how to declare, extend, and implement a covariant generic interface. It also shows how to use implicit conversion for classes that implement a covariant interface.

// Covariant interface.
interface ICovariant<out R> { }

// Extending covariant interface.
interface IExtCovariant<out R> : ICovariant<R> { }

// Implementing covariant interface.
class Sample<R> : ICovariant<R> { }

class Program
{
    static void Test()
    {
        ICovariant<Object> iobj = new Sample<Object>();
        ICovariant<String> istr = new Sample<String>();

        // You can assign istr to iobj because
        // the ICovariant interface is covariant.
        iobj = istr;
    }
}