Variant types as parameters and return values

If a covariant type appears as an output, the containing type is covariant. Producing a producer of Ts is like producing Ts.

interface IReturnCovariant<out T>
{
    IEnumerable<T> GetTs();
}

If a contravariant type appears as an output, the containing type is contravariant. Producing a consumer of Ts is like consuming Ts.

interface IReturnContravariant<in T>
{
    IComparer<T> GetTComparer();
}

If a covariant type appears as an input, the containing type is contravariant. Consuming a producer of Ts is like consuming Ts.

interface IAcceptCovariant<in T>
{
    void ProcessTs(IEnumerable<T> ts);
}

If a contravariant type appears as an input, the containing type is covariant. Consuming a consumer of Ts is like producing Ts.

interface IAcceptContravariant<out T>
    {
        void CompareTs(IComparer<T> tComparer);
    }