Invariance, Contravariance, Covariance C#

TierHandler.cs

internal class HightTier { }
internal class MediumTier : HightTier { }
internal class LowTier : MediumTier { }

internal delegate Covariance TierHandler<Invariance, in Contravariance, out Covariance>(Contravariance arg);

/// <summary>
/// if 1st_op = 2nd_op 
/// then 
///     <in> is tantamount to Going UP (emerged from MediumTier to HightTier)
///     <out> is tantamount to Going DOWN (settled down to the depths from MediumTier to LowTier)
/// </summary>
class Test
{
    void Main()
    {
        TierHandler<MediumTier, MediumTier, MediumTier> tierHandler1 = default;
        TierHandler<MediumTier, HightTier, LowTier> tierHandler2 = default;
        tierHandler1 = tierHandler2;

        TierHandler<MediumTier, MediumTier, MediumTier> tierHandler3 = default;
        TierHandler<MediumTier, LowTier, HightTier> tierHandler4 = default;
        tierHandler4 = tierHandler3;
    }
}