Program.cs

using System;
using System.Collections.Generic;

namespace GenericCovariantAndContravariant
{
    // Ref: https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/concepts/covariance-contravariance/
    class Program
    {
        static void SetObject(object o) { }

        static void Main(string[] args)
        {
            // Assignment compatibility.
            string str = "test";
            // An object of a more derived type is assigned to an object of a less derived type.
            object obj = str;

            // Covariance.
            IEnumerable<string> strings = new List<string>();
            // An object that is instantiated with a more derived type argument
            // is assigned to an object instantiated with a less derived type argument.
            // Assignment compatibility is preserved.
            IEnumerable<object> objects = strings;

            // Contravariance.
            // Assume that the following method is in the class:
            // static void SetObject(object o) { }
            Action<object> actObject = SetObject;
            // An object that is instantiated with a less derived type argument
            // is assigned to an object instantiated with a more derived type argument.
            // Assignment compatibility is reversed.
            Action<string> actString = actObject;

            actObject(new object());
            // actString(new object()); will meet a compile error.
            actString(new string(""));
        }
    }
}