Covariance

Example.cs

using System.Collections.Generic;
using System.Linq;

namespace Covariance
{
    interface IMyFace { string Title { get; } }
    class MyFace:  IMyFace { public string Title => "This is my face"; }
    class YourFace :  IMyFace { public string Title => "And your face"; }
    public class Example 
    {
        private static void AddToMe(List<IMyFace> list) => list.Add(new YourFace() ); 
        private static IMyFace FirstOrNull(IEnumerable<IMyFace> list) => list.FirstOrDefault(); 

        public void Examples()
        {
            var foo = new List<IMyFace> {new MyFace(), new MyFace()};
            var foo2 = new List<IMyFace> {new MyFace(), new MyFace(), new YourFace()};
            var foo3 = new List<MyFace> {new MyFace(), new MyFace() };
            AddToMe(foo);
            AddToMe(foo2);
            FirstOrNull(foo3); // OK because it is only pulling things out
            // AddToMe(foo3); Not OK because we can put YourFace in a MyFace list
        }
    }
}