gistfile1.cs

    public abstract class AbstractBoxOfFruit<TFruit> where TFruit : Fruit
    {
        public abstract void Add(TFruit fruit);
    }

    public class BoxOfApple : AbstractBoxOfFruit<Apple>
    {

        public override void Add(Apple fruit)
        {
            throw new NotImplementedException();
        }
    }

    public class Fruit
    {
    }

    public class Apple : Fruit
    { 
    
    }