See http://ericlippert.com/2015/04/27/wizards-and-warriors-part-one/

WizardsAndWarriors.cs

using System;

namespace WizardsAndWarriors
{
    public class Program
    {
        public static void Main(string[] args)
        {
            IPlayer player = new Wizard();
            Console.WriteLine(player.Weapon.Damage);
            Console.ReadLine();
        }
    }

    public abstract class Weapon
    {
        public abstract int Damage { get; }
    }
    public sealed class Sword : Weapon
    {
        public override int Damage { get { return 10; } }
    }
    public sealed class Staff: Weapon
    {
        public override int Damage { get { return 100; } }
    }

    public interface IPlayer
    {
        Weapon Weapon { get; }
    }

    public abstract class Player<TWeapon> : IPlayer where TWeapon : Weapon
    {
        public abstract TWeapon Weapon { get; set; }
        Weapon IPlayer.Weapon { get { return Weapon; } }
    }

    public sealed class Warrior : Player<Sword>
    {
        public override Sword Weapon { get; set; } = new Sword();
    }

    public sealed class Wizard : Player<Staff>
    {
        public override Staff Weapon { get; set; } = new Staff();
    }
}