C# 泛型接口的抗变和协变

/// <summary>
 /// 泛型接口
 /// </summary>
 /// <typeparam name="T"></typeparam>
 public interface IDisplay< T >
 {
  void Show(T item);
 }
 /// <summary>
 /// 实现泛型接口IDisaplay
 /// </summary>
 /// <typeparam name="T"></typeparam>
 public class ShapDisplay<T> : IDisplay<T>
 {
  public void Show(T item)
  {
   Console.WriteLine("测试成功！");
  }
 }
 /// <summary>
 /// 父类
 /// </summary>
 public class ParentClass
 {
 }
 /// <summary>
 /// 子类
 /// </summary>
 public class SubClass : ParentClass
 {
 }

class Program
 {
  static void Main(string[] args)
  {
   // 用子类实例化泛型类(简称子类对象)
   IDisplay<SubClass> sub1 = new ShapDisplay<SubClass>();

   // 用父类实例化泛型类(简称父类对象)
   IDisplay<ParentClass> par1 = new ShapDisplay<ParentClass>();

   // 用父类类型接收子类对象(子类对象→父类类型)协变
   IDisplay<ParentClass> parent = sub1;

   // 用子类类型接收父类对象(父类对象→子类类型)抗变
   IDisplay<SubClass> sub = par1;

   Console.ReadKey();
  }
 }

/// <summary>
 /// 泛型接口
 /// </summary>
 /// <typeparam name="T"></typeparam>
 public interface IDisplay<out T>
 {
  void Show(T item);
 }

/// <summary>
 /// 泛型接口
 /// </summary>
 /// <typeparam name="T"></typeparam>
 public interface IDisplay<in T>
 {
  void Show(T item);
 }