C#中，接口转换实验

在C#中，接口是一个比较特殊的存在，一个接口的实例，可以转换为没有实现该接口的类，并且不会发生编译错误。

为此，我们新建一个.NET Core控制台项目，然后使用如下代码来进行实验：

using System;

namespace NetCoreInterfaceTypeConversion
{

    //接口IPeople
    interface IPeople
    {

    }

    //结构体PeopleStruct，没有实现接口IPeople
    struct PeopleStruct //: IPeople
    {

    }

    //抽象类PeopleAbstractClass，没有实现接口IPeople
    abstract class PeopleAbstractClass
    {

    }

    //类PeopleClass，没有实现接口IPeople
    class PeopleClass //: PeopleAbstractClass
    {

    }

    //类PeopleImplementedIPeopleClass，实现了接口IPeople
    class PeopleImplementedIPeopleClass : IPeople
    {

    }


    class Program
    {
        static void Main(string[] args)
        {
            ConsoleColor defaultColor = Console.ForegroundColor;

            IPeople iPeople = null;

            Console.WriteLine("=============================================");
            Console.WriteLine("接口IPeople和结构体PeopleStruct转换测试：");
            Console.WriteLine("=============================================");

            //PeopleStruct peopleStruct = (PeopleStruct)iPeople;//如果结构体PeopleStruct没有实现接口IPeople，那么接口IPeople的实例iPeople，无法使用类型转换为结构体类型PeopleStruct，编译时会报错
            Console.WriteLine("iPeople是PeopleStruct结构体吗？{0}", (iPeople is PeopleStruct).ToString());//False

            PeopleStruct peopleStruct = new PeopleStruct();
            //iPeople = (IPeople)peopleStruct;//同理，因为结构体PeopleStruct没有实现接口IPeople，所以结构体PeopleStruct的实例peopleStruct，无法使用类型转换为接口类型IPeople，编译时会报错
            //iPeople = peopleStruct as IPeople;//同理，因为结构体PeopleStruct没有实现接口IPeople，所以结构体PeopleStruct的实例peopleStruct，无法使用as关键字转换为接口类型IPeople，编译时会报错
            Console.WriteLine("peopleStruct是IPeople类型吗？{0}", (peopleStruct is IPeople).ToString());//False

            Console.WriteLine();

            Console.WriteLine("=============================================");
            Console.WriteLine("iPeople为null，iPeople使用类型转换为PeopleAbstractClass和PeopleClass类型：");
            Console.WriteLine("=============================================");
            iPeople = null;

            PeopleAbstractClass peopleAbstractClass = (PeopleAbstractClass)iPeople;
            PeopleClass peopleClass = (PeopleClass)iPeople;

            Console.WriteLine("peopleAbstractClass为null吗？{0}", (peopleAbstractClass == null).ToString());//True
            Console.WriteLine("peopleClass为null吗？{0}", (peopleClass == null).ToString());//True

            Console.WriteLine();

            Console.WriteLine("=============================================");
            Console.WriteLine("iPeople为null，iPeople使用as关键字转换为PeopleAbstractClass和PeopleClass类型：");
            Console.WriteLine("=============================================");
            iPeople = null;

            peopleAbstractClass = iPeople as PeopleAbstractClass;
            peopleClass = iPeople as PeopleClass;

            Console.WriteLine("peopleAbstractClass为null吗？{0}", (peopleAbstractClass == null).ToString());//True
            Console.WriteLine("peopleClass为null吗？{0}", (peopleClass == null).ToString());//True

            Console.WriteLine();

            Console.WriteLine("=============================================");
            Console.WriteLine("iPeople为null，iPeople使用is关键字判断是否为PeopleAbstractClass和PeopleClass类型：");
            Console.WriteLine("=============================================");
            iPeople = null;

            Console.WriteLine("iPeople是PeopleAbstractClass类型吗？{0}", (iPeople is PeopleAbstractClass).ToString());//False
            Console.WriteLine("iPeople是PeopleClass类型吗？{0}", (iPeople is PeopleClass).ToString());//False

            Console.WriteLine();

            Console.WriteLine("=============================================");
            Console.WriteLine("iPeople为PeopleImplementedIPeopleClass类型的实例对象，iPeople使用类型转换为PeopleAbstractClass和PeopleClass类型：");
            Console.WriteLine("=============================================");
            iPeople = new PeopleImplementedIPeopleClass();

            try
            {
                peopleAbstractClass = (PeopleAbstractClass)iPeople;//此时因为iPeople不为null了，所以类型转换为没有继承关系的PeopleAbstractClass类时，会发生InvalidCastException异常
            }
            catch (InvalidCastException)
            {
                Console.ForegroundColor = ConsoleColor.Red;
                Console.WriteLine("iPeople为PeopleImplementedIPeopleClass类型的实例对象，使用类型转换为PeopleAbstractClass类型，发生了InvalidCastException异常");
                Console.ForegroundColor = defaultColor;
            }

            try
            {
                peopleClass = (PeopleClass)iPeople;//此时因为iPeople不为null了，所以类型转换为没有继承关系的PeopleClass类时，会发生InvalidCastException异常
            }
            catch (InvalidCastException)
            {
                Console.ForegroundColor = ConsoleColor.Red;
                Console.WriteLine("iPeople为PeopleImplementedIPeopleClass类型的实例对象，使用类型转换为PeopleClass类型，发生了InvalidCastException异常");
                Console.ForegroundColor = defaultColor;
            }

            Console.WriteLine();

            Console.WriteLine("=============================================");
            Console.WriteLine("iPeople为PeopleImplementedIPeopleClass类型的实例对象，iPeople使用as关键字转换为PeopleAbstractClass和PeopleClass类型：");
            Console.WriteLine("=============================================");
            iPeople = new PeopleImplementedIPeopleClass();

            peopleAbstractClass = iPeople as PeopleAbstractClass;
            peopleClass = iPeople as PeopleClass;

            Console.WriteLine("peopleAbstractClass为null吗？{0}", (peopleAbstractClass == null).ToString());//True
            Console.WriteLine("peopleClass为null吗？{0}", (peopleClass == null).ToString());//True

            Console.WriteLine();

            Console.WriteLine("=============================================");
            Console.WriteLine("iPeople为PeopleImplementedIPeopleClass类型的实例对象，iPeople使用is关键字判断是否为PeopleAbstractClass和PeopleClass类型：");
            Console.WriteLine("=============================================");
            iPeople = new PeopleImplementedIPeopleClass();

            Console.WriteLine("iPeople是PeopleAbstractClass类型吗？{0}", (iPeople is PeopleAbstractClass).ToString());//False
            Console.WriteLine("iPeople是PeopleClass类型吗？{0}", (iPeople is PeopleClass).ToString());//False

            Console.WriteLine();

            Console.WriteLine("=============================================");
            Console.WriteLine("peopleAbstractClass为null，peopleClass为null，peopleAbstractClass和peopleClass使用类型转换为IPeople类型：");
            Console.WriteLine("=============================================");
            peopleAbstractClass = null;
            peopleClass = null;

            iPeople = (IPeople)peopleAbstractClass;
            Console.WriteLine("iPeople为null吗？{0}", (iPeople == null).ToString());//True

            iPeople = (IPeople)peopleClass;
            Console.WriteLine("iPeople为null吗？{0}", (iPeople == null).ToString());//True

            Console.WriteLine();

            Console.WriteLine("=============================================");
            Console.WriteLine("peopleAbstractClass为null，peopleClass为null，peopleAbstractClass和peopleClass使用as关键字转换为IPeople类型：");
            Console.WriteLine("=============================================");
            peopleAbstractClass = null;
            peopleClass = null;

            iPeople = peopleAbstractClass as IPeople;
            Console.WriteLine("iPeople为null吗？{0}", (iPeople == null).ToString());//True

            iPeople = peopleClass as IPeople;
            Console.WriteLine("iPeople为null吗？{0}", (iPeople == null).ToString());//True

            Console.WriteLine();

            Console.WriteLine("=============================================");
            Console.WriteLine("peopleAbstractClass为null，peopleClass为null，peopleAbstractClass和peopleClass使用is关键字判断是否为IPeople类型：");
            Console.WriteLine("=============================================");
            peopleAbstractClass = null;
            peopleClass = null;

            Console.WriteLine("peopleAbstractClass是IPeople类型吗？{0}", (peopleAbstractClass is IPeople).ToString());//False
            Console.WriteLine("peopleClass是IPeople类型吗？{0}", (peopleClass is IPeople).ToString());//False

            Console.WriteLine();

            Console.WriteLine("=============================================");
            Console.WriteLine("peopleClass为PeopleClass类型的实例对象，peopleClass使用类型转换为IPeople类型：");
            Console.WriteLine("=============================================");
            peopleClass = new PeopleClass();

            try
            {
                iPeople = (IPeople)peopleClass;//此时因为peopleClass不为null了，所以类型转换为没有继承关系的IPeople接口时，会发生InvalidCastException异常
            }
            catch (InvalidCastException)
            {
                Console.ForegroundColor = ConsoleColor.Red;
                Console.WriteLine("peopleClass为PeopleClass类型的实例对象，使用类型转换为IPeople类型，发生了InvalidCastException异常");
                Console.ForegroundColor = defaultColor;
            }

            Console.WriteLine();

            Console.WriteLine("=============================================");
            Console.WriteLine("peopleClass为PeopleClass类型的实例对象，peopleClass使用as关键字转换为IPeople类型：");
            Console.WriteLine("=============================================");
            peopleClass = new PeopleClass();

            iPeople = peopleClass as IPeople;
            Console.WriteLine("iPeople为null吗？{0}", (iPeople == null).ToString());//True

            Console.WriteLine();

            Console.WriteLine("=============================================");
            Console.WriteLine("peopleClass为PeopleClass类型的实例对象，peopleClass使用is关键字判断是否为IPeople类型：");
            Console.WriteLine("=============================================");
            peopleClass = new PeopleClass();

            Console.WriteLine("peopleClass是IPeople类型吗？{0}", (peopleClass is IPeople).ToString());//False

            Console.WriteLine();

            //peopleAbstractClass = null;
            //peopleClass = (PeopleClass)peopleAbstractClass;//因为抽象类PeopleAbstractClass和类PeopleClass之间没有继承关系，所以抽象类PeopleAbstractClass的实例peopleAbstractClass，无法使用类型转换为类PeopleClass，编译时会报错
            //peopleClass = peopleAbstractClass as PeopleClass;//因为抽象类PeopleAbstractClass和类PeopleClass之间没有继承关系，所以抽象类PeopleAbstractClass的实例peopleAbstractClass，无法使用as关键字转换为类PeopleClass，编译时会报错

            //peopleClass = null;
            //peopleAbstractClass= (PeopleAbstractClass)peopleClass;//因为抽象类PeopleAbstractClass和类PeopleClass之间没有继承关系，所以类PeopleClass的实例peopleClass，无法使用类型转换为抽象类PeopleAbstractClass，编译时会报错
            //peopleAbstractClass = peopleClass as PeopleAbstractClass;//因为抽象类PeopleAbstractClass和类PeopleClass之间没有继承关系，所以类PeopleClass的实例peopleClass，无法使用as关键字转换为抽象类PeopleAbstractClass，编译时会报错

            Console.WriteLine("=============================================");
            Console.WriteLine("peopleClass为null，peopleClass使用is关键字判断是否为PeopleAbstractClass类型：");
            Console.WriteLine("=============================================");
            peopleClass = null;
            Console.WriteLine("peopleClass是PeopleAbstractClass类型吗？{0}", (peopleClass is PeopleAbstractClass).ToString());//False

            Console.WriteLine();

            Console.WriteLine("=============================================");
            Console.WriteLine("peopleClass为PeopleClass类型的实例对象，peopleClass使用is关键字判断是否为PeopleAbstractClass类型：");
            Console.WriteLine("=============================================");
            peopleClass = new PeopleClass();
            Console.WriteLine("peopleClass是PeopleAbstractClass类型吗？{0}", (peopleClass is PeopleAbstractClass).ToString());//False

            Console.WriteLine();

            Console.WriteLine("=============================================");
            Console.WriteLine("peopleAbstractClass为null，peopleAbstractClass使用is关键字判断是否为PeopleClass类型：");
            Console.WriteLine("=============================================");
            peopleAbstractClass = null;
            Console.WriteLine("peopleAbstractClass是PeopleClass类型吗？{0}", (peopleAbstractClass is PeopleClass).ToString());//False

            Console.WriteLine();

            Console.WriteLine("Press any key to end...");
            Console.ReadKey();
        }
    }
}

该代码详细测试了接口与类、抽象类、结构体之间的相互转换。