C#中，接口轉換實驗

在C#中，接口是一個比較特殊的存在，一個接口的實例，可以轉換為沒有實現該接口的類，並且不會發生編譯錯誤。

為此，我們新建一個.NET Core控制台項目，然后使用如下代碼來進行實驗：

using System;

namespace NetCoreInterfaceTypeConversion
{

    //接口IPeople
    interface IPeople
    {

    }

    //結構體PeopleStruct，沒有實現接口IPeople
    struct PeopleStruct //: IPeople
    {

    }

    //抽象類PeopleAbstractClass，沒有實現接口IPeople
    abstract class PeopleAbstractClass
    {

    }

    //類PeopleClass，沒有實現接口IPeople
    class PeopleClass //: PeopleAbstractClass
    {

    }

    //類PeopleImplementedIPeopleClass，實現了接口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和結構體PeopleStruct轉換測試：");
            Console.WriteLine("=============================================");

            //PeopleStruct peopleStruct = (PeopleStruct)iPeople;//如果結構體PeopleStruct沒有實現接口IPeople，那么接口IPeople的實例iPeople，無法使用類型轉換為結構體類型PeopleStruct，編譯時會報錯
            Console.WriteLine("iPeople是PeopleStruct結構體嗎？{0}", (iPeople is PeopleStruct).ToString());//False

            PeopleStruct peopleStruct = new PeopleStruct();
            //iPeople = (IPeople)peopleStruct;//同理，因為結構體PeopleStruct沒有實現接口IPeople，所以結構體PeopleStruct的實例peopleStruct，無法使用類型轉換為接口類型IPeople，編譯時會報錯
            //iPeople = peopleStruct as IPeople;//同理，因為結構體PeopleStruct沒有實現接口IPeople，所以結構體PeopleStruct的實例peopleStruct，無法使用as關鍵字轉換為接口類型IPeople，編譯時會報錯
            Console.WriteLine("peopleStruct是IPeople類型嗎？{0}", (peopleStruct is IPeople).ToString());//False

            Console.WriteLine();

            Console.WriteLine("=============================================");
            Console.WriteLine("iPeople為null，iPeople使用類型轉換為PeopleAbstractClass和PeopleClass類型：");
            Console.WriteLine("=============================================");
            iPeople = null;

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

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

            Console.WriteLine();

            Console.WriteLine("=============================================");
            Console.WriteLine("iPeople為null，iPeople使用as關鍵字轉換為PeopleAbstractClass和PeopleClass類型：");
            Console.WriteLine("=============================================");
            iPeople = null;

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

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

            Console.WriteLine();

            Console.WriteLine("=============================================");
            Console.WriteLine("iPeople為null，iPeople使用is關鍵字判斷是否為PeopleAbstractClass和PeopleClass類型：");
            Console.WriteLine("=============================================");
            iPeople = null;

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

            Console.WriteLine();

            Console.WriteLine("=============================================");
            Console.WriteLine("iPeople為PeopleImplementedIPeopleClass類型的實例對象，iPeople使用類型轉換為PeopleAbstractClass和PeopleClass類型：");
            Console.WriteLine("=============================================");
            iPeople = new PeopleImplementedIPeopleClass();

            try
            {
                peopleAbstractClass = (PeopleAbstractClass)iPeople;//此時因為iPeople不為null了，所以類型轉換為沒有繼承關系的PeopleAbstractClass類時，會發生InvalidCastException異常
            }
            catch (InvalidCastException)
            {
                Console.ForegroundColor = ConsoleColor.Red;
                Console.WriteLine("iPeople為PeopleImplementedIPeopleClass類型的實例對象，使用類型轉換為PeopleAbstractClass類型，發生了InvalidCastException異常");
                Console.ForegroundColor = defaultColor;
            }

            try
            {
                peopleClass = (PeopleClass)iPeople;//此時因為iPeople不為null了，所以類型轉換為沒有繼承關系的PeopleClass類時，會發生InvalidCastException異常
            }
            catch (InvalidCastException)
            {
                Console.ForegroundColor = ConsoleColor.Red;
                Console.WriteLine("iPeople為PeopleImplementedIPeopleClass類型的實例對象，使用類型轉換為PeopleClass類型，發生了InvalidCastException異常");
                Console.ForegroundColor = defaultColor;
            }

            Console.WriteLine();

            Console.WriteLine("=============================================");
            Console.WriteLine("iPeople為PeopleImplementedIPeopleClass類型的實例對象，iPeople使用as關鍵字轉換為PeopleAbstractClass和PeopleClass類型：");
            Console.WriteLine("=============================================");
            iPeople = new PeopleImplementedIPeopleClass();

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

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

            Console.WriteLine();

            Console.WriteLine("=============================================");
            Console.WriteLine("iPeople為PeopleImplementedIPeopleClass類型的實例對象，iPeople使用is關鍵字判斷是否為PeopleAbstractClass和PeopleClass類型：");
            Console.WriteLine("=============================================");
            iPeople = new PeopleImplementedIPeopleClass();

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

            Console.WriteLine();

            Console.WriteLine("=============================================");
            Console.WriteLine("peopleAbstractClass為null，peopleClass為null，peopleAbstractClass和peopleClass使用類型轉換為IPeople類型：");
            Console.WriteLine("=============================================");
            peopleAbstractClass = null;
            peopleClass = null;

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

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

            Console.WriteLine();

            Console.WriteLine("=============================================");
            Console.WriteLine("peopleAbstractClass為null，peopleClass為null，peopleAbstractClass和peopleClass使用as關鍵字轉換為IPeople類型：");
            Console.WriteLine("=============================================");
            peopleAbstractClass = null;
            peopleClass = null;

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

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

            Console.WriteLine();

            Console.WriteLine("=============================================");
            Console.WriteLine("peopleAbstractClass為null，peopleClass為null，peopleAbstractClass和peopleClass使用is關鍵字判斷是否為IPeople類型：");
            Console.WriteLine("=============================================");
            peopleAbstractClass = null;
            peopleClass = null;

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

            Console.WriteLine();

            Console.WriteLine("=============================================");
            Console.WriteLine("peopleClass為PeopleClass類型的實例對象，peopleClass使用類型轉換為IPeople類型：");
            Console.WriteLine("=============================================");
            peopleClass = new PeopleClass();

            try
            {
                iPeople = (IPeople)peopleClass;//此時因為peopleClass不為null了，所以類型轉換為沒有繼承關系的IPeople接口時，會發生InvalidCastException異常
            }
            catch (InvalidCastException)
            {
                Console.ForegroundColor = ConsoleColor.Red;
                Console.WriteLine("peopleClass為PeopleClass類型的實例對象，使用類型轉換為IPeople類型，發生了InvalidCastException異常");
                Console.ForegroundColor = defaultColor;
            }

            Console.WriteLine();

            Console.WriteLine("=============================================");
            Console.WriteLine("peopleClass為PeopleClass類型的實例對象，peopleClass使用as關鍵字轉換為IPeople類型：");
            Console.WriteLine("=============================================");
            peopleClass = new PeopleClass();

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

            Console.WriteLine();

            Console.WriteLine("=============================================");
            Console.WriteLine("peopleClass為PeopleClass類型的實例對象，peopleClass使用is關鍵字判斷是否為IPeople類型：");
            Console.WriteLine("=============================================");
            peopleClass = new PeopleClass();

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

            Console.WriteLine();

            //peopleAbstractClass = null;
            //peopleClass = (PeopleClass)peopleAbstractClass;//因為抽象類PeopleAbstractClass和類PeopleClass之間沒有繼承關系，所以抽象類PeopleAbstractClass的實例peopleAbstractClass，無法使用類型轉換為類PeopleClass，編譯時會報錯
            //peopleClass = peopleAbstractClass as PeopleClass;//因為抽象類PeopleAbstractClass和類PeopleClass之間沒有繼承關系，所以抽象類PeopleAbstractClass的實例peopleAbstractClass，無法使用as關鍵字轉換為類PeopleClass，編譯時會報錯

            //peopleClass = null;
            //peopleAbstractClass = (PeopleAbstractClass)peopleClass;//因為抽象類PeopleAbstractClass和類PeopleClass之間沒有繼承關系，所以類PeopleClass的實例peopleClass，無法使用類型轉換為抽象類PeopleAbstractClass，編譯時會報錯
            //peopleAbstractClass = peopleClass as PeopleAbstractClass;//因為抽象類PeopleAbstractClass和類PeopleClass之間沒有繼承關系，所以類PeopleClass的實例peopleClass，無法使用as關鍵字轉換為抽象類PeopleAbstractClass，編譯時會報錯

            Console.WriteLine("=============================================");
            Console.WriteLine("peopleClass為null，peopleClass使用is關鍵字判斷是否為PeopleAbstractClass類型：");
            Console.WriteLine("=============================================");
            peopleClass = null;
            Console.WriteLine("peopleClass是PeopleAbstractClass類型嗎？{0}", (peopleClass is PeopleAbstractClass).ToString());//False

            Console.WriteLine();

            Console.WriteLine("=============================================");
            Console.WriteLine("peopleClass為PeopleClass類型的實例對象，peopleClass使用is關鍵字判斷是否為PeopleAbstractClass類型：");
            Console.WriteLine("=============================================");
            peopleClass = new PeopleClass();
            Console.WriteLine("peopleClass是PeopleAbstractClass類型嗎？{0}", (peopleClass is PeopleAbstractClass).ToString());//False

            Console.WriteLine();

            Console.WriteLine("=============================================");
            Console.WriteLine("peopleAbstractClass為null，peopleAbstractClass使用is關鍵字判斷是否為PeopleClass類型：");
            Console.WriteLine("=============================================");
            peopleAbstractClass = null;
            Console.WriteLine("peopleAbstractClass是PeopleClass類型嗎？{0}", (peopleAbstractClass is PeopleClass).ToString());//False

            Console.WriteLine();

            Console.WriteLine("=============================================");
            Console.WriteLine("iPeople為null，peopleClass為null，peopleAbstractClass為null，使用is關鍵字判斷它們是否分別為IPeople、PeopleClass、PeopleAbstractClass類型：");
            Console.WriteLine("=============================================");
            iPeople = null;
            peopleClass = null;
            peopleAbstractClass = null;

            Console.WriteLine("iPeople是IPeople類型嗎？{0}", (iPeople is IPeople).ToString());//False
            Console.WriteLine("peopleClass是PeopleClass類型嗎？{0}", (peopleClass is PeopleClass).ToString());//False
            Console.WriteLine("peopleAbstractClass是PeopleAbstractClass類型嗎？{0}", (peopleAbstractClass is PeopleAbstractClass).ToString());//False

            Console.WriteLine();

            iPeople = (IPeople)null;
            iPeople = null as IPeople;

            peopleClass = (PeopleClass)null;
            peopleClass = null as PeopleClass;

            peopleAbstractClass = (PeopleAbstractClass)null;
            peopleAbstractClass = null as PeopleAbstractClass;

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

該代碼詳細測試了接口與類、抽象類、結構體之間的相互轉換。