在前面的文章中，我們介紹了編譯期多態(tài)、params關(guān)鍵字、實例化、base關(guān)鍵字等。本節(jié)我們來關(guān)注另外一種多態(tài)：運行時多態(tài)， 運行時多態(tài)也叫遲綁定。
深入理解OOP(一):多態(tài)和繼承(初期綁定和編譯時多態(tài))
深入理解OOP(二):多態(tài)和繼承(繼承)
深入理解OOP(三):多態(tài)和繼承(動態(tài)綁定和運行時多態(tài))
深入理解OOP(四):多態(tài)和繼承(C#中的抽象類)
深入理解OOP(五):C#中的訪問修飾符(Public/PRivate/Protected/Internal/Sealed/Constants/Static and Readonly Fields)
深入理解OOP(六):枚舉(實用方法)
深入理解OOP(七):屬性(實用方法)
深入理解OOP(八):索引器(實用方法)
深入理解OOP(九):事件(深入理解)
在C#語音中，運行時多態(tài)也叫方法重寫（overriding），我們可以在子類中overriding基類的同簽名函數(shù)，使用“virtual & override”關(guān)鍵字即可。
創(chuàng)建一個console 示例工程，命名為InheritanceAndPolymorphism。在Program.cs基礎(chǔ)上，再添加2個類文件，分別命名為ClassA.cs、ClassB.cs。拷貝如下代碼：
public class ClassA { public void AAA() { Console.WriteLine("ClassA AAA"); } public void BBB() { Console.WriteLine("ClassA BBB"); } public void CCC() { Console.WriteLine("ClassA CCC"); } }
ClassB：
public class ClassB { public void AAA() { Console.WriteLine("ClassB AAA"); } public void BBB() { Console.WriteLine("ClassB BBB"); } public void CCC() { Console.WriteLine("ClassB CCC"); } }
在上面的代碼中，我們可以看到ClassA、ClassB有同樣簽名的方法，可以在program.cs中直接使用。
我們對代碼再做休整，結(jié)構(gòu)如下：
/// <summary> /// ClassB, acting as a base class /// </summary> public class ClassB { public void AAA() { Console.WriteLine("ClassB AAA"); } public void BBB() { Console.WriteLine("ClassB BBB"); } public void CCC() { Console.WriteLine("ClassB CCC"); } } /// <summary> /// Class A, acting as a derived class /// </summary> public class ClassA : ClassB { public void AAA() { Console.WriteLine("ClassA AAA"); } public void BBB() { Console.WriteLine("ClassA BBB"); } public void CCC() { Console.WriteLine("ClassA CCC"); } }
Program.cs
/// <summary> /// Program: used to execute the method. /// Contains Main method. /// </summary> public class Program { private static void Main(string[] args) { ClassA x = new ClassA(); ClassB y=new ClassB(); ClassB z=new ClassA(); x.AAA(); x.BBB(); x.CCC(); y.AAA(); y.BBB();y.CCC(); z.AAA(); z.BBB(); z.CCC(); } }
F5，運行代碼，結(jié)果如下：
ClassA AAA
ClassA BBB
ClassA CCC
ClassB AAA
ClassB BBB
ClassB CCC
ClassB AAA
ClassB BBB
ClassB CCC
但同時，在VS的Output窗口，我們獲得了3個Warnings：
'InheritanceAndPolymorphism.ClassA.AAA()' hides inherited member
'InheritanceAndPolymorphism.ClassB.AAA()'. Use the new keyWord if hiding was intended.
'InheritanceAndPolymorphism.ClassA.BBB()' hides inherited member
'InheritanceAndPolymorphism.ClassB.BBB()'. Use the new keyword if hiding was intended.
'InheritanceAndPolymorphism.ClassA.CCC()' hides inherited member
'InheritanceAndPolymorphism.ClassB.CCC()'. Use the new keyword if hiding was intended.
這些Warnings的原因是因為子類和基類的AAA、BBB、CCC方法簽名相同，盡管從執(zhí)行上看優(yōu)先執(zhí)行子類同簽名的方法，但是可能會有潛在的問題，故Warnings提出。
基于上面的Warning，我們手動修改代碼，看看如何消除這些Warnings。
先給子類添加new、override關(guān)鍵字試試：
/// <summary> /// Class A, acting as a derived class /// </summary> public class ClassA : ClassB { public override void AAA() { Console.WriteLine("ClassA AAA"); } public new void BBB() { Console.WriteLine("ClassA BBB"); } public void CCC() { Console.WriteLine("ClassA CCC"); } }
執(zhí)行的結(jié)果是報錯了：
Error: 'InheritanceAndPolymorphism.ClassA.AAA()': cannot override inherited member 'InheritanceAndPolymorphism.ClassB.AAA()' because it is not marked virtual, abstract, or override
從這個錯誤提示信息看，我們需要修改基類方法，如添加virtual關(guān)鍵字。
/// <summary> /// ClassB, acting as a base class /// </summary> public class ClassB { public virtual void AAA() { Console.WriteLine("ClassB AAA"); } public virtual void BBB() { Console.WriteLine("ClassB BBB"); } public virtual void CCC() { Console.WriteLine("ClassB CCC"); } } /// <summary> /// Class A, acting as a derived class /// </summary> public class ClassA : ClassB { public override void AAA() { Console.WriteLine("ClassA AAA"); } public new void BBB() { Console.WriteLine("ClassA BBB"); } public void CCC() { Console.WriteLine("ClassA CCC"); } } /// <summary> /// Program: used to execute the method. /// Contains Main method. /// </summary> public class Program { private static void Main(string[] args) { ClassB y = new ClassB(); ClassA x = new ClassA(); ClassB z = new ClassA(); y.AAA(); y.BBB(); y.CCC(); x.AAA(); x.BBB(); x.CCC(); z.AAA(); z.BBB(); z.CCC(); Console.ReadKey(); } }
執(zhí)行，則無Warning了，通過這個實例，我們得知通過在基類添加Virtual關(guān)鍵字授權(quán)其子類可override基類同簽名方法的權(quán)限，方便了OOP的擴展。
/// <summary> /// ClassB, acting as a base class /// </summary> public class ClassB { public void AAA() { Console.WriteLine("ClassB AAA"); } public virtual void BBB() { Console.WriteLine("ClassB BBB"); } public virtual void CCC() { Console.WriteLine("ClassB CCC"); } } /// <summary> /// Class A, acting as a derived class /// </summary> public class ClassA : ClassB { public virtual void AAA() { Console.WriteLine("ClassA AAA"); } public new void BBB() { Console.WriteLine("ClassA BBB"); } public override void CCC() { Console.WriteLine("ClassA CCC"); } } /// <summary> /// Class C, acting as a derived class /// </summary> public class ClassC : ClassA { public override void AAA() { Console.WriteLine("ClassC AAA"); } public void CCC() { Console.WriteLine("ClassC CCC"); } } /// <summary> /// Program: used to execute the method. /// Contains Main method. /// </summary> public class Program { private static void Main(string[] args) { ClassB y = new ClassA(); ClassB x = new ClassC(); ClassA z = new ClassC(); y.AAA(); y.BBB(); y.CCC(); x.AAA(); x.BBB(); x.CCC(); z.AAA(); z.BBB(); z.CCC(); Console.ReadKey(); } }
運行結(jié)果：
ClassB AAA
ClassB BBB
ClassA CCC
ClassB AAA
ClassB BBB
ClassA CCC
ClassC AAA
ClassA BBB
ClassA CCC
如果基類聲明了virtual 關(guān)鍵字，子類可使用override修飾符實現(xiàn)運行時多態(tài)：只有在編譯器動態(tài)決定是否被調(diào)用。
如果未標明virtual或非virtual，則方法是否被調(diào)用在編譯期就能決定。
再看看下面的例子：
internal class A { public virtual void X() { } } internal class B : A { public new void X() { } } internal class C : B { public override void X() { } }
F5運行，結(jié)果報錯了：
Error: 'InheritanceAndPolymorphism.C.X()': cannot override inherited member 'InheritanceAndPolymorphism.B.X()' because it is not marked virtual, abstract, or override
錯誤的原因是A中定義了virtual的X函數(shù)，在B中用new關(guān)鍵字隱藏了A中的X函數(shù)。當C嘗試通過override關(guān)鍵字的時候，是獲得不了A中的virtual關(guān)鍵字X函數(shù)的，既在C中X函數(shù)為非Virtual的，故不能override。
internal class A { public virtual void X() { Console.WriteLine("Class: A ; Method X"); } } internal class B : A { public new virtual void X() { Console.WriteLine("Class: B ; Method X"); } } internal class C : B { public override void X() { Console.WriteLine("Class: C ; Method X"); } } /// <summary> /// Program: used to execute the method. /// Contains Main method. /// </summary> public class Program { private static void Main(string[] args) { A a = new C(); a.X(); B b = new C(); b.X(); Console.ReadKey(); } }
執(zhí)行結(jié)果如下：
Class: A ; Method XClass: C ; Method X
在這里，我們通過在B類中添加new Virtual修飾符，然后在C中即可使用B中Virtual的X函數(shù)了。
在上面繼承上，在運行時多態(tài)中添加第四個類：ClassD。
/// <summary> /// Class A /// </summary> public class ClassA { public virtual void XXX() { Console.WriteLine("ClassA XXX"); } } /// <summary> /// ClassB /// </summary> public class ClassB:ClassA { public override void XXX() { Console.WriteLine("ClassB XXX"); } } /// <summary> /// Class C /// </summary> public class ClassC : ClassB { public virtual new void XXX() { Console.WriteLine("ClassC XXX"); } } /// <summary> /// Class D /// </summary> public class ClassD : ClassC { public override void XXX() { Console.WriteLine("ClassD XXX"); } } /// <summary> /// Program: used to execute the method. /// Contains Main method. /// </summary> public class Program { private static void Main(string[] args) { ClassA a = new ClassD(); ClassB b = new ClassD(); ClassC c=new ClassD(); ClassD d=new ClassD(); a.XXX(); b.XXX(); c.XXX(); d.XXX(); Console.ReadKey(); } }
執(zhí)行結(jié)果如下：
ClassB XXX
ClassB XXX
ClassD XXX
ClassD XXX
第一行輸出中，來自a.XXX（）函數(shù) , 我們在 ClassA中定義了XXX函數(shù)，然后在ClassB中使用new關(guān)鍵字切斷了virtual關(guān)系--對子類而言。因此XXX函數(shù)從ClassC開始成為新的virtual函數(shù)，在這個代碼中a是ClassD的實例,但是聲明的為ClassA，故從下往上找，找到ClassB的XXX函數(shù)，打印并輸出結(jié)果。
/// <summary> /// Class A /// </summary> public class ClassA { public virtual void XXX() { Console.WriteLine("ClassA XXX"); } } /// <summary> /// ClassB /// </summary> public class ClassB:ClassA { public override void XXX() { ((ClassA)this).XXX(); Console.WriteLine("ClassB XXX"); } } /// <summary> /// Program: used to execute the method. /// Contains Main method. /// </summary> public class Program { private static void Main(string[] args) { ClassA a = new ClassB(); a.XXX(); } }
運行報錯：
Error: {Cannot evaluate expression because the current thread is in a stack overflow state.}
在這個例子中，((ClassA)this).XXX(); 導(dǎo)致了循環(huán)調(diào)用，修改為base.XXX即可修復(fù)這個強轉(zhuǎn)導(dǎo)致的循環(huán)調(diào)用。
在C#中，子類對象可賦值給一個基類對象；相反需要強轉(zhuǎn)。
override關(guān)鍵字用于子類重寫同簽名的基類virtual函數(shù)
用new和override可重寫基類virtual的同簽名函數(shù)
virtual修飾符的函數(shù)，只能在運行時決定是否被執(zhí)行
函數(shù)未用virtual修飾，則在編譯期即可決定是否被調(diào)用
譯文鏈接：http://www.survivalescaperooms.com/powertoolsteam/p/Diving-in-OOP-Day-Polymorphism-and-Inheritance-Dyn.html
原文鏈接：Diving in OOP (Day 3): Polymorphism and Inheritance (Dynamic Binding/Run Time Polymorphism)
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