C# 语言参考
sealed(C# 参考)
当对一个类应用 sealed 修饰符时,此修饰符会阻止其他类从该类继承。在下面的示例中,类 B 从类 A 继承,但是任何类都不能从类 B 继承。
1class A {}
2sealed class B : A {}
还可以在重写基类中的虚方法或虚属性的方法或属性上使用 sealed 修饰符。这将使您能够允许类从您的类继承,并防止它们重写特定的虚方法或虚属性。在下面的示例中,C 从 B 继承,但 C 无法重写在 A 中声明并在 B 中密封的虚函数 F。
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class A
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{
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protected virtual void F() { Console.WriteLine("A.F");}
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protected virtual void F2() { Console.WriteLine("A.F2");}
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}
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class B : A
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{
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sealed protected override void F() { Console.WriteLine("B.F");}
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protected override void F2() {Console.WriteLine("A.F3");}
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}
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class C : B
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{
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// Attempting to override F causes compiler error CS0239.
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// protected override void F() { Console.WriteLine("C.F"); }
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// Overriding F2 is allowed.
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protected override void F2() { Console.WriteLine("C.F2"); }
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}
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说明: |
---|
当在类中定义新的方法或属性时,通过不将这些方法或属性声明为 virtual,可防止派生类重写这些方法或属性。 |
重点:
1. 将 abstract 修饰符用于密封类是错误的做法,因为抽象类必须由提供抽象方法或属性的实现的类继承。
2 .当应用于方法或属性时,sealed 修饰符必须始终与 override 一起使用。
关于这一点见下面代码:
public class ClassA
{
public ClassA()
{ }
public sealed void MethodA()
{
Console.WriteLine("Return A");
}
public void MethodB()
{
Console.WriteLine("Return B");
}
}
{
public ClassA()
{ }
public sealed void MethodA()
{
Console.WriteLine("Return A");
}
public void MethodB()
{
Console.WriteLine("Return B");
}
}
以上程序,将产生如下错误信息:
'ClassA.MethodA()' cannot be sealed because it is not an override
3.由于结构是隐式密封的,因此它们不能被继承。
有关更多信息,请参见(C# 编程指南)。
示例:
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// cs_sealed_keyword.cs
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using System;
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sealed class SealedClass
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{
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public int x;
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public int y;
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}
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class MainClass
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{
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static void Main()
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{
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SealedClass sc = new SealedClass();
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sc.x = 110;
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sc.y = 150;
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Console.WriteLine("x = {0}, y = {1}", sc.x, sc.y);
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}
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}
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输出