c# - 在 C# 中使用 new 关键字

Question

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Answer 1

new 关键字让代码阅读者知道此方法将隐藏其基类中具有相同名称的方法。即使您忽略了它，编译器也会以同样的方式对待它，但会警告您。

由于您将该方法声明为virtual，因此您在编译时会收到此警告:

'B.Metod1()' hides inherited member 'A.Metod1()'. To make the current member override that implementation, add the override keyword. Otherwise add the new keyword.

但是，如果您通过删除 virtual 来更改声明:

• 在A

  中没有virtual 的声明

  class A {
      public void Metod1() {
          Console.WriteLine("A.Method1");
      }
  }
  
  class B: A {
      public void Metod1() {
          Console.WriteLine("B.Method1");
      }
  }
  

仍会编译，但警告消息将是:

'B.Metod1()' hides inherited member 'A.Metod1()'. Use the new keyword if hiding was intended.

以上述任一方式，如下:

• 测试代码

  var b=new B();
  b.Metod1();
  (b as A).Metod1();
  

将输出:

B.Method1
A.Method1

But, if you declare as the following:

• Declaration with override in B

  class A {
      public virtual void Metod1() {
          Console.WriteLine("A.Method1");
      }
  }
  
  class B: A {
      public override void Metod1() {
          Console.WriteLine("B.Method1");
      }
  }
  

那么输出将是:

B.Method1
B.Method1

That is because Method1 of the base class is not just hidden but overridden by the derived class.

A case that you cannot just use new is that if Method1 is a abstract method in a abstract class:

• Code that must use override in derived classes

  abstract class A {
      public abstract void Metod1();
  }
  
  class B: A {
      public override void Metod1() { // use `new` instead of `override` would not compile
          Console.WriteLine("B.Method1");
      }
  }
  

在这种情况下，您不能只使用 new，因为 A 需要派生类来覆盖 Method1。