Detailed analysis of the usage of new in C# and the difference between it and override

This article mainly introduces the usage of new in C# and the difference with override. Friends in need can refer to the usage of new in

C# There are three types:

(1) new is the operator , used to create objects and call constructors . For example, Class1=new Class1(); you can also call the default constructor for the value type, such as int a=new int(); at this time a=0.

(2)new Is a modifier used to hide inherited members of base class members. Override cannot hide inherited members of accumulated members. For example:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace A
{
  
  class Program
  {
    abstract public class Test
    {      
      public virtual void Prinf() 
      {
        Console.WriteLine("Abstract Printf...");
      }
    }
    public class Class1 : Test
    {
      
      public override void Prinf() 
      {

        Console.WriteLine("Class One Override Printf...");
      }
       
    }

    public class Class2 : Test
    {
      public new void Prinf()
      {

        Console.WriteLine("Class Two New Printf...");
      }
    }
 
    
    static void Main(string[] args)
    {

      Class1 cls1 = new Class1();
      Class2 cls2 = new Class2();
      cls1.Prinf();
      cls2.Prinf();
      ((Test)cls1).Prinf();
      ((Test)cls2).Prinf();

      Console.ReadLine();    

    }
  }
}

The running results show that: the new modifier hides the overridden method, and calls the base class method by converting the derived object into a base class object. Override completely rewrites the method. Even if it is converted to a base class object, the overridden method of the derived class is also called.

That is: new hidden base class method, override rewrite base class method.

(3)new is constraint. Used to add type constraints to generics.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace A
{
  class Test<T>
    where T : new()//定义类型T的约束，表示T类型必须有不带参数的构造函数
  {
    public T GetItem()
    {
      return new T();//如果不添加new()约束，编译错误：变量类型“T”没有 new() 约束，因此无法创建该类型的实例
              //想一下，T类型不知道，编译器不知道分配多大的空间，所以会通过反射技术实现
    }
  }
  class TClass
  {
    private int a;
    
    public TClass() //如果不添加无参构造函数，编译错误：TClass必须是具有公共的无参数构造函数的非抽象类型，才能用作泛型类型或方法“A.Test<T>”中的参数“T”
    {

    }
    public TClass(int a)
    {
      this.a = a;
    }
  }

  class Program
  {
  
    static void Main(string[] args)
    {
      Test<TClass> test = new Test<TClass>();

      Console.ReadLine();    

    }
  }
}

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