在文档的解释和示例之后(代码很长,但很容易理解,直接从文档中复制粘贴),我发现了以下内容:
using System;
public class IdInfo
{
public int IdNumber;
public IdInfo(int IdNumber)
{
this.IdNumber = IdNumber;
}
}
public class Person
{
public int Age;
public string Name;
public IdInfo IdInfo;
public Person ShallowCopy()
{
return (Person) this.MemberwiseClone();
}
public Person DeepCopy()
{
Person other = (Person) this.MemberwiseClone();
other.IdInfo = new IdInfo(IdInfo.IdNumber);
other.Name = String.Copy(Name);
return other;
}
}
public class Example
{
public static void Main()
{
// Create an instance of Person and assign values to its fields.
Person p1 = new Person();
p1.Age = 42;
p1.Name = "Sam";
p1.IdInfo = new IdInfo(6565);
// Perform a shallow copy of p1 and assign it to p2.
Person p2 = p1.ShallowCopy();
// Display values of p1, p2
Console.WriteLine("Original values of p1 and p2:");
Console.WriteLine(" p1 instance values: ");
DisplayValues(p1);
Console.WriteLine(" p2 instance values:");
DisplayValues(p2);
// Change the value of p1 properties and display the values of p1 and p2.
p1.Age = 32;
p1.Name = "Frank";
p1.IdInfo.IdNumber = 7878;
Console.WriteLine("nValues of p1 and p2 after changes to p1:");
Console.WriteLine(" p1 instance values: ");
DisplayValues(p1);
Console.WriteLine(" p2 instance values:");
DisplayValues(p2);
// Make a deep copy of p1 and assign it to p3.
Person p3 = p1.DeepCopy();
// Change the members of the p1 class to new values to show the deep copy.
p1.Name = "George";
p1.Age = 39;
p1.IdInfo.IdNumber = 8641;
Console.WriteLine("nValues of p1 and p3 after changes to p1:");
Console.WriteLine(" p1 instance values: ");
DisplayValues(p1);
Console.WriteLine(" p3 instance values:");
DisplayValues(p3);
}
public static void DisplayValues(Person p)
{
Console.WriteLine(" Name: {0:s}, Age: {1:d}", p.Name, p.Age);
Console.WriteLine(" Value: {0:d}", p.IdInfo.IdNumber);
}
}
在第二个输出中,我期望p2.Name
是Frank,因为根据文档string
是一个引用类型,因此应该受到原始对象名称更改的影响。
// The example displays the following output:
// Original values of p1 and p2:
// p1 instance values:
// Name: Sam, Age: 42
// Value: 6565
// p2 instance values:
// Name: Sam, Age: 42
// Value: 6565
//
// Values of p1 and p2 after changes to p1:
// p1 instance values:
// Name: Frank, Age: 32
// Value: 7878
// p2 instance values:
// Name: Sam, Age: 42 ---> EXPECTED FRANK HERE
// Value: 7878
//
// Values of p1 and p3 after changes to p1:
// p1 instance values:
// Name: George, Age: 39
// Value: 8641
// p3 instance values:
// Name: Frank, Age: 32
// Value: 7878
实际上这两个字段都是在DeepCopy方法中处理的:
other.IdInfo = new IdInfo(IdInfo.IdNumber);
other.Name = String.Copy(Name);
如果两者都是引用类型,为什么IdInfo.Value
会受到原始对象变化的影响,而Name
则不会?
此处不依赖于string是引用类型还是值类型。您创建了Person
的克隆——此时,p1
和p2
都将.Name
指向同一个string
对象。然后在p1
上更改.Name
。重要的是:*这根本不影响字符串-它只是改变p1
指向一个完全不同的字符串。这就是这里发生的一切。
因为字符串是外部不可变的,你几乎总是改变值来指向完全不同的字符串;你(几乎)从来没有真正改变一个存在的字符串,所以:无论它是引用类型还是值类型都很少相关,除了在理解内存使用方面。
出于同样的原因:真的没有必要调用string.Copy
,string.Copy
可以被实现为return value;
(它不是,虽然-我检查过)