为什么要整零不等于长期零？ [英] Why integer zero does not equal long zero?

问题描述

一个奇怪的一块code我刚刚发现在C＃中（也应该适用于使用其他CLI语言.NET的结构）。

A strange piece of code I've just discovered in C# (should also be true for other CLI languages using .NET's structs).

using System;

public class Program
{
    public static void Main(string[] args)
    {
    int a;
    long b;

    a = 0;
    b = 0;

    Console.WriteLine(a.Equals(b)); // False
    Console.WriteLine(a.Equals(0L)); // False
    Console.WriteLine(a.Equals((long)0)); // False
    Console.WriteLine(a.Equals(0)); // True
    Console.WriteLine(a.Equals(a)); // True
    Console.WriteLine(a == b); // True
    Console.WriteLine(a == 0L); // True

    Console.WriteLine();

    Console.WriteLine(b.Equals(a)); // True
    Console.WriteLine(b.Equals(0)); // True
    Console.WriteLine(b.Equals((int)0)); // True
    Console.WriteLine(b.Equals(b)); // True
    Console.WriteLine(b == a); // True
    Console.WriteLine(b == 0); // True
    }
}

下面两个有趣的点（假定 A 是 INT 和 B 是长）：

Two interesting points here (assuming that a is int and b is long):

1. A = B ，而 B == A ;！
2. （a.Equals（B））！=（A == B）

1. a != b, but b == a;
2. (a.Equals(b)) != (a == b)

是否有任何理由，比较了这种方式实现？

Is there any reason why comparison was implemented this way?

注：.NET 4中使用，如果这有什么差别

Note: .NET 4 was used if it makes any difference.

推荐答案

在一般情况下，等于（）方法不应该返回true，针对不同类型的对象。

In general, Equals() methods are not supposed to return true for objects of different types.

a.Equals（B）要求 int.Equals（对象），只能返回true盒装的Int32 S：

a.Equals(b) calls int.Equals(object), which can only return true for boxed Int32s:

public override bool Equals(Object obj) { 
    if (!(obj is Int32)) {
        return false;
    }
    return m_value == ((Int32)obj).m_value; 
}  

b.Equals（一）要求 long.Equals（长）后隐含转换 INT 到长。
因此，它比较两个长取值直接，返回真。

b.Equals(a) calls long.Equals(long) after implicitly converting the int to a long.
It therefore compares the two longs directly, returning true.

要了解更清楚，看看这个简单的例子中产生的IL（它打印真假真）：

To understand more clearly, look at the IL generated by this simpler example (which prints True False True):

int a = 0;
long b = 0L;

Console.WriteLine(a == b);
Console.WriteLine(a.Equals(b));
Console.WriteLine(b.Equals(a));

---

IL_0000:  ldc.i4.0    
IL_0001:  stloc.0     
IL_0002:  ldc.i4.0    
IL_0003:  conv.i8     
IL_0004:  stloc.1     

IL_0005:  ldloc.0     //Load a
IL_0006:  conv.i8     //Cast to long
IL_0007:  ldloc.1     //Load b
IL_0008:  ceq         //Native long equality check
IL_000A:  call        System.Console.WriteLine    //True

IL_000F:  ldloca.s    00            //Load the address of a to call a method on it
IL_0011:  ldloc.1                   //Load b
IL_0012:  box         System.Int64  //Box b to an Int64 Reference
IL_0017:  call        System.Int32.Equals
IL_001C:  call        System.Console.WriteLine    //False

IL_0021:  ldloca.s    01  //Load the address of b to call a method on it
IL_0023:  ldloc.0         //Load a
IL_0024:  conv.i8         //Convert a to Int64
IL_0025:  call        System.Int64.Equals
IL_002A:  call        System.Console.WriteLine    //True

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