== 使用反射的运算符 [英] == operator using reflection

问题描述

使用反射后的对象比较

var a = new A
   {
      a = "aa",
      b = 1
   };

   var b = new A { 
      a = "aa",
      b = 2
   };

   Type type = typeof(A);

   object old = type.GetProperty("a").GetValue(a);
   object Oldold = type.GetProperty("a").GetValue(b);

   int one = 1;
   int oneOne = 1;

   object oneO = one;
   object oneOneO = oneOne


   // old == Oldold  - true
   // one == oneOne - true
   // oneO == oneOneO - false
}

我希望 oneO == oneOneO 是真的.有人可以解释一下这里发生了什么吗?

I would expect that oneO == oneOneO is true. Can someone explain me what is going on here?

推荐答案

您是 装箱 int 通过将其分配给 Object 变量.这会创建一个新实例，并且 System.Object 中的 == 比较引用，它们是不同的，所以它返回 false.

You are boxing the int by assigning it to an Object variable. That creates a new instance and == in System.Object compares references, they are different, so it returns false.

如果您将其转换回(拆箱)为 int，== 将按预期工作:

If you would cast it back(unbox) to an int the == would work as expected:

object oneO = one;
object oneOneO = oneOne;
int newOne = (int) oneO;
int newOneOne = (int) oneOneO;
Console.WriteLine(newOne == newOneOne); // true

如果您使用 Equals 而不是 ==，它们也会按预期进行比较，因为 System.Int32 覆盖Equals 有意义.

If you would use Equals instead of == they would also be compared as expected because System.Int32 overrides Equals meaningfully.

• old 和 Oldold 是没有装箱的引用类型(字符串)，只有值类型
• 但是字符串是一种特殊的引用类型，它重载了相等运算符(进一步阅读)

• old and Oldold are reference types(string) which are not boxed, only value types
• But string is a special reference type which overloads the equality operator(read further)

根据经验:如果您使用引用类型，请小心使用 == 运算符.System.String 重载相等运算符 例如.但这是一个例外.通常你只是比较参考.装箱基本上使值类型成为引用类型，但这是一个隐藏的实现细节(即使 oneO.GetType().IsValueType 仍然返回 true).

As a rule of thumb: if you use reference types be careful with the == operator. System.String overloads the equality operator for example. But that is an exception. Normally you are just comparing references. Boxing basically makes the value type a reference type, but that's a hidden implementation detail(even oneO.GetType().IsValueType still returns true).

另请注意，如果您有这样的方法，也会发生这种装箱转换:

Also note that this boxing conversion also takes place if you have a method like this:

public static bool SameThings(object obj1, object obj2)
{
    return obj1 == obj2;
}

我希望你不会再对输出 false 感到惊讶:

I hope you are not anymore surprised that this outputs false:

Console.WriteLine(SameThings(1, 1));  // false

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