值类型的引用相等 [英] Reference equality of value types

问题描述

我已经做了一些 ref 关键字测试，有一件事我无法理解:

I have made some ref keyword tests and there is one thing I can't understand:

static void Test(ref int a, ref int b)
{
    Console.WriteLine(Int32.ReferenceEquals(a,b));
}

static void Main(string[] args)
{
    int a = 4;
    Test(ref a, ref a);
    Console.ReadLine();
}

为什么此代码显示 False ?我知道 int 是一个值类型，但是在这里它应该将引用传递给同一对象.

Why does this code display False? I know that int is a value type but here it should pass references to the same object.

推荐答案

为什么此代码显示 False ?

因为 int a 和 int b

Because int a and int b are being boxed when you call object.ReferenceEquals. Each integer is boxed inside an object instance. Thus, you are actually comparing references between two boxed values, which clearly aren't equal.

如果查看为该方法生成的 CIL ，您可以轻松地看到此内容:

You can easily see this if you look at the generated CIL for the method:

Test:
IL_0000:  nop
IL_0001:  ldarg.0     Load argument a
IL_0002:  ldind.i4
IL_0003:  box         System.Int32
IL_0008:  ldarg.1     Load argument b
IL_0009:  ldind.i4
IL_000A:  box         System.Int32
IL_000F:  call        System.Object.ReferenceEquals
IL_0014:  call        System.Console.WriteLine
IL_0019:  nop
IL_001A:  ret

可以使用可验证的CIL(例如，在 @leppie的答案中)来检查存储位置是否相等，或者通过以下方式来实现:不安全代码:

Checking for storage location equality can be achieved either by using verifiable CIL (such as in @leppie's answer) or by unsafe code:

unsafe static void Main(string[] args)
{
    int a = 4;
    int b = 5;
    Console.WriteLine(Test(ref a, ref a)); // True
    Console.WriteLine(Test(ref a, ref b)); // False;
}

unsafe static bool Test(ref int a, ref int b)
{
    fixed (int* refA = &a)
    fixed (int* refB = &b)
    {
        return refA == refB;
    }
}

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