[英]Unexpected behavior in c# generic method on .Equals
Why does the Equals method return a different result from within the generic method? 为什么Equals方法从泛型方法中返回不同的结果? I think that there's some automatic boxing here that I don't understand.
我认为这里有一些我不明白的自动拳击。
Here's an example that reproduces the behavior with .net 3.5 or 4.0: 这是一个使用.net 3.5或4.0重现行为的示例:
static void Main(string[] args)
{
TimeZoneInfo tzOne = TimeZoneInfo.Local;
TimeZoneInfo tzTwo = TimeZoneInfo.FindSystemTimeZoneById(tzOne.StandardName);
Console.WriteLine(Compare(tzOne, tzTwo));
Console.WriteLine(tzOne.Equals(tzTwo));
}
private static Boolean Compare<T>(T x, T y)
{
if (x != null)
{
return x.Equals(y);
}
return y == null;
}
Output: 输出:
False
True
Edit: This code works as desired without many compromises: 编辑:此代码按预期工作,没有多少妥协:
private static Boolean Compare<T>(T x, T y)
{
if (x != null)
{
if (x is IEquatable<T>)
{
return (x as IEquatable<T>).Equals(y);
}
return x.Equals(y);
}
return y == null;
}
Followup : I filed a bug via MS Connect and it has been resolved as fixed, so it's possible this will be fixed in the next version of the .net framework. 跟进 :我通过MS Connect提交了一个错误 ,它已被解决为已修复,因此可能会在下一版本的.net框架中修复。 I'll update with more details if they become available.
如果可用,我会更新更多细节。
PS : This appears to be fixed in .net 4.0 and later (by looking at the disassembly of TimeZoneInfo in mscorlib). PS :这似乎是在.net 4.0及更高版本中修复的(通过查看mscorlib中TimeZoneInfo的反汇编)。
TimeZoneInfo does not override the Object Equals method, so it calls the default Object Equals, which apparently does not work as expected. TimeZoneInfo不会覆盖Object Equals方法,因此它调用默认的Object Equals,这显然不能按预期工作。 I would consider this a bug in TimeZoneInfo.
我会认为这是TimeZoneInfo中的一个错误。 This should work:
这应该工作:
private static Boolean Compare<T>(T x, T y)
where T: IEquatable<T>
{
if (x != null)
{
return x.Equals(y);
}
return false;
}
The above will cause it to call Equals<T>
, which is the method you were calling above (it implicitly preferred the generic call because it was more specific to the parameter type than the Object Equals; inside the generic method, however, it had no way to be sure that such a generic Equals existed, since there was no constraint guaranteeing this). 以上将导致它调用
Equals<T>
,这是你上面调用的方法(它隐含地优先使用泛型调用,因为它比参数类型更具体于参数类型;但是在泛型方法中,它有无法确定这样的通用Equals是否存在,因为没有约束保证这一点)。
FWIW, on mono 2.8+ both return values are False, outputting FWIW,在单声道2.8+上,两个返回值均为False,输出
False
False
Amazingly, csc.exe from VS2010 produces different results, indeed, outputting: 令人惊讶的是,来自VS2010的csc.exe产生了不同的结果,实际上,输出:
False
True
Even more interestingly, the problem appears not with the generated IL code , but with the Framework implementation/JIT engine; 更有趣的是,问题不在于生成的IL代码 ,而在于Framework实现/ JIT引擎;
False/False
, like the mono compiled version False/False
,就像单声道编译版本一样 False/True
, like the MS compiled version False/True
,就像MS编译版本一样 For your interest, here are the disassemblies of Microsoft's CSC.exe compiler ( csc.exe /optimize+ test.cs
): 为了您的兴趣,以下是Microsoft的CSC.exe编译器(
csc.exe /optimize+ test.cs
)的反汇编:
.method private static hidebysig
default bool Compare<T> (!!T x, !!T y) cil managed
{
// Method begins at RVA 0x2087
// Code size 30 (0x1e)
.maxstack 8
IL_0000: ldarg.0
IL_0001: box !!0
IL_0006: brfalse.s IL_001c
IL_0008: ldarga.s 0
IL_000a: ldarg.1
IL_000b: box !!0
IL_0010: constrained. !!0
IL_0016: callvirt instance bool object::Equals(object)
IL_001b: ret
IL_001c: ldc.i4.0
IL_001d: ret
} // end of method Program::Compare
and Mono's gmcs.exe compiler ( dmcs -optimize+ test.cs
): 和Mono的gmcs.exe编译器(
dmcs -optimize+ test.cs
):
.method private static hidebysig
default bool Compare<T> (!!T x, !!T y) cil managed
{
// Method begins at RVA 0x212c
// Code size 33 (0x21)
.maxstack 4
IL_0000: ldarg.0
IL_0001: box !!0
IL_0006: brfalse IL_001f
IL_000b: ldarga.s 0
IL_000d: ldarg.1
IL_000e: box !!0
IL_0013: constrained. !!0
IL_0019: callvirt instance bool object::Equals(object)
IL_001e: ret
IL_001f: ldc.i4.0
IL_0020: ret
} // end of method Program::Compare
TimezoneInfo defines it's own overload of Equals(TimeZoneInfo) . TimezoneInfo定义它自己的Equals重载(TimeZoneInfo) 。 In the Compare method, the object equals is used (it's a virtual method call of Object.Equals), whereas in Console.WriteLine(tzOne.Equals(tzTwo)) the overloaded (new) TimeZoneInfo.Equals method is called.
在Compare方法中,使用对象equals(它是Object.Equals的虚方法调用),而在Console.WriteLine(tzOne.Equals(tzTwo)中)调用重载(新)TimeZoneInfo.Equals方法。
TimeZoneInfo obviously hasn't overridden the Object.Equals method correctly... TimeZoneInfo显然没有正确覆盖Object.Equals方法......
声明:本站的技术帖子网页,遵循CC BY-SA 4.0协议,如果您需要转载,请注明本站网址或者原文地址。任何问题请咨询:yoyou2525@163.com.