我试图查看这个旧问题中发布的代码的反汇编情况,并发现了一些奇怪的东西。
为了清晰起见,这里是源代码:
class ThreadTest
{
static void Main(string[] args)
{
for (int i = 0; i < 10; i++)
new Thread(() => Console.WriteLine(i)).Start();
}
}
当然,这个程序的行为是出乎意料的,但这不是问题所在。
以下是我查看反汇编代码时看到的内容:
internal class ThreadTest
{
private static void Main(string[] args)
{
int i;
int j;
for (i = 0; i < 10; i = j + 1)
{
new Thread(delegate
{
Console.WriteLine(i);
}).Start();
j = i;
}
}
}
这里“j”是做什么用的?以下是字节码:
.method private hidebysig static
void Main (
string[] args
) cil managed
{
// Method begins at RVA 0x2050
// Code size 64 (0x40)
.maxstack 2
.entrypoint
.locals init (
[0] class ConsoleApplication2.ThreadTest/'<>c__DisplayClass0_0' 'CS$<>8__locals0',
[1] int32
)
IL_0000: newobj instance void ConsoleApplication2.ThreadTest/'<>c__DisplayClass0_0'::.ctor()
IL_0005: stloc.0
IL_0006: ldloc.0
IL_0007: ldc.i4.0
IL_0008: stfld int32 ConsoleApplication2.ThreadTest/'<>c__DisplayClass0_0'::i
IL_000d: br.s IL_0035
// loop start (head: IL_0035)
IL_000f: ldloc.0
IL_0010: ldftn instance void ConsoleApplication2.ThreadTest/'<>c__DisplayClass0_0'::'<Main>b__0'()
IL_0016: newobj instance void [mscorlib]System.Threading.ThreadStart::.ctor(object, native int)
IL_001b: newobj instance void [mscorlib]System.Threading.Thread::.ctor(class [mscorlib]System.Threading.ThreadStart)
IL_0020: call instance void [mscorlib]System.Threading.Thread::Start()
IL_0025: ldloc.0
IL_0026: ldfld int32 ConsoleApplication2.ThreadTest/'<>c__DisplayClass0_0'::i
IL_002b: ldc.i4.1
IL_002c: add
IL_002d: stloc.1
IL_002e: ldloc.0
IL_002f: ldloc.1
IL_0030: stfld int32 ConsoleApplication2.ThreadTest/'<>c__DisplayClass0_0'::i
IL_0035: ldloc.0
IL_0036: ldfld int32 ConsoleApplication2.ThreadTest/'<>c__DisplayClass0_0'::i
IL_003b: ldc.i4.s 10
IL_003d: blt.s IL_000f
// end loop
IL_003f: ret
} // end of method ThreadTest::Main
但是最奇怪的事情是,如果我像这样更改原始代码,用i = i + 1
替换i++
:
class ThreadTest
{
static void Main(string[] args)
{
for (int i = 0; i < 10; i = i + 1)
new Thread(() => Console.WriteLine(i)).Start();
}
}
我知道您需要的是什么:
internal class ThreadTest
{
private static void Main(string[] args)
{
int i;
for (i = 0; i < 10; i++)
{
new Thread(delegate
{
Console.WriteLine(i);
}).Start();
}
}
}
这正是我所期望的。
以下是字节码:
.method private hidebysig static
void Main (
string[] args
) cil managed
{
// Method begins at RVA 0x2050
// Code size 62 (0x3e)
.maxstack 3
.entrypoint
.locals init (
[0] class ConsoleApplication2.ThreadTest/'<>c__DisplayClass0_0' 'CS$<>8__locals0'
)
IL_0000: newobj instance void ConsoleApplication2.ThreadTest/'<>c__DisplayClass0_0'::.ctor()
IL_0005: stloc.0
IL_0006: ldloc.0
IL_0007: ldc.i4.0
IL_0008: stfld int32 ConsoleApplication2.ThreadTest/'<>c__DisplayClass0_0'::i
IL_000d: br.s IL_0033
// loop start (head: IL_0033)
IL_000f: ldloc.0
IL_0010: ldftn instance void ConsoleApplication2.ThreadTest/'<>c__DisplayClass0_0'::'<Main>b__0'()
IL_0016: newobj instance void [mscorlib]System.Threading.ThreadStart::.ctor(object, native int)
IL_001b: newobj instance void [mscorlib]System.Threading.Thread::.ctor(class [mscorlib]System.Threading.ThreadStart)
IL_0020: call instance void [mscorlib]System.Threading.Thread::Start()
IL_0025: ldloc.0
IL_0026: ldloc.0
IL_0027: ldfld int32 ConsoleApplication2.ThreadTest/'<>c__DisplayClass0_0'::i
IL_002c: ldc.i4.1
IL_002d: add
IL_002e: stfld int32 ConsoleApplication2.ThreadTest/'<>c__DisplayClass0_0'::i
IL_0033: ldloc.0
IL_0034: ldfld int32 ConsoleApplication2.ThreadTest/'<>c__DisplayClass0_0'::i
IL_0039: ldc.i4.s 10
IL_003b: blt.s IL_000f
// end loop
IL_003d: ret
} // end of method ThreadTest::Main
为什么编译器在第一个场景中添加了j
?
注意:我正在使用VS 2015 Update 3,.NET Framework 4.5.2,在发布模式下编译。
for (i = 0; i < 10; ++i)
做同样的事情。 - wake-0++i
而不是i++
,看看会发生什么。++i
很像i = i + 1
,而i++
被定义为增加i
但返回它在增加之前的值。当然,由于您正在丢弃i++
或i = i + 1
的结果,除了额外生成的IL代码(可能会被JIT编译器删除)外,这没有任何区别,但如果您正在使用此表达式的结果,则它们将是两个不同的事物。 - Michael Gearyi++
解释为后置自增运算符,因此生成的代码会在递增之前保存值 - 即使该值最终将被丢弃。它留给 JIT 编译器来清理和优化它。另一方面,你可能是对的,ILSpy 可能正在做一些有趣的事情!直接查看字节码而不是反编译将揭示真正发生的事情。 - Michael Geary