c# – CIL – 拳击/拆箱与Nullable

void C<T>(object o) where T : struct {
    if (o is T t)
        Console.WriteLine($"Argument is {typeof(T)}: {t}");
}

编译成以下CIL

IL_0000: ldarg.0
IL_0001: isinst valuetype [mscorlib]System.Nullable`1<!!T>
IL_0006: unbox.any valuetype [mscorlib]System.Nullable`1<!!T>
IL_000b: stloc.1
IL_000c: ldloca.s 1
IL_000e: call instance !0 valuetype [mscorlib]System.Nullable`1<!!T>::GetValueOrDefault()
IL_0013: stloc.0
IL_0014: ldloca.s 1
IL_0016: call instance bool valuetype [mscorlib]System.Nullable`1<!!T>::get_HasValue()
IL_001b: brfalse.s IL_003c

IL_001d: ldstr "Argument is {0}: {1}"
IL_0022: ldtoken !!T
IL_0027: call class [mscorlib]System.Type [mscorlib]System.Type::GetTypeFromHandle(valuetype [mscorlib]System.RuntimeTypeHandle)
IL_002c: ldloc.0
IL_002d: box !!T
IL_0032: call string [mscorlib]System.String::Format(string,object,object)
IL_0037: call void [mscorlib]System.Console::WriteLine(string)

IL_003c: ret

但是下面的C#

void D<T>(object o) where T : struct {
    if (o is T)
        Console.WriteLine($"Argument is {typeof(T)}: {(T) o}");
}

编译成以下CIL

IL_0000: ldarg.0
IL_0001: isinst !!T
IL_0006: brfalse.s IL_002c

IL_0008: ldstr "Argument is {0}: {1}"
IL_000d: ldtoken !!T
IL_0012: call class [mscorlib]System.Type [mscorlib]System.Type::GetTypeFromHandle(valuetype [mscorlib]System.RuntimeTypeHandle)
IL_0017: ldarg.0
IL_0018: unbox.any !!T
IL_001d: box !!T
IL_0022: call string [mscorlib]System.String::Format(string,object)
IL_0027: call void [mscorlib]System.Console::WriteLine(string)

IL_002c: ret

我认为发生了什么：查看第一种方法的CIL,似乎(1)检查参数是否为[盒装？] Nullable< T>,如果是,则将其推入堆栈,否则为null,( 2)取消它(如果它为空？),(3)尝试获取其值,否则默认(T),(4)然后检查它是否有值,如果没有则分支出来.第二种方法的CIL足够简单,它只是试图取消打包参数.

如果两个代码的语义是等价的,为什么前一种情况涉及拆箱到Nullable< T>.而前一种情况“只是拆箱”？其次,在第一个CIL中,如果对象参数是一个盒装的int,我现在认为它正是它在tin上所说的(即盒装的int而不是盒装的Nullable< int>),不会isinst指令总是失败？ Nullable< T>即使在CIL级别也能得到特殊待遇？

更新：在手写一些MSIL之后,看起来该对象(如果它确实是一个盒装的int)可以解包到int或Nullable< int>中.

.method private static void Foo(object o) cil managed {
    .maxstack 1
    ldarg.0
    isinst int32
    brfalse.s L_00
    ldarg.0
    unbox.any int32
    call void [mscorlib]System.Console::WriteLine(int32)
L_00:
    ldarg.0
    isinst valuetype [mscorlib]System.Nullable`1<int32>
    brfalse.s L_01
    ldarg.0
    unbox valuetype [mscorlib]System.Nullable`1<int32>
    call instance !0 valuetype [mscorlib]System.Nullable`1<int32>::GetValueOrDefault()
    call void [mscorlib]System.Console::WriteLine(int32)
L_01:
    ldarg.0
    unbox valuetype [mscorlib]System.Nullable`1<int32>
    call instance bool valuetype [mscorlib]System.Nullable`1<int32>::get_HasValue()
    brtrue.s L_02
    ldstr "No value!"
    call void [mscorlib]System.Console::WriteLine(string)
L_02:
    ret
}