为什么 using(null) 在 C# 中是有效的情况？
有人可以向我解释为什么下面显示的代码在 C# 中有效并执行对 Console.WriteLine 的调用吗？
using (null)
{
Console.WriteLine ("something is here")
}
它编译成（最后显示块）。正如您所看到的，编译器决定不执行 Dispose() 方法并跳转到 endfinally 指令。
IL_0013: ldnull
IL_0014: ceq
IL_0016: stloc.1
IL_0017: ldloc.1
IL_0018: brtrue.s IL_0021 // branches here and decide not to execute Dispose()
IL_001a: ldnull
IL_001b: callvirt instance void [mscorlib]System.IDisposable::Dispose()
IL_0020: nop
IL_0021: endfinally
但是，如果我运行以下代码，它将失败并出现 NullReferenceException（这是预期的）：
((IDisposable)null).Dispose();
IL_0023: ldnull
IL_0024: callvirt instance void [mscorlib]System.IDisposable::Dispose()
为什么第一个版本可以编译？为什么编译器决定不执行Dispose()？是否存在其他情况下编译器可能决定不在 using 块中调用 Dispose() ？
Could someone please explain to me why the code shown below is valid in C# and executes the call to Console.WriteLine?
using (null)
{
Console.WriteLine ("something is here")
}
It compiles into (finally block is shown). As you can see compiler decides not to execute the Dispose() method and jumps to the endfinally instruction.
IL_0013: ldnull
IL_0014: ceq
IL_0016: stloc.1
IL_0017: ldloc.1
IL_0018: brtrue.s IL_0021 // branches here and decide not to execute Dispose()
IL_001a: ldnull
IL_001b: callvirt instance void [mscorlib]System.IDisposable::Dispose()
IL_0020: nop
IL_0021: endfinally
However, if I run the following code, it will fail with a NullReferenceException (which is expected):
((IDisposable)null).Dispose();
IL_0023: ldnull
IL_0024: callvirt instance void [mscorlib]System.IDisposable::Dispose()
Why does the first version compile? Why does the compiler decide not to execute Dispose()? Are there any other cases when compiler may decide not to call Dispose() in a using block?
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评论（3）
语言规范明确指出（8.13），如有必要，捕获的值将被测试为空，即
finally本质上是（对不可为空类型的警告）我经常使用它来发挥我的优势，因为可能为空，但如果不是：需要处置。事实上，这是一个有用的场景（手动枚举
IEnumerable）：因为
IEnumerator的非泛型版本不一定是IDisposable，但是当是的，应该处理掉。The language spec explicitly states (8.13) that the captured value is tested for null if necessary, i.e. the
finallyis essentially (with caveats around non-nullable types)I frequently use this to my advantage, for things that might be null, but when they aren't: need disposing. In fact, here's a useful scenario (manually enumerating
IEnumerable):as the non-generic version of
IEnumeratorisn't necessarilyIDisposable, but when it is, should be disposed.我认为这是
using(some_expression)更一般情况的自然结果，其中some_expression允许计算为null。需要制定一项特殊规则来区分这种情况与更一般的情况。
I think it's a natural outcome of the more general case of
using(some_expression), wheresome_expressionis allowed to evaluate tonull.It would have required a special rule to distinguish this case from the more general one.
如果对象为空，它将忽略 - http://msdn.microsoft.com/ en-us/library/yh598w02.aspx
It will ignore if the Object is null - http://msdn.microsoft.com/en-us/library/yh598w02.aspx