【原创】Using and IL

Abstract: 通过查看IL来了解下声明在using语句中的对象是在try语句外面实例化还是在try里面实例化。

举例来说，下面的c# code

using(FileStream aFs = new FileStream(@"c:\test.txt", FileMode.Open))
{
    // Do noting...
}

是对应下面的那种情况呢？

(1)

FileStream aFs = null;
try
{
   aFs = new FileStream(@"c:\test.txt", FileMode.Open);
}
finally
{
   if (aFs != null)
   {
       aFs.Close();
   }
}

(2)

FileStream aFs = new FileStream(@"c:\test.txt", FileMode.Open);
try
{

}
finally
{
    if (aFs != null)
    {
        aFs.Close();
    }
}

其实通过查看Using语句对应的IL就很清楚了

.locals init (
     [0] class [mscorlib]System.IO.FileStream aFs,
     [1] bool CS$4$0000)
 L_0000: nop
 L_0001: ldstr "c:\\test.txt"
 L_0006: ldc.i4.3
 L_0007: newobj instance void [mscorlib]System.IO.FileStream::.ctor(string, valuetype [mscorlib]System.IO.FileMode)
 L_000c: stloc.0
 L_000d: nop
 L_000e: nop
 L_000f: leave.s L_0021
 L_0011: ldloc.0
 L_0012: ldnull
 L_0013: ceq
 L_0015: stloc.1
 L_0016: ldloc.1
 L_0017: brtrue.s L_0020
 L_0019: ldloc.0
 L_001a: callvirt instance void [mscorlib]System.IDisposable::Dispose()
 L_001f: nop
 L_0020: endfinally
 L_0021: nop
 L_0022: ret
 .try L_000d to L_0011 finally handler L_0011 to L_0021

注意L_0007对应的实例化FileStream对象aFs在try语句之外(L_00d to L_0011)，因此很显然using语句中的对象初始化其实是在进入try语句之前执行的。

Conclusion:  Dig a little deeper to IL to see what happens behind the scenes.

--End--