c# - "as"关键字在内部是如何工作的？

Question

我知道这个关键字的作用，但我想知道它在较低层次上是如何工作的。

哪个更快？它们总是产生相同的结果吗？如果他们这样做，为什么会有两种不同的方式？

// Is there an overhead? An internal try catch?
Class123 obj = someobject as Class123;

if (Class123 != null)
{
    //OK
}

或

Class123 obj = null;

if (someobject is Class123)
{
    obj = (Class123)someobject;
}

Answer 1

根据 MSDN: as (C# Reference) :

The as operator is like a cast operation. However, if the conversion is not possible, as returns null instead of raising an exception. Consider the following expression:

expression as type

It is equivalent to the following expression except that expression is evaluated only one time.

expression is type ? (type)expression : (type)null

---

第一个变体(作为操作数)...

string str1 = strAsObject as string;
if (str1 != null)
{
    this.blabla(str1);
}

...编译成这个 IL 代码:

L_0009: ldloc.1 
L_000a: isinst string
L_000f: stloc.2 
L_0010: ldloc.2 
L_0011: ldnull 
L_0012: ceq 
L_0014: stloc.s CS$4$0000
L_0016: ldloc.s CS$4$0000
L_0018: brtrue.s L_0024
L_001a: nop 
L_001b: ldarg.0 
L_001c: ldloc.2 
L_001d: call instance void TestWinFormsApplication001.Form1::blabla(string)
L_0022: nop 
L_0023: nop 

... 和第二个变体(是操作数 + cast)...

if (strAsObject is string)
{
    string str2 = (string) strAsObject;
    this.blabla(str2);
}

...编译成这个 IL 代码:

L_0024: ldloc.1 
L_0025: isinst string
L_002a: ldnull 
L_002b: cgt.un 
L_002d: ldc.i4.0 
L_002e: ceq 
L_0030: stloc.s CS$4$0000
L_0032: ldloc.s CS$4$0000
L_0034: brtrue.s L_0047
L_0036: nop 
L_0037: ldloc.1 
L_0038: castclass string
L_003d: stloc.3 
L_003e: ldarg.0 
L_003f: ldloc.3 
L_0040: call instance void TestWinFormsApplication001.Form1::blabla(string)
L_0045: nop 
L_0046: nop 

...所以您看到的唯一区别是 L_0038 行中的附加 castclass 代码。