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835

answers:

3

A colleague asked me an interesting question today - is the C# keyword/operator "is" considered reflection?

object tmp = "a string";
if(tmp is String)
{
}

How is this operator implemented behind the scenes? Does it require reflection or introspection? Or because of the strongly typed nature of the language, is the Type of the object immediately accessable as a top-level attribute of the object in memory?

MSDN states that:

Note that the is operator only considers reference conversions, boxing conversions, and unboxing conversions. Other conversions, such as user-defined conversions, are not considered by the is operator.

The ability to consider boxed and unboxed conversions seems to imply to me some sort of introspection.

+5  A: 

The is operator essentially determines if a cast is possible, but instead of throwing an exception when the cast is impossible it returns false. If you consider casting reflection then this is also reflection.

EDIT:

After some research I have discovered that a cast is performed in IL på the castclass instruction while the is operator maps to the isinst instruction. FxCop has a rule that warns you if you are doing unecessary casts by first using the isinst and then the castclass instruction. Even though the operations are efficient they still have a performance cost.

Martin Liversage
'is' operator returns true/false and not null.
SolutionYogi
I think you're thinking of the "as" operator
Matt
What you've described is the 'as' operator. The 'is' operator is essentially the same operation, just with a different return value.
Charlie
You are right about me mixing 'is' and 'as' up in my answer. I have corrected my mistake.
Martin Liversage
+1  A: 

Other languages have runtime time information sufficient to support dynamic casting, and yet nothing that could be described as reflection (C++ being an obvious example).

So reflection refers to additional capabilities beyond merely discovering the type of an object. To "reflect" on an object implies the ability to walk its members, for example.

Daniel Earwicker
+14  A: 

Referencing ECMA-335, the is operator generates the isinst object model IL instruction (Partition III §4.6), which is part of the base instruction set as opposed to being part of the Reflection library (Partition IV §5.5).

Edit: The is operator is extremely efficient compared to the reflection library. You could perform basically the same test much more slowly via reflection:

typeof(T).IsAssignableFrom(obj.GetType())

Edit 2: You are not correct about the efficiency of the castclass and isinst instructions (which you've now edited out of the post). They are highly optimized in any practical VM implementation. The only real performance issue involved is the potential for castclass to throw an exception, which you avoid by using the C# as operator and a test for null (for reference types) or the is operator followed by a cast (for value types).

280Z28
I haven't edited anything out, but perhaps I should clarify what I mean by "expensive". I'm don't know much about IL efficiency, but FxCop has a warning if your create code that emits both an isinst and castclass instruction as they consider it ineffecient or "expensive".
Martin Liversage
As I mentioned in the second edit, you should use `as` followed by a `null` check for reference types. You instead used `is` followed by a cast, which is what FxCop is alerting you to.
280Z28
For what it's worth, on the Compact Framework, IL that needs to explicitly check RTTI (such as `isinst` and `castclass`) is as slow as reflection, while callvirt is highly optimized. The moral: try to avoid using type-case constructs where virtual/abstract methods can work.
Jeffrey Hantin