Instantiate "AS" keyword

Question

I've recently started working with JSON and the ExtJs framework and I've come across the following code in an example.

we retrieve the information from the frontend using this:

object updatedConfig = JavaScriptConvert.DeserializeObject(Request["dataForm"]);

Then in the example they do the following:

JavaScriptObject jsObj = updatedConfig as JavaScriptObject;

I've never seen the "as" keyword used like that before. Is this just another form of explicitly boxing the updatedConfig variable as a JavaScriptObject or is there something I'm not understanding about this?

Thanks

score 4 · Accepted Answer · edited May 23 '17 at 12:30

This is known as a safe cast. What is does is it attempts to cast from one type to another and if the cast fails it returns null instead of throwing an InvalidCastException.

There are actually two separate IL instructions to handle the difference between "as" casting and normal static casting. The following C# code contains both types of casting:

using System;

class Program
{
    static void Main()
    {
        Object o = null;

        String s0 = (String)o;
        String s1 = o as String;
    }
}

The first cast uses the castclass IL instruction and the second cast uses the isinst instruction.

Please see Casting vs using the 'as' keyword in the CLR for a more detailed explanation.

score 3 · Answer 2 · edited Nov 02 '18 at 00:01

3

The as keyword is a safer way to cast objects in C#.

SomeType a = obj as SomeType;

Means that if obj is of type SomeType, obj will be cast into that type. If obj is null or is not of type SomeType, a will be null.

edited Nov 02 '18 at 00:01

HSchmale

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answered Mar 04 '09 at 12:02

Szymon Rozga

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It's also faster to use 'as' and check for null than to check the type with 'is' and then do an hard cast. The type will be checked twice in the latter case. – Dave Van den Eynde Mar 04 '09 at 12:05
@DaveVandenEynde Interestingly, Jon Skeet mentioned in his C# in Depth 2nd Edition that he did some timing of `as` versus `is` and a cast for an array of objects where 1/3 of them weren't boxed integers and found that `is` plus the cast was 20x faster. (pg. 121) Seemed counter-intuitive to me... but my tests confirmed that. – itsmatt Oct 11 '12 at 17:01
@itsmatt Could you add a chapter/section reference? page 121 is about nullable types. – Dave Van den Eynde Oct 11 '12 at 17:21
@DaveVandenEynde 4.3.5 is the section. In the "Surprising Performance Trap" paragraph. – itsmatt Oct 11 '12 at 17:31
@itsmatt Thanks. That is surprising indeed, but I should add that it involves a lot of unboxing which may affect the performance of the casting. – Dave Van den Eynde Oct 11 '12 at 17:38
@DaveVandenEynde Agreed. When I ran my tests with my own class instead of ints, is+cast was still 1/3 faster. Anyhow, thought I'd share. – itsmatt Oct 11 '12 at 17:58

score 0 · Answer 3 · answered Mar 04 '09 at 12:18

0

Another advantage of the as keyword is that it will throw a compile time exception if the type can not be cast where as (cast) doesn't break until run-time.

answered Mar 04 '09 at 12:18

Sruly

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score 0 · Answer 4 · answered Mar 04 '09 at 12:56

Also, it's important to remember that "as" operates in the reference not in the object itself. That's why it can return null instead of throwing an exception, because the object will remain intact. And that's why you can only do it on reference types.

Normally it does not matter that much, but if you implement a casting function (like here in MSDN) it will not be invoked by using as operator.

So, the as operator is useful to "move up and down the inheritance hiearchy": If you have a class Person, you can do: Person p = new Person(); ojbect o = p as object; p = o as Person; But in all cases the object in memory won't be modified in any way, just the reference to it.

Hope that helps

Instantiate "AS" keyword

4 Answers4