CSharpBits:The is and as Operators

re: CSharpBits:The is and as Operators

Jerry Pisk — Tue, 16 Mar 2004 20:26:00 GMT

Why exactly does "is" return false when the object is null? Did the C# specification change and null is no longer a valid object value?

re: CSharpBits:The is and as Operators

Ron — Tue, 16 Mar 2004 23:21:00 GMT

Jeff,

As always, thanks for these articles.

1) how does the "is" operator check that the type of object is the same?

2) Why do we need a cast operator? Is it not better to always use the "as" operator (and check for null)?

re: CSharpBits:The is and as Operators

Jeff Adkins — Wed, 17 Mar 2004 00:49:00 GMT

1)I'm not certain, but I assume it's doing a typeOf call or a reflection runtime GetType() call. Here's a reference..
http://msdn.microsoft.com/library/en-us/csref/html/vclrftypeofpg.asp?frame=true

2)The cast will raise the InvalidCastException for you to catch and you have to handle your own exception handling with the "as" operator. Otherwise, you're making a good point when it comes to reference types. Casting is used more for value types.

re: CSharpBits:The is and as Operators-Why exactly does "is" return false when the object is null? Did the C# specification change and null is no longer a valid object value?

Jeff Adkins — Wed, 17 Mar 2004 01:13:00 GMT

The "is" Operator is trying to determine if a cast can be made without error. If the object reference is null, that means it is not pointing to a current object in memory and an error would be raised when the framework tries to perform a typeOf operator or a GetType() call on a null value. Also, I believe a cast on reference variable with a current null value would raise an exception. These are my assumptions. If you're still curious, try testing these two scenarios. See if you can do a GetType or use the typeOf operator on a ref variable with a null value. And, separtely, try to do a cast on this variable with a null value.