using "IS" is better or checking for "NOT NULL"

Question

In C#.NET language: This style of coding is recommended or the one below it?

 if (sheet.Models.Data is GroupDataModel)
 {
 GroupDataModel gdm = (GroupDataModel)sheet.Models.Data;
 Group group = gdm.GetGroup(sheet.ActiveCell.Row.Index); 
 if (group!=null && controller != null)
 {
 controller.CheckApplicationState();
 }
 }

or this one:

 var gdm = sheet.Models.Data as GroupDataModel;
 if (gdm != null)
 {
 Group group = gdm.GetGroup(sheet.ActiveCell.Row.Index); 
 if (@group!=null && controller != null)
 {
 controller.CheckApplicationState();
 }
 }

• Robert Harvey: Relevant: stackoverflow.com/q/132445/102937

Answer 1

If you intend to cast (or use as) if the result of calling is is true, you should just call as and avoid two cast attempts.

I prefer the second version for this reason.

• Blake: hmm what is that "@" that is being added in second version to the group variable? it is now @group
• Oded: @BDotA - Looks like a typo to me.
• Oded: @BDotA - The @ before a variable name is ignored - it allows you to use reserved keywords as variable names. Like var @class, for instance.
• David Anderson: The usage of @ is called a verbatim identifier. stackoverflow.com/questions/11851540/…
• jk.: FxCop will actually warn against casting twice in the first one

Answer 2

In short, if you already know what type it can cast to use a C-style cast:

var o = (string) iKnowThisIsAString; 

Note that only with a C-style cast can you perform explicit type coercion. If you don't know whether it's the desired type and you're going to use it if it is, use as keyword:

var txt = o as string;
if (txt != null) return txt.Replace("_","-");
//or for early return:
if (txt == null) return;

Note that as will not call any type conversion operators. It will only be non-null if the object is not null and natively of the specified type.

Answer 3

Fundamentally, they are two different things. But what do you think reads better?

if (sheet.Models.Data is GroupDataModel) 

or

var gdm = sheet.Models.Data as GroupDataModel;
if (gdm != null)

You've got your answer here. Oded made a very valid point, but programming isn't much different from writing articles. It needs to be easily readable by other engineers.