Random Nerd Debates : Episode 2 VAR

In the second episode of Random Nerd debates we are doing to discuss the use of the C# keyword var.

First, what is var? MSDN has a very succinct explanation of the keyword:

“Beginning in Visual C# 3.0, variables that are declared at method scope can have an implicit type var. An implicitly typed local variable is strongly typed just as if you had declared the type yourself, but the compiler determines the type.”

And the simple code example:

var i = 10; // implicitly typed
int i = 10; //explicitly typed

Ok so now that we are both on the same page let’s talk about usage of this keyword. I have a very specific rule when it comes to the usage of var.

I will only use the var keyword to shorten my code without obscuring the clarity. This means I will only use the var keyword if the type can be inferred directly by viewing the right side of the operation. By “directly” I mean that the type is explicitly specified in the right side of the operation. I don’t want to have to jump to the method declaration or hover over the expression to determine the type. The best way to show this is through code examples:

//Since foo is clearly specified on the right side I WILL use var in this case
var foo = new Foo();

//Since the return type of the method GetBar() is not clear I WILL NOT use var in this case
var bar = GetBar();

//Other places where I WILL use var
var foo = (Foo)bar;
var bar = foo as Bar;

The benefit of the var keyword is allowing to write more succinct code. To me, succinctness can not come at the cost of clarity. In my examples above where I WILL use var I don’t feel like I am sacrificing any clarity but I am reducing noise.

There are two exceptions to my rule. When writing a LINQ query you almost always see the use of var and so I will follow that convention

var files = from file in enumerableFiles
            select new Uri(file.FullName,UriKind.Absolute);

The other exception is similar to the LINQ example but more specific. There are cases which I want to deliberately de-emphasize the underlying storage mechanism and keep it ambiguous. In these cases I will use var to purposely obscure the underlying type. Eric Lippert explained this idea well in a blog post. The tidbit from Lippert was this:

   “Another subtle point here: notice how when I changed the type of the variable "racks" from "array of string" to "set of string", I didn't have to redundantly change the type thanks to implicit typing of local variables. I want to emphasize the semantics here, not the storage mechanism. If I felt that communicating the storage mechanism was an important part of this code -- because it has such a strong effect on performance -- perhaps I would choose to emphasize the storage by eschewing the "var"."

If you don’t already follow Eric’s blog I highly recommend it.

So I pose the question to you? Do you use var? Do you have a specific rule you follow when it comes to using it? Did you read this whole post and realize you just wasted the last 20 minutes of your life?