Alex Rivera | Logout

Will using 'var' affect performance?

Asked 2008-12-10T17:19:45.113
248

Earlier I asked a question about why I see so many examples use the varkeyword and got the answer that while it is only necessary for anonymous types, that it is used nonetheless to make writing code 'quicker'/easier and 'just because'.

Following this link ("C# 3.0 - Var Isn't Objec") I saw that var gets compiled down to the correct type in the IL (you will see it about midway down article).

My question is how much more, if any, IL code does using the var keyword take, and would it be even close to having a measurable level on the performance of the code if it was used everywhere?

c# performance variables var
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2 Answers

32

As nobody has mentioned reflector yet...

If you compile the following C# code:

static void Main(string[] args)
{
    var x = "hello";
    string y = "hello again!";
    Console.WriteLine(x);
    Console.WriteLine(y);
}

Then use reflector on it, you get:

// Methods
private static void Main(string[] args)
{
    string x = "hello";
    string y = "hello again!";
    Console.WriteLine(x);
    Console.WriteLine(y);
}

So the answer is clearly no runtime performance hit!

answered 2009-11-18T14:42:02.213
8

I don't think you properly understood what you read. If it gets compiled to the correct type, then there is no difference. When I do this:

var i = 42;

The compiler knows it's an int, and generate code as if I had written 

int i = 42;

As the post you linked to says, it gets compiled to the same type. It's not a runtime check or anything else requiring extra code. The compiler just figures out what the type must be, and uses that.

answered 2008-12-10T17:30:41.113

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