Calling ToString() To Prevent Boxing

Question

With the following code snippet:

public static void Main()
{
    int v = 2;
    Console.WriteLine("number" + "," + v);
}

Apparently it's better to replace v with v.ToString() in the call to WriteLine() to prevent the value type from being boxed. However calling ToString() still allocates an object on the heap, as would boxing the value type.

So what is the benefit of using v.ToString() rather than letting it be boxed?

Update: Looks like int.ToString() is called before passing the value to string.Concat() with or without explicitly calling int.ToString() yourself. I checked the CIL for with and without ToString() and they're identical.

Answer 1

ToString() will only box a variable if its type has no override of ToString() (so it has to go to the implementation on the Object class). However, int does have an overridden ToString() (see http://msdn.microsoft.com/en-us/library/6t7dwaa5(v=vs.110).aspx), so this does not happen.

The boxing is actually performed by the + operator, because that just calls string.Concat, which expects parameters of type Object (or string, depending on which overload is used). Therefore, integers have to be boxed for the call. Then, the string.Concat method unboxes the int again and calls ToString(). Therefore, if you call it yourself, you'll save time by not having to do the boxing and unboxing.

There is a performance gain, although in most cases it will be quite marginal.