overhead to unused "using" declarations?

Question

I've just installed resharper and it's letting me know the namespaces i'm not actually using in each of my classes.

which lead me to the question - is there actually any overhead in leaving these, unused, using declarations in?

is it just a matter of tight code, or is there a performance hit in invoking these namespaces when i don't need to?

Answer 1

From The C# Team's answers to frequently asked questions:

When you add assembly references or make use of the 'using' keyword, csc.exe will ignore any assembly which you have not actually made use of in your code ... Don't [waste] your time stripping out unused 'using' statements or assembly references from your application. The C# compiler will do so for you automatically.

You can verify that this is actually the case by calling Assembly.GetReferencedAssemblies(); you'll see that anything that isn't used won't actually be included in the list.

The main utility in stripping out unused ones is

• It is easier to see what your code is actually using
• It will keep your Intellisense from being polluted with things that you aren't actually going to use.

Answer 2

There are some performance reasons to strip down using statements:

• VS's IntelliSense runs faster when there is less data to look through

However, note all these reasons are compile-time specific. The compiler strips unused usings automatically.