Why should you remove unnecessary C# using directives?

Question

There are few reasons for removing unused using(s)/namespaces, besides coding preference:

• removing the unused using clauses in a project, can make the compilation faster because the compiler has fewer namespaces to look-up types to resolve. (this is especially true for C# 3.0 because of extension methods, where the compiler must search all namespaces for extension methods for possible better matches, generic type inference and lambda expressions involving generic types)
• can potentially help to avoid name collision in future builds when new types are added to the unused namespaces that have the same name as some types in the used namespaces.
• will reduce the number of items in the editor auto completion list when coding, posibly leading to faster typing (in C# 3.0 this can also reduce the list of extension methods shown)

What removing the unused namespaces won't do:

• alter in any way the output of the compiler.
• alter in any way the execution of the compiled program (faster loading, or better performance).

The resulting assembly is the same with or without unused using(s) removed.

It won't change anything when your program runs. Everything that's needed is loaded on demand. So even if you have that using statement, unless you actually use a type in that namespace / assembly, the assembly that using statement is correlated to won't be loaded.

Mainly, it's just to clean up for personal preference.

Code cleanliness is important.

One starts to get the feeling that the code may be unmaintained and on the browfield path when one sees superfluous usings. In essence, when I see some unused using statements, a little yellow flag goes up in the back of my brain telling me to "proceed with caution." And reading production code should never give you that feeling.

So clean up your usings. Don't be sloppy. Inspire confidence. Make your code pretty. Give another dev that warm-fuzzy feeling.

There's no IL construct that corresponds to using. Thus, the using statements do not increase your application memory, as there is no code or data that is generated for it.

Using is used at compile time only for the purposes of resolving short type names to fully qualified type names. Thus, the only negative effect unnecessary using can have is slowing the compile time a little bit and taking a bit more memory during compilation. I wouldn't be worried about that though.

Thus, the only real negative effect of having using statements you don't need is on intellisense, as the list of potential matches for completion while you type increases.

You may have name clashes if you call your classes like the (unused) classes in the namespace. In the case of System.Text, you'll have a problem if you define a class named "Encoder".

Anyways this is usually a minor problem, and detected by the compiler.