Why should you remove unnecessary C# using directives?

For example, I rarely need:

using System.Text;

but it's always there by default. I assume the application will use more memory if your code contains unnecessary using directives. But is there anything else I should be aware of?

Also, does it make any difference whatsoever if the same using directive is used in only one file vs. most/all files?

Edit: Note that this question is not about the unrelated concept called a using statement, designed to help one manage resources by ensuring that when an object goes out of scope, its IDisposable.Dispose method is called. See Uses of "using" in C#.

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.