Why should you remove unnecessary C# using directives?

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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.

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