== Why is this an issue?

Unnecessary `using` directives refer to importing namespaces, types or creating aliases that are not used or referenced anywhere in the code.

include::../description.adoc[]

Starting with C# 10, it's possible to define https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/using-directive#global-modifier[global usings] for an entire project. They reduce the need for repetitive namespace inclusions, but can also mask which namespaces are truly necessary for the code at hand. Over-relying on them can lead to less transparent code dependencies, especially for newcomers to the project.

=== Exceptions

The rule will not raise a warning for `global using` directives, even if none of the types of that namespace are used in the project:

[source,csharp]
----
global using System.Net.Sockets; // Compliant by exception
----

Unnecessary `using` directives are ignored in ASP.NET Core projects in the following files:

* ``++_Imports.razor++``
* ``++_ViewImports.cshtml++``

== How to fix it

While it's not difficult to remove these unneeded lines manually, modern code editors support the removal of every unnecessary `using` directive with a single click from every file of the project.

=== Code examples

==== Noncompliant code example

[source,csharp,diff-id=1,diff-type=noncompliant]
----
using System.IO;
using System.Linq;
using System.Collections.Generic;   // Noncompliant - no types are used from this namespace    
using MyApp.Helpers;                // Noncompliant - FileHelper is in the same namespace
using MyCustomNamespace;            // Noncompliant - no types are used from this namespace

namespace MyApp.Helpers
{
    public class FileHelper
    {
        public static string ReadFirstLine(string filePath) =>
            File.ReadAllLines(filePath).First();
    }
}
----

==== Compliant solution

[source,csharp,diff-id=1,diff-type=compliant]
----
using System.IO;
using System.Linq;

namespace MyApp.Helpers
{
    public class FileHelper
    {
        public static string ReadFirstLine(string filePath) =>
            File.ReadAllLines(filePath).First();
    }
}
----

== Resources
=== Documentation

* https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/using-directive[MSDN - using directives]
* https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/namespace[MSDN - namespaces]

=== Related rules

* S1144 - Unused private types or members should be removed
* S1481 - Unused local variables should be removed

ifdef::env-github,rspecator-view[]

'''
== Implementation Specification
(visible only on this page)

=== Message

* Remove this unnecessary 'using'.


'''
== Comments And Links
(visible only on this page)

include::../comments-and-links.adoc[]

=== on 23 Sep 2019, 16:37:14 Nicolas Harraudeau wrote:
*OUT OF SCOPE*


Duplicate imports are out of scopes as Roslyn already raises an issue in this case. As Roslyn issues are enabled by default this would simply create duplicates.

=== on 18 Dec 2020, 10:06:15 Andrei Epure wrote:
We are removing this rule from SonarWay due to its performance issues. After the rule gets re-designed to avoid perf issues, (see https://github.com/SonarSource/sonar-dotnet/issues/3761[#3761]), we should bring it back to SonarWay.

endif::env-github,rspecator-view[]