https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/using

The using keyword has two major uses:

• As a directive, when it is used to create an alias for a namespace or to import types defined in other namespaces.

• As a statement, when it defines a scope at the end of which an object will be disposed.

In addition, the using static directive lets you define a type whose static members you can access without specifying a type name.