The using directive has two uses:
- Create an alias for a namespace (a using alias).
- Permit the use of types in a namespace, such that, you do not have to qualify the use of a type in that namespace (a using directive).
using [alias = ]class_or_namespace;
where:
- alias (optional)
- A user-defined symbol that you want to represent a namespace. You will then be able to use alias to represent the namespace name.
- class_or_namespace
- The namespace name that you want to either use or alias, or the class name that you want to alias.
Remarks
Create a using alias to make it easier to qualify an identifier to a namespace or class.
Create a using directive to use the types in a namespace without having to specify the namespace. A using directive does not give you access to any namespaces that may be nested in the namespace you specify.
Namespaces come in two categories: user-defined and system-defined. User-defined namespaces are namespaces defined in your code. See the documentation for the .NET Framework for a list of the system-defined namespaces.
For examples on referencing methods in other assemblies, see Creating and Using C# DLLs.
Example
The following sample shows how to define and use a using alias for a namespace:
// cs_using_directive.cs
using MyAlias = MyCompany.Proj.Nested; // define an alias to represent a namespace
namespace MyCompany.Proj
{
public class MyClass
{
public static void DoNothing()
{
}
}
namespace Nested // a nested namespace
{
public class ClassInNestedNameSpace
{
public static void SayHello()
{
System.Console.WriteLine("Hello");
}
}
}
}
public class UnNestedClass
{
public static void Main()
{
MyAlias.ClassInNestedNameSpace.SayHello(); // using alias
}
}Output
Example
The following sample shows how to define a using directive and a using alias for a class:
// cs_using_directive2.cs
using System; // using directive
using AliasToMyClass = NameSpace1.MyClass; // using alias for a class
namespace NameSpace1
{
public class MyClass
{
public override string ToString()
{
return "You are in NameSpace1.MyClass";
}
}
}
namespace NameSpace2
{
class MyClass
{
}
}
namespace NameSpace3
{
using NameSpace1; // using directive
using NameSpace2; // using directive
class Test
{
public static void Main()
{
AliasToMyClass somevar = new AliasToMyClass();
Console.WriteLine(somevar);
}
}
}Output
You are in NameSpace1.MyClass
See Also
C# Keywords | namespace | using Statement