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using Directive (C# Reference) 

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The using directive has two uses:

  • To permit the use of types in a namespace so you do not have to qualify the use of a type in that namespace.

  • To create an alias for a namespace.

The using keyword is also be used to create using statements, which define when an object will be disposed. See using Statement for more information.

using namespace;
using alias = type|namespace;

Parameters

  • Alias
    A user-defined symbol that you want to represent a namespace or type. You will then be able to use alias to represent the namespace name.
  • Type
    The type that you want to be represented by alias.
  • namespace
    The namespace that you want to be represented by alias.-or-The namespace containing types you want to use without having to specify the fully qualified name.

Remarks

The scope of a using directive is limited to the file in which it appears.

Create a using alias to make it easier to qualify an identifier to a namespace or type.

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 are 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. For a list of the system-defined namespaces, see .NET Framework Class Library Reference.

For examples on referencing methods in other assemblies, see Creating and Using C# DLLs.

Example 1

The following example shows how to define and use a using alias for a namespace:

using MyAlias = MyCompany.Proj.Nested;
// Define an alias to represent a namespace.

namespace MyCompany.Proj 
{
    public class MyClass 
    {
        public static void DoNothing() 
        {
        }
    }
}

Example 2

The following example shows how to define a using directive and a using alias for a class:

// cs_using_directive2.cs
// Using directive.
using System;   
// Using alias for a class.
using AliasToMyClass = NameSpace1.MyClass;   

namespace NameSpace1 
{
    public class MyClass 
    {
        public override string ToString() 
        {
            return "You are in NameSpace1.MyClass";
        }
    }
}

namespace NameSpace2 
{
    class MyClass 
    {
    }
}

namespace NameSpace3 
{
    // Using directive:
    using NameSpace1;
    // Using directive:
    using NameSpace2;   

    class MainClass
    {
        static void Main() 
        {
            AliasToMyClass somevar = new AliasToMyClass();
            Console.WriteLine(somevar);
        }
    }
}

Output

You are in NameSpace1.MyClass

C# Language Specification

For more information, see the following sections in the C# Language Specification:

  • 9.3 Using directives

See Also

Reference

C# Keywords
Namespace Keywords (C# Reference)
using Statement (C# Reference)

Concepts

C# Programming Guide
Namespaces (C# Programming Guide)

Other Resources

C# Reference