extern alias (C# Reference)

You might need to reference two versions of assemblies that have the same fully qualified type names. For example, you might need to use two or more versions of an assembly in the same application. By using an external assembly alias, you can wrap the namespaces from each assembly inside root-level namespaces named by the alias. This approach enables you to use both versions in the same file.

The C# language reference documents the most recently released version of the C# language. It also contains initial documentation for features in public previews for the upcoming language release.

The documentation identifies any feature first introduced in the last three versions of the language or in current public previews.

Tip

To find when a feature was first introduced in C#, consult the article on the C# language version history.

Note

The extern keyword is also used as a method modifier, declaring a method written in unmanaged code.

To reference two assemblies with the same fully qualified type names, specify an alias in your .csproj file, and add the following code:

<Reference Include="grid.dll"> 
    <Aliases>GridV1</Aliases>
</Reference>
<Reference Include="grid20.dll">
    <Aliases>GridV2</Aliases>
</Reference>

You can learn more in the article on the CSC task in the Visual Studio documentation.

This command creates the external aliases GridV1 and GridV2. To use these aliases from within a program, reference them by using the extern keyword. For example:

extern alias GridV1; extern alias GridV2;

Each extern alias declaration introduces an additional root-level namespace that parallels (but doesn't lie within) the global namespace. You can refer to types from each assembly without ambiguity by using their fully qualified name, rooted in the appropriate namespace-alias.

In the previous example, GridV1::Grid is the grid control from grid.dll, and GridV2::Grid is the grid control from grid20.dll.

Using Visual Studio

If you're using Visual Studio, you can provide aliases in a similar way.

Add references to grid.dll and grid20.dll to your project in Visual Studio. Open the property tab and change the Aliases from global to GridV1 and GridV2 respectively.

Use these aliases the same way as described earlier.

extern alias GridV1;  
extern alias GridV2;  

Now you can create an alias for a namespace or a type by using the using alias directive. For more information, see using directive.

using Class1V1 = GridV1::Namespace.Class1;
using Class1V2 = GridV2::Namespace.Class1;

C# Language Specification

For more information, see the C# Language Specification. The language specification is the definitive source for C# syntax and usage.

See also

• C# Keywords
• :: Operator
• References (C# Compiler Options)