C# Extern Alias Example

Extern eliminates conflicts. In very large C# projects, you may have two class libraries that contain a class that has the same name. For example, ClassLibrary1 and ClassLibrary2 both introduce the same class. With the extern keyword, you can use both of those classes at once.

This C# article shows the extern keyword. It provides an example program that uses extern.

Setup

This example requires some setup. First, create two Class Library projects and have them specify the same name class with no containing namespace. Next, compile them and in Visual Studio change the Aliases fields to X and Y.

Then: You can create the main program, the console application, that specifies the extern aliases. If the compilation options in Visual Studio match the extern aliases, you can use those types in the same program.

Example

Let's look at the code for the extern alias example. The extern alias X and Y are meaningless unless they match the settings you specify in Visual Studio. There, they are mapped to the ClassLibrary1 and ClassLibrary2 DLL files.

Program that uses extern alias syntax [C#]

extern alias X;
extern alias Y;

using System;

class Program
{
    static void Main()
    {
	X.A._b = 1;
	Y.A._b = 2;

	Console.WriteLine(X.A._b);
	Console.WriteLine(Y.A._b);
    }
}

Contents of ClassLibrary1 [C#]

public class A
{
    public static int _b;
}

Contents of ClassLibrary2 [C#]

public class A
{
    public static int _b;
}

Output

1
2

Usage

In simple programs, the extern alias syntax is rarely useful. However, if you are Microsoft and are developing a huge framework, it would definitely be useful. It could also be handy in very large programs or ones that use multiple versions of the same library at once. For most of us, I wouldn't worry too much about it.

Summary

The extern alias directive provides a way for you to differentiate between different physical locations of the same class name. This means you can use a class A from many different files with no conflicts. You must specify the extern alias in your C# program, and also in the compilation system (Visual Studio).