Extern Keyword in C#

The extern modifier is used to declare a method that is implemented externally. A common use of the extern modifier is with the DllImport attribute when you are using Interop services to call into unmanaged code. In this case, the method must also be declared as static, as shown in the following example:

[DllImport("avifil32.dll")]
private static extern void AVIFileInit();

NOTE:

The extern keyword can also define an external assembly alias, which makes it possible to reference different versions of the same component from within a single assembly. For more information, see extern alias (C# Reference).

It is an error to use the abstract (C# Reference) (MSDN LINK) and extern modifiers together to modify the same member. Using the extern modifier means that the method is implemented outside the C# code, whereas using the abstract modifier means that the method implementation is not provided in the class.

NOTE:

The extern keyword has more limited uses than in C++. To compare with the C++ keyword, see Using extern to Specify Linkage in the C++ Language Reference.

In this example, the program receives a string from the user and displays it inside a message box. The program uses the MessageBox method imported from the User32.dll library.

// Add the following directive to your file:
// using System.Runtime.InteropServices;
class ExternTest
{
    [DllImport("User32.dll", CharSet=CharSet.Unicode)] 
    public static extern int MessageBox(int h, string m, string c, int type);

    static int Main()
    {
        string myString;
        Console.Write("Enter your message: ");
        myString = Console.ReadLine();
        return MessageBox(0, myString, "My Message Box", 0);
    }

}
This example creates a DLL from a C program that is invoked from within the C# program in the next example

// cmdll.c

// Compile with: /LD
int __declspec(dllexport) SampleMethod(int i)
{
   return i*10; 

}

This example uses two files, CM.cs and Cmdll.c, to demonstrate extern. The C file is the external DLL created in Example 2 that is invoked from within the C# program.

// cm.cs
using System;
using System.Runtime.InteropServices;
public class MainClass 
{
   [DllImport("Cmdll.dll")]
   public static extern int SampleMethod(int x);

   static void Main() 
   {
      Console.WriteLine("SampleMethod() returns {0}.", SampleMethod(5));
   }
}