Use of Extern keyword

Use the extern modifier in a method declaration to indicate that the method is implemented externally. A common use of the extern modifier is with the DllImport attribute.

For example

using System; 

using System.Runtime.InteropServices; 

class MyClass 

{

     [DllImport("User32.dll")]    

     public static extern int MessageBox(int h, string m, string c, int type);   

     public static int Main()  

     {  

	     string myString;

             Console.Write("Enter your message: ");   

             myString = Console.ReadLine();

             return MessageBox(0, myString, "My Message Box", 0);   

     }

 }

 In this above 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  Extern is nothing but a memory storage type.If we declare extern before the function than the variable can be accessed by that function not only locally but externally too. 

 extern int a=4 

 Int fun (int x) 

 { 

    x=a;

}

  print gives a=4