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.

//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));
   }
}

SampleMethod() returns 50.

To build the project:

• Compile Cmdll.c to a DLL by using the Visual C++ command line:

  cl /LD Cmdll.c

• Compile CM.cs using the command line:

  csc CM.cs

This will create the executable file CM.exe. When you run this program, SampleMethod will pass the value 5 to the DLL file, which returns the value multiplied by 10.

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

• 10.6.7 External methods

Concepts

Reference

Other Resources