The extern modifier C# Reference

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 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();

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

Example

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;

using System.Runtime.InteropServices;
class MainClass
{
[DllImport("User32.dll")]
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));
}
}

Output

SampleMethod() returns 50.

Remarks

To build the project:

* Compile Cmdll.c to a DLL 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.

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

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