[C #] extern

ExternModifier is used to declare methods for external implementation.ExternThe common usage of modifiers isDllimportIn this case, the method must also be declaredStatic, As shown in the following example:

[Dllimport ("avifil32.dll")]
Private Static extern void avifileinit ();

ExternYou can also define the alias of an external Assembly so that different versions of the same component can be referenced from a single assembly. For more information, see external alias (C # reference ).

Abstract (C # reference) andExternIt is wrong to use modifiers together to modify the same member. UseExternModifier means that the method is implemented outside the C # code, andAbstractModifier means that method implementation is not provided in the class.

ExternKeyword usage has more restrictions than C ++. To compare with the c ++ keyword, see using extern to specify linkage in C ++ language reference.

Example:

 

In this example, the program receives a string from the user and displays the string in the message box. Program usageUser32.dllLibrary importedMessageBoxMethod.

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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 uses the C program to create a DLL. In the next example, the DLL will be called from the C # program.

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// cmdll.c// compile with: /LDint __declspec(dllexport) SampleMethod(int i){   return i*10;}

This example uses two filesCm. CSAndCmdll. cTo describeExtern. The C file is the external DLL created in example 2, which is called from the C # program.

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// cm.csusing 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.

Note:

Generate a project:

• Use the Visual C ++ command lineCmdll. cCompile as dll:

  CL/LD cmdll. c

• Compile with command lineCm. CS:

  CSC cm. CS

This will create an executable fileCm.exe. When running this program,SamplemethodPass the value 5 to the DLL file, which is multiplied by 10 to return the value.