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How to: Build a Multifile Assembly

This section describes the procedure used to create a multifile assembly and provides a complete example that illustrates each of the steps in the procedure.

To create a multifile assembly

  1. Compile all files that contain namespaces referenced by other modules in the assembly into code modules. The default extension for code modules is .netmodule. For example, if a file called Stringer creates a namespace called myStringer that is referenced in the Client file code, Stringer should be compiled into a code module first.

  2. Compile all other modules, using the necessary compiler options to indicate the other modules that are referenced in the code.

  3. Use the Assembly Linker (Al.exe) to create the output file that contains the assembly manifest. This file contains reference information for all modules or resources that are part of the assembly.


    The Visual Studio 2005 IDE for C# and Visual Basic can only be used to create single-file assemblies. If you want to create multifile assemblies, you must use the command-line compilers or Visual Studio 2005 with Visual C++.

The following example illustrates step 1 of the procedure above, by compiling files with namespaces referenced by other files. This example starts with some simple code for the Stringer file. Stringer has a namespace called myStringer with a class called Stringer. The Stringer class contains a method called StringerMethod that writes a single line to the console.

// Assembly building example in the .NET Framework.
using System;
namespace myStringer 
   public class Stringer
      public void StringerMethod() 
         System.Console.WriteLine("This is a line from StringerMethod.");

Use the following command to compile this code:

csc /t:module Stringer.cs

Specifying the module parameter with the /t: compiler option indicates that the file should be compiled as a module rather than as an assembly. The compiler produces a module called Stringer.netmodule, which can be added to an assembly.

In step two of the procedure above, you must compile modules with references to other modules. This step uses the /addmodule compiler option. In the following example, a code module called Client has an entry point Main method that references a method in the Stringer.dll module created in Step 1.

The following example shows the code for Client.

using System;
using myStringer; //The namespace created in Stringer.netmodule.
class MainClientApp 
   // Static method Main is the entry point method.
   public static void Main() 
      Stringer myStringInstance = new Stringer();
      Console.WriteLine("Client code executes");

Use the following command to compile this code:

csc /addmodule:Stringer.netmodule /t:module Client.cs

Specify the /t:module option because this module will be added to an assembly in a future step. Specify the /addmodule option because the code in Client references a namespace created by the code in Stringer.netmodule. The compiler produces a module called Client.netmodule that contains a reference to another module, Stringer.netmodule.


The C# and Visual Basic compilers support directly creating multifile assemblies using the following two different syntaxes.

  • Two compilations create a two-file assembly:

    vbc /t:module Stringer.vb
    vbc Client.vb /addmodule:Stringer.netmodule

    csc /t:module Stringer.cs
    csc Client.cs /addmodule:Stringer.netmodule

  • One compilation creates a two-file assembly:

    vbc /out:Stringer.netmodule Stringer.vb /out:Client.exe Client.vb

    csc /out:Client.exe Client.cs /out:Stringer.netmodule Stringer.cs

You can use the Assembly Linker (Al.exe) to create an assembly from a collection of compiled code modules.

To create a multifile assembly using the Assembly Linker

  • At the command prompt, type the following command:

    al <module name> <module name> … /main:<method name> /out:<file name> /target:<assembly file type>

    In this command, the module name arguments specify the name of each module to include in the assembly. The /main: option specifies the method name that is the assembly's entry point. The /out: option specifies the name of the output file, which contains assembly metadata. The /target: option specifies that the assembly is a console application executable (.exe) file, a Windows executable (.win) file, or a library (.lib) file.

In the following example, Al.exe creates an assembly that is a console application executable called myAssembly.exe. The application consists of two modules called Client.netmodule and Stringer.netmodule, and the executable file called myAssembly.exe, which contains only assembly metadata . The entry point of the assembly is the Main method in the class MainClientApp, which is located in Client.dll.

al Client.netmodule Stringer.netmodule /main:MainClientApp.Main /out:myAssembly.exe /target:exe 

You can use the MSIL Disassembler (Ildasm.exe) to examine the contents of an assembly, or determine whether a file is an assembly or a module.