How to build custom COM interface marshalers in Visual C++

MORE INFORMATION

To generate a standard marshaler, write an IDL file that describes your custom interface. The Microsoft Interface Definition Language (MIDL) compiler is used to generate source files for the marshaler DLL from the IDL file. Once these files are generated, you compile and link the code to produce the marshaler DLL.

Versions of Visual C++ prior to 5.0 do not provide built-in rules for compiling IDL files using the MIDL compiler. However, you can use the custom build feature to add the necessary rules to your project. Beginning with version 6.0, the build engine automatically calls MIDL.exe to compile IDL files. The Project/Settings dialog box has a MIDL tab to control the settings.

To generate a standard marshaler for the interface IMyInterface:

1. Create a new empty Dynamic-Link Library project using Developer Studio. This can either be a new project workspace or a subproject of an existing workspace.
   
   Note The project name is also used as the name of the marshaler DLL. For example, PS<interface>.
2. Create a new file called IMyInterface.IDL, and enter the description of your new interface.
3. Insert the file IMyInterface.IDL into the project.
4. Go to the project Build Settings dialog box and select IMyInterface.IDL in all of the project's configurations.
   
   Note Make sure that you have selected the file in all of your project's configurations.
5. For Versions of Visual C++ prior to 6.0, click the Custom Build tab and type the following entries, and then close the Build Settings dialog box:

      Description:
         Compiling Interface Definition
   
      Build Commands:
         midl.exe /ms_ext /char unsigned /c_ext /out "$(InputDir)"
         "$(InputPath)"
   
      Output File(s):
         $(InputDir)\$(InputName).h
         $(InputDir)\dlldata.c
         $(InputDir)\$(InputName)_i.c
         $(InputDir)\$(InputName)_p.c

   For version 6.0 and later, set the appropriate MIDL settings for IMyinterface.IDL in the MIDL section of either the Project/Settings dialog box or the Properties dialog box.
6. Add the file RPCHelp.c to your project by typing the following code:

         #pragma comment(lib, "rpcndr.lib")
         #pragma comment(lib, "rpcdce4.lib")
         #pragma comment(lib, "rpcns4.lib")
         #pragma comment(lib, "rpcrt4.lib")

   Note The file Rpcdce4.dll is no longer included with Microsoft operating systems. Its functions have been incorporated into the other DLLs. If you receive a link error on Rpcdce4.lib, remove its comment pragma from the Rpchelp.c file.
7. Add the following three files to your project:

   dlldata.c
   IMyInterface_i.c
   IMyInterface_p.c

   Since the files don't exist yet, you need to type the names in the File Name field on the Insert File dialog box. When you get the warning message that the file doesn't exist, click Yes.
   
   In Visual C++ .NET, the easiest way to do this is to click Add New Item on the Project menu for each file. In the Add New Item dialog box, click to select the Text File (.txt) template, and then type one of the names that is listed (make sure that you include the '.c' extension).
8. Use the following PSSample.def file as a model to create a DEF file for your project:

         LIBRARY     PSSAMPLE
         DESCRIPTION 'Sample custom interface marshaler'
         EXPORTS
            DllGetClassObject PRIVATE
            DllCanUnloadNow PRIVATE
            DllRegisterServer PRIVATE
            DllUnregisterServer PRIVATE

   Note Make sure you change the library name to match the name of your marshaler DLL.
9. Add your new DEF file to the project.
10. Open the project Build Settings dialog box (or Properties dialog box) and select your project (make sure that you select all configurations). Click the C++ tab and add the value REGISTER_PROXY_DLL to the Preprocessor Definitions field.
    
    In Visual C++ .NET, you must also add a Preprocessor definition that identifies the target operating system. The following definition will compile for Windows NT 4.0, Windows 2000, and later:

    _WIN32_WINNT=0x400

    For more information, see the "Targeting Stubs for Specific 32-bit or 64-bit Platforms" topic in the Platform SDK Books Online.
11. In the Custom Build Step section of the project properties (or settings), type the following values, and then close the Build Settings dialog box:

       Description:
          Registering Custom Interface Marshaler
    
       Build Command(s):
          regsvr32 /s /c "$(TargetPath)"
          echo regsvr32 exe. time > "$(OutDir)\regsvr32.trg"
    
       Output File(s):
          $(OutDir)\regsvr32.trg

Your project is now configured to properly build and register the custom interface marshaler.