HOWTO: Build Custom COM Interface Marshalers with Visual C++

• Microsoft Visual C++, 32-bit Editions, versions 4.0, 5.0, 6.0

SUMMARY

Custom COM interfaces that must be used cross-process or cross-apartment require marshaling support. This article explains how to set up a Visual C++ 4.0 project to build a self-registering standard marshaler from an IDL script that describes your custom interface.

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 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:

   
      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 

   Close the Build Settings dialog box.
   
   For version 6.0 and later, set the appropriate MIDL settings for IMyinterface.IDL on the MIDL tab of the Project/Settings 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") 




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.



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 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.



11. Click the Custom Build tab and enter the following values:

    
       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 

    Close the Build Settings dialog box.

REFERENCES

Visual C++ Help file; Topic: "Specifying Custom Build Tools"

Visual C++ InfoViewer; Topic: "SDKs\Win32 SDK\RPC"

Additional query words: MIDL marshal custom interface

Keywords : kbide kbVC kbVC400 kbVC500 kbVC600 TlsMisc
Version : winnt:4.0,5.0,6.0
Platform : winnt
Issue type : kbhowto