1 Loading Mz COM
3 Methods
4 Events
5 Errors
6 Evaluation thread
7 Acknowledgments
Version: 4.1

MzCOM: Scheme as a Windows COM Object

MzCOM.exe is a Windows COM (i.e., Component Object Model) class wrapper for PLT Scheme.

During normal installation of MzCOM, the executable is registered as a COM object automatically. If you move the PLT Scheme installation folder, re-register MzCOM.exe with

  mzcom.exe /RegServer

The MzCOM.exe executable find DLLs and PLT Scheme library collections relative to its own path.

1 Loading MzCOM

To load a COM object, COM hosts require a COM class name or a ProgID. MzCOM has the class name "MzObj Class" and the ProgID "MzCOM.MzObj.version", where ‹version› is 4.1.

In the Visual BASIC 6 environment, from the Project|References (VB6), check MzCOM 1.0 Type Library. In Visual BASIC .NET, choose Project|Add Reference, and from the COM tab, select MzCOM 1.0 Type Library. In your code, declare a variable, then assign to it:

  DIM schemeObject AS MzObj

  SET schemeObject = NEW MzObj

From Visual C++:

  #include "mzcom.h"


  CLSID clsid;

  IMzObj *pIMzObj;




  CoCreateInstance(clsid,NULL,CLSCTX_SERVER,IID_IMzObj, (void **)&pIMzObj);

where <version> is the version number. You’ll need the definition of IID_IMzObj (see GUIDs). The header file "mzcom.h" is generated as "src\worksp\mzcom\" when building from the PLT Scheme source distribution. The above C/C++ code is for illustration; your actual code should check return values, of course.

Using mysterx to manipulate COM objects within Scheme, you can load MzCOM with either

  (cci/coclass "MzObj Class")


  (cci/progid "MzCOM.MzObj.<version>")

Consult your documentation for loading MzCOM into other COM environments. MzCOM is compiled as a “dual-mode” class, meaning its methods may be called directly or by using OLE Automation.


When compiled from the PLT Scheme source distibrution, the directory "src\worksp\mzcom\" contains the file "MzCOM_i.c" that contains GUIDs for MzCOM. Those GUIDs are as follows:

  const IID IID_IMzObj =



  const IID LIBID_MZCOMLib =



  const IID DIID__IMzObjEvents =



  const CLSID CLSID_MzObj =


which represent the IMzObj interface, the MzCOM type library, the IMzObjEvents interface, and the MzObj class, respectively.

3 Methods

MzCOM support three COM methods:

4 Events

MzCOM supports a single event.

5 Errors

When an error occurs in MzCOM, it creates a COM error object. C and C++ clients can use GetErrorInfo to retrieve error information. Clients implemented in other languages typically have some equivalent means to obtain COM error information.

6 Evaluation thread

The PLT Scheme evaluator runs in a Win32 thread created when MzCOM is loaded. If an expression kills the primary MzScheme thread, as in

  (kill-thread (current-thread))

then the evaluator Win32 thread is also killed. When that happens, subsequent calls to Eval() will fail.

7 Acknowledgments

MzCOM was developed in response to a query by Andre Van Meulebrouck. Andre also did extensive testing with Visual BASIC.