MzCOM: Racket as a Windows COM Object

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

During normal installation of MzCOM, the executable is registered as a COM object automatically. If that registration fails or if you move the Racket installation, re-register MzCOM.exe with

  ‹installation›\lib\MzCOM.exe /RegServer /v

The MzCOM.exe executable will find DLLs and 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 7.3.

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:

From Visual C++:

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 Racket 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 Racket, you can load MzCOM with either

(cci/coclass "MzObj Class")

or

(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.

2 GUIDs

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

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

3 Methods

MzCOM support three COM methods:

• void About()

  Takes no arguments and displays an informational dialog.

• BSTR Eval(BSTR input)

  Takes and returns strings (specifically, BSTRs). The returned value is the result of evaluating the input expression, formatted as a string. The input string may contain several S-expressions. The embedded Racket updates its environment with each evaluation. Therefore, it is possible to define procedures in a call to Eval, and use the procedures in subsequent calls.

• void Reset()

  Resets the Racket environment to the initial environment. Also, the custodian for the primary Racket thread is invoked, shutting all its managed values.

4 Events

MzCOM supports a single event.

• SchemeError()

  Passed a string (specifically, a BSTR) that explains the error.

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 Racket evaluator runs in a Win32 thread created when MzCOM is loaded. If an expression kills the primary Racket 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.