DCOM using Visual Basic
DS420, Fall 1998, Dr. Clark Elliott, Depaul University
Project: Bangalore,  Student: Suresh Sreerama
Exam Questions The visual basic code

Introduction

As part of exploring the capabilities offered by Visual Basic for developing DCOM applications, I have implemented the class DCOM assignment in Visual Basic (VB).  This document attempts to document the steps followed and the things that happen behind the scenes in VB.

Development Environment

Operating System Windows NT 4.0 Server, Service Pack 3.0
Hardware Intel Dual Pentium P200, 128MB RAM
Language/Tools Microsoft Visual Basic 6.0
Browser Internet Explorer 4.72.xx

Result

The three parts of the class DCOM Assignment TheServer, ThePusher, TheControl have been implemented in VB.  Currently the implementation has been tested on a single machine (across processes).

In terms of COM/DCOM development, Visual Basic looks like a good tool for prototyping COM/DCOM designs and seems more ideal for rapidly developing windows graphical controls.

References

1.    Microsoft's COM site
2.    Visual Basic Books Online
3.    Inside COM - Dale Rogerson
4.    HardCode Visual Basic - Bruce McKinney
5.    DS 420, Fall DCOM Assignment, Dr. Clark Elliott's Class Notes/Handouts

TheServer

The sole purpose of the TheServer in life is to track a single long value.  TheServer is a DCOM server (created via File->New Project->ActiveX EXE) that supports the following interfaces:

IDataServer

  • UpdateData()
 The UpdateData() is invoked by external applications (for ex. ThePusher) that  change the value, that is served by the Data Server.
IConnectionMgr
  • Connect()
  • Disconnect()
 This interface has two methods Connect() and Disconnect(), which can be called by the consumer DCOM applications or Controls (TheControl).  As the names suggest, the Connect() and Disconnect() methods serve to establish and terminate a connection with the Server
IDataConsumer
  • OnDataChange
This is an Interface that must be implemented by the consumer DCOM applications or Controls that intend to consume the data served by the TheServer
The Class TheServerComObject (TheServerComObject.cls) implements the interfaces IDataServer and IConnectionMgr.

Declaring Interfaces in VB

To create an interface in VB, you simply create a class module (vis Project->Add Class Module) with empty methods and properties.  For example, the IDataServer interface is declared as follows:

                        Function UpdateData(lValue As Long) As Long
                        End Function

Implementing an Interface in VB

In the class that you chose to implement an interface, just type Implements <my interface name> and Visual Basic updates the Code Window with an entry for the interface.  For example, the TheServerComObject has the following two implement statements:

                    Implements IDataServer
                    Implements IConnectionMgr

Debugging

I did not use any sophisticated debugging aids other than the MsgBox() API.  There is an option in the project properties to make the DCOM server run in Unattended Execution mode.  When the DCOM server is set to run in the unattended mode the MsgBox() API (which throws up a dialog box) has no effect.   Therefore, the calls to MsgBox() API have been left intact for future debugging.

server.gif (43570 bytes)

The Pusher

ThePusher is a regular VB Windows application. (Select File->New Project->Standard Exe)

Upon Startup (In the Form_Load Event) the ThePusher application creates an instance of the IDataServer Interface by calling the VB operator New()   API (Use could also use the CreateObject() API). 

The Pusher application sports a edit box and a button that invokes a method UpdateData(lNewValue) in the IDataServer interface supported by the DataServerApplication.

The following brief list of operations/funtions performed by the VB Pusher applicaton and the corresponding ones that were used in the C/C++ DCOM assignment:

Function/Operation VB C/C++
Creating an Instance of the TheServerComObject: (Or  for IUnknown)  Used the New() Operator.  Ex.

Set m_TheServer = New TheServerComObject

VB does the equivalent of CoCreateInstance() behind the scenes.

 Used CoCreateInstance() API
Checking for Success/Failure Compare the return value of New() with Nothing.  Ex.

      If m_TheServer Is Nothing Then
                 ' Failure
      Else
                 ' Success
      End If

 Compare the returned HRESULT for S_OK.
Query Interface for the IDataServer Interface Use the Set statement and VB performs the QueryInterface() behind the scenes.  Ex.

      Set m_IDataServer = m_TheServer

 Invoke QueryInterface() or use ATL's CComPtr<T> wrapper class.

pusher.gif (17878 bytes)

TheControl

TheControl is an ActiveX control (created via File->New Project->ActiveX DLL).  This control implements the IDataConsumer interface and makes use of the IConnectionMgr interface implemented by TheServer.  The IConnectionMgr implementation mimics the behaviour of the IConnectionPoint interface that the class implemented in the C++ version of the TheServer.  TheControl (in response to the Connect to Server button) passes an instance of itself to the TheServer (via Connect(....) method.

Whenever the value in the Pusher application is changed the new value is reflected in the ActiveX control hosted by IE 4.0. The control looks in IE 4.0 as shown below:

control.gif (24238 bytes)