How To Create a DCOM Client/Server with Events by Using Visual Basic

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Article ID: 267836 - View products that this article applies to.
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SUMMARY

This article shows you how to create, package, and deploy a Distributed Component Object Model (DCOM) client/server application with events by using Visual Basic. To create a DCOM client/server application, you need the Enterprise Edition of Visual Basic. Microsoft assumes that the reader is already familiar with creating client/server applications with events that run on the same computer.

MORE INFORMATION

You do not need to change your code to enable a client application to instantiate a remote server by using DCOM. The difference is the way in which you package and deploy the client. In addition, there are some security settings you need to make after the client and the server are installed. You can make these settings by using an utility called DCOMCNFG. In particular, when your server raises events, not only are the server settings important, but the client ones are too.

The following steps show you how to distribute and configure a simple client/server application. Call the server DCOMDemoEvents_Svr and the client DCOMDemoEvents_Cli. Create a separate folder for each of them and, for the purpose of this article, name the folders c:\DCOMDemoEvents\Server, and c:\DCOMDemoEvents\Client.

Create the Server

  1. Start a new Visual Basic Project. In the New Project dialog box, select ActiveX EXE, and then click Open. Class1 is created by default.
  2. Add the following code to the Class1 module:
    Option Explicit
    
    Public Event TimeUpdate(sTime As String)
    
    Public Sub CallMeBack()
        RaiseEvent TimeUpdate(Time())
        
    End Sub
    
    Public Function ServerTime() As String
        
        ' this method was included just in case we need
        ' to test a method without call back.
        ServerTime = Time
        
    End Function
    					
  3. On the Project menu, click the Project Properties option.
  4. Select the General tab.
  5. In the Project Name field, type DCOMDemoEvents_Svr.
  6. In the Project Description field, type DCOMDemoEvents_Svr - Server.
  7. Select the Component tab, and then select the Remote Server Files option.

    NOTE: Selecting this option makes the Visual Basic compiler generate the VBR and TLB files needed for packaging the client applications that use this server. The files contain registry entries that need to be included on the client computer.
  8. Close the Project Properties dialog box.
  9. On the File menu, select Save As, and then save this project to the c:\DCOMDemoEvents\Server folder.
  10. On the File menu, select Make DCOMDemoEvents_Svr and compile the server.
  11. On the Project menu, select the Project Properties option, and then select the Component tab. On the Component tab, select Version Compatibility, select the Binary Compatibility option, and then make the project binary compatible with the server's executable file you just created (DCOMDemoEvents_Svr.exe). By making this option, you are assuring that all GUIDs are kept the same if you recompile the server.

Create the Client

  1. On the File menu, select the New Project option, select Standard EXE, and click OK. Form1 is created by default.
  2. On the Project menu, click the Project Properties option, and then select the General tab.
  3. In the Project Name field, type DCOMDemoEvents_Cli.
  4. In the Project Description field, type DCOMDemoEvents_Cli Project - Client, and then click OK to close the Project Properties dialog box.
  5. On the Project menu, select References, and add a reference to DCOMDemoEvents_Svr - Server.
  6. In the General Declarations section of Form1, include the following line:
    Dim WithEvents MyEventObj As DCOMDemoEvents_Svr.Class1
    					
    This declares the server object with events. After typing in this line, you should see MyEventObj in the list of objects for this form. The list of objects of the form is found in the upper-left-hand combo box in the form's code module.

  7. From the list of objects of the form, select MyEventObj. The empty Sub MyEventObj_TimeUpdate is created. Type in the following line of code:
    MsgBox "Event from Server received. Time=" & sTime
    					
  8. Place a command button on Form1, and change the button's caption to Run with Events.
  9. Place the following code in the button's click event:
        On Error GoTo err1
        
        Set MyEventObj = CreateObject("DCOMDemoEvents_Svr.Class1")
        MyEventObj.CallMeBack
        Exit Sub
    
    err1:
        MsgBox "Error: " & Err.Number & " - " & Err.Description
    					
  10. Place a second command button on Form1, and change the button's caption to Run without Events.
  11. Place the following code in the button's click event:
        Dim MyObj As DCOMDemoEvents_Svr.Class1
        
        On Error GoTo err1
        
        Set MyObj = CreateObject("DCOMDemoEvents_Svr.Class1")
        MsgBox "Server time without events: " & MyObj.ServerTime
        Exit Sub
    err1:
        MsgBox "Error: " & Err.Number & " - " & Err.Description
    					
  12. Once you have done this, the form's code should look like the following:
    Option Explicit
    Dim WithEvents MyEventObj As DCOMDemoEvents_Svr.Class1
    
    Private Sub Command1_Click()
    
        On Error GoTo err1
        
        Set MyEventObj = CreateObject("DCOMDemoEvents_Svr.Class1")
        MyEventObj.CallMeBack
        Exit Sub
    err1:
        MsgBox "Error: " & Err.Number & " - " & Err.Description
           
    End Sub
    
    Private Sub Command2_Click()
        Dim MyObj As DCOMDemoEvents_Svr.Class1
        
        On Error GoTo err1
        
        Set MyObj = CreateObject("DCOMDemoEvents_Svr.Class1")
        MsgBox "Server time without events: " & MyObj.ServerTime
        Exit Sub
    err1:
        MsgBox "Error: " & Err.Number & " - " & Err.Description
        
    End Sub
    
    Private Sub MyEventObj_TimeUpdate(sTime As String)
        MsgBox "Event from Server received. Time=" & sTime
    
    End Sub
    					
  13. On the File menu, select Save As, and then save the project to the client's folder, c:\DCOMDemoEvents\Client.
  14. Press the F5 button to run the client in the IDE and test it out. Be aware that this test is local to the development computer at this point. It is running the local ActiveX EXE Server.
  15. On the File menu, select Make DCOMDemoEvents_Cli to compile the client, and then close Visual Basic.

Package the Server

Use the Package and Deployment Wizard to package your server for distribution as usual. The server is instantiated by a remote client using DCOM. While creating the package for your server, you get a dialog box asking you if this server will be used as a Remote Automation server and if you want to include support files for this purpose. Just click NO because DCOM is not Remote Automation. Remote Automation is an older technology which was replaced by DCOM.

Package the Client

When you package the client, there are some specific steps that need to be taken, considering that the server does not run on the same computer as the client. The changes made to the client's package assure that only the type library (.tlb file) is installed and some additional registry entries are included instead of the server's executable on the client's computer. You do not need the server's executable installed in the client's computer because it is not going to run there.

Package the client following these steps:
  1. Start the Package and Deployment Wizard, and then select the Client's project. Click the Package button.
  2. In the Package Type dialog box, select Standard Setup Package, and then click Next.
  3. In the Package Folder dialog box, select the folder where the package is stored, and then click Next. In this case, it is c:\DCOMDemoEvents\Client\Package.

    NOTE: You may get a dialog box saying that there is no dependency information for your server. Click OK because your server doesn't have any dependencies.

    You should now be in the Included Files dialog box.

    Deselect the server's executable file, DCOMDemoEvents_Svr.exe. You do not want to distribute the server's executable.
  4. Click the Add button, and change the Files of Type combo box to Remote Server Files (*.vbr).
  5. Point to the folder where you have your Server's project, in this case, c:\DCOMDemoEvents\Server, and then select the related VBR file DCOMDemoEvents_Svr.VBR.
  6. Click Open and the Add File dialog box closes. Note that two files are included, DCOMDemoEvents_Svr.VBR and DCOMDemoEvents_Svr.TLB. Click Next.
  7. In the Remote Servers dialog box, define the name of the computer (Net Address) where the server will be running.

    Usually you keep this field blank because you may not know in advance where the server is installed. If you keep it blank, you are prompted for it when you install the client. For this example, keep it blank.

    Click Next to proceed. You can now proceed with the standard procedures for Package and Deployment Wizard. In our example, click Next to all remaining dialog boxes.

Install the Server

Install the server on the computer on which you would like to run it, using the distribution package you created earlier. If you want to use your development computer to run the server, you don't need to install it because Visual Basic makes the registration for you when it compiles the server.

Install the Client

Install the Client on the computer on which you would like to run it, using the distribution package you created earlier. Because this client uses a DCOM server and you left the actual location of the server blank when you created the distribution package, you now need to provide this location. When Setup prompts you for this location, provide the name of the computer where you installed the server.

Set the Server's Security

If you installed the server on a Microsoft Windows NT or Microsoft Windows 2000 computer, you need to configure security for it. Do this by using DCOMCNFG, as shown in the following steps, which assume that both client and server computers are part of a domain, and the user logged on to the client computer is logged on as a domain user. The suggested settings are just one possible configuration. They are very generic and give wide access to the server. Remember that this is just an example. When you deploy your real applications, and security is a concern for your environment, you should select more restrictive options. Also, if the computer you are using to test this example server is used to run other servers, make a note of the current settings before you make the following changes, and return to the original settings as soon as you are done with your tests.
  1. On the server computer, click the Start button, and then select Run. In the Run dialog box, type DCOMCNFG, and then click OK. You need to have Administrator rights to be able to run DCOMCNFG.
  2. Select the Default Properties tab, and verify that Enable Distributed COM on this computer is checked.
  3. Set the Default Authentication Level to Connect, and set the Default Impersonation Level to Identify.
  4. Select the Default Security tab.
  5. Click the Edit Default button in the Default Access Permissions panel.
  6. Verify that Everyone and System are included in the list with Allow Access rights. If they are not, you can use the Add button to add them to the list. Click OK when the list is complete.
  7. Click the Edit Default button in the Default Launch Permissions panel.
  8. Verify that Everyone and System are included in the list with Allow Launch permissions. If they are not, use the Add button to add them to the list. Click OK when the list is complete.
  9. Select the Applications tab, highlight your server, DCOMDemo_Svr.Class1, and then click the Properties button.
  10. Select the General tab, set the Authentication Level to Default, and then select the Location tab. The only option checked should be Run application on this computer.
  11. Select the Security tab and verify that the Use default access permissions and the Use default launch permissions options are checked.
  12. Select the Identity tab, check The launching user option, click OK to close the Server's Properties dialog box, and then click OK again to close DCOMCNFG. As you can see, the test server uses all of the default settings. When deploying your own servers, you should define settings specific to your application. All custom settings take precedence over the default ones.

    You are now ready to test your server. On the client computer, launch the client, and then click the Run button. You should see a message box indicating the server's time. If you are unable to test this sample successfully, please see the troubleshooting article, Q269330, listed in the "References" section.

Set the Client Security

Because the DCOM server exposes events that the client application consumes, you must also set DCOM permissions on the client computer to allow the server to make callbacks to the client. If your DCOM server does not expose events, or if your client application does not consume me events, you do not have to perform this step.
  1. On the client computer, click Start, and then click to select Run.
  2. In the Run dialog box, type DCOMCNFG, and then click OK.

    You must have administrator rights to run DCOMCNFG.
  3. Click the Default Properties tab, and then verify that Enable Distributed COM on this computer is checked.
  4. Set the Default Authentication Level to Connect, and then set the Default Impersonation Level to Identify.
  5. Click the Default Security tab.
  6. Click the Edit Default button in the Default Access Permissions panel.
  7. Verify that Everyone and System are included in the list with Allow Access Rights. If they are not included in the list, use the Add button to add them to the list. Click OK when the list is complete.

REFERENCES

For additional information regarding DCOM client/server applications, click the article numbers below to view the articles in the Microsoft Knowledge Base:
266717 How To Create a DCOM Client/Server Application by Using Visual Basic
268550 How To Use DCOMCNFG for a Visual Basic DCOM Client/Server Application
182248 How To Use DCOMCNFG (DCOMCNFG.EXE) with Windows 95 and Windows 98
269330 How To Troubleshoot DCOM for Visual Basic Client/Server Applications
183607 How To Configure DCOM for Visual Basic Using DCOMCNFG.EXE

Properties

Article ID: 267836 - Last Review: June 29, 2004 - Revision: 3.1
APPLIES TO
  • Microsoft Visual Basic 6.0 Enterprise Edition
  • Microsoft Visual Basic 6.0 Professional Edition
Keywords: 
kbhowto kbdcom kbdevsecurity KB267836

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