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


This article shows you how to create, package, and deploy a Distributed Component Object Model (DCOM) client/server application 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 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.

The following steps show you how to distribute and configure a simple client/server application. Name the server DCOMDemo_Svr, and name the client DCOMDemo_Cli. Create a separate folder for each of them. For the purpose of this article, call these folders c:\DCOMDemo\Server and c:\DCOMDemo\Client.

Create the Server

  1. Start a new Visual Basic Project. On 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:
    Public Function ServerTime() As String
    ServerTime = Time
    End Function
  3. On the Project menu, click the Project Properties option, and then select the General tab.
  4. In the Project Name field, type in DCOMDemo_Svr.
  5. On the Project Description field, type in DCOMDemo_Svr - Server. Check the Unattended Execution option.

    NOTE: This option should always be checked in servers that don't have any UI to assure that no dialog boxes of any type are displayed while the server is running. If you have any type of user interaction while your server is running under an identity that is not the Interactive User, your server may appear to hang.
  6. Select the Component tab, and check the Remote Server Files option.

    NOTE: Checking this option makes the Visual Basic compiler generate the VBR and TLB files needed for packaging the client applications that use this server. These files contain registry entries that need to be included on the client computer.
  7. Close the Project Properties dialog box.
  8. On the File menu, select Save As, and then save this project to the c:\DCOMDemo\Server folder.
  9. On the File menu, select Make DCOMDemo_Svr and compile the server.
  10. On the Project menu, select the Project Properties option, and then select the Component tab.
  11. 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 (DCOMDemo_Svr.exe). By selecting 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 then 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 in DCOMDemo_Cli.
  4. In the Project Description field, type in DCOMDemo_Cli Project - Client.
  5. On the Project menu, select References. From the list of available references, select DCOMDemo_Svr - Server.
  6. Place a command button on Form1, and change the button's caption to Run.
  7. Place the following code in the button's click event:
        Dim MyObj As DCOMDemo_Svr.Class1

    On Error GoTo err1

    Set MyObj = CreateObject("DCOMDemo_Svr.Class1")
    MsgBox "Server Time=" & MyObj.ServerTime & " Client Time=" & Time

    Exit Sub
    MsgBox "Connection failed: Error " & Err.Number & " - " & Err.Description

  8. On the File menu, select Save As, and then save the project to the client's folder c:\DCOMDemo\Client.
  9. Press the F5 key to run the client in the IDE and test it out.
  10. On the File menu, select Make DCOMDemo_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 the No button, because DCOM is not Remote Automation. Remote Automation is an older technology that was replaced by DCOM.

Package the Client

When packaging 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 just the type library (.tlb file) is installed and some additional registry entries are included instead of the server's executable, which is not necessary on the client's computer because it's not going to run there.

Package the client by using the following steps:

  1. Start Package and Deployment Wizard, and then select the Client's project.
  2. Click the Package button. In the Package Type dialog box, select Standard Setup Package, and then click Next.
  3. In the Package Folder dialog box, select the folder to store the package, and then click Next. In this case, it is c:\DCOMDemo\Client\Package.

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

    You should now be in the Included Files dialog box.
  4. Deselect the server's executable file, DCOMDemo_Svr.exe, because you do note want to distribute the server's executable, and then click the Add button.
  5. Change the Files of Type combo box to Remote Server Files (*.vbr).
  6. Point to the folder where you have your Server's project (in this case c:\DCOMDemo\Server), and select the related VBR file, DCOMDemo_Svr.VBR. Click Open, and the Add File dialog box closes. Notice that two files are included, DCOMDemo_Svr.VBR and DCOMDemo_Svr.TLB. Click the Next button.

    NOTE: In the Remote Servers dialog box you can define the name of the computer (Net Address) where the server is running. Usually you keep this field blank because you may not know in advance where the server will be installed. If you keep it blank, you are prompted for it when you install the client. For this sample, keep it blank.
  7. Click Next to proceed. You can now proceed with the standard procedures for Package and Deployment Wizard. In this case, just 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 that is 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.


For additional information regarding DCOM client/server applications, click the article numbers below to view the articles in the Microsoft Knowledge Base:

267836 How To Create a DCOM Client/Server with Events by Using Visual Basic
268550 How To Use Dcomcnfg for a Visual Basic DCOM Client/Server Application
182248 How To Use Dcomcnfg with Windows 95 or Windows 98
269330 How To Troubleshoot DCOM for Visual Basic Client/Server Applications
161837 How To Create a DCOM Client/Server Application with VB5

Artikelnummer: 266717 – Letzte Überarbeitung: 19.06.2014 – Revision: 1