Common Technical Questions and Answers to Delphi 3 and BDE/SQL Links 4.x - DCOM/Multi Tier/ActiveForms

DCOM/MULTI TIER/ACTIVEFORMS SECTION

Q: OnActivate and OnDeactivate events: Why doesn't the OnActivate and OnDeactivate fire for my ActiveForm?

A: The container (MSIE) takes care of (de)activation for you, so you'll never see these messages. Therefore you shouldn't do anything in your OnActivate/OnDeactivate events.

Q: OnDestroy event: Why doesn't the OnDestroy fire for my ActiveForm?

A: MSIE kills the OCX (ActiveForm) without letting it getting a chance of cleaning up properly. Therefore you shouldn't do anything in OnDestroy.

Q: Missing ProviderName in TClientDataSet: I don't see entries in the TClientDataSet ProviderName property. What's wrong?

A: On your server application, in the remote data module, right click your dataset (TTable, Tquery, etc) and select "Export [MyDataset] from data module". Rebuild the server application. You should now see the provider supplied in the list of the TClientDataset.

"Exporting from data module" adds a "Get_MyDataset" function to the protected section of the remote datamodule's CoClass that returns an IProvider interface.

Q: Propigating constraints from middle-tier to client: How do I propagate constraints from Middle-tier to Client?

A: This example uses the demos found in \Delphi 3\Demos\DB\CLIENTDS\Empedit.dpr and \server.dpr. Empedit.dpr represents the client tier, server.dpr runs on the middle tier, and the database, DBDEMOS, is located also on the middle tier, but in a real world scenario would be an SQL database running on yet a third machine. This example uses the Employee.db table. In order to use the following, you will need to set up DCOM and have the two examples running before adding the complication of constraints to the mix.
The general steps are

  1. Define constraints in the data dictionary
  2. Associate the data dictionary attributes to a TField
  3. Access the TField from a ClientDataSet.
First, define an attribute set in the data dictionary that includes a constraint. For instance, an attribute set called EmpNoA could have a CustomConstraint of "EmpNo > 100" and a ConstraintErrorMessage of "EmpNo is less than 100". Define static TFields in Server.dpr. Bring up the EmpServer datamodule and double click EmpQuery to bring up the fields editor. In the fields editor, right-click the EmpNo TField, select Associate Attributes, and select EmpNoA from the Associate Attributes dialog. Now when selecting the EmpNo TField in the fields editor you should see CustomConstraint and ConstraintErrorMessage matching the data dictionary entry. Open EmpEdit.dpr and add a TDBEdit to EmployeeForm. Set the DataField property to EmpNo and the DataSource property to EmpData (the TClientDataSet). Compile and run Server.dpr on the middle tier machine, then on the client machine run the EmpEdit example application. Attempt to post a record where EmpNo is greater than 100; your custom error message will appear.

Q: "DAX Error: Unexpected failure." error: How do I resolve this error?

A: This usually happens when the application server has raised an exception during initialization. Add exception handling to your server.

Q: Client launching: What do I need to do to make my client launch and access my server through DCOM?

A: One setup that usually work is if you install Service Pack 3 on all NT 4.0 machines. If you are going to use any Win95 clients then you'll have to install DCOM for Win95 on those machines. You'll also need Microsoft Internet Explorer 3.02 on the client machines. On the machine were the application server is going to run (an NT 4.0 machine) you'll do each of the following:

Q: "DAX Error: Server execution failed" error: What am I doing wrong and how do I resolve this error?

A: Make sure you followed the instructions in "What do I need to do to make my client launch and access my server through DCOM?". If you did and it still doesn't work, try running the server once manually.

Q: "DAX Error: Access is denied" error: What am I doing wrong and how do I resolve this error?

A: Make sure you followed the instructions in "What do I need to do to make my client launch and access my server through DCOM?". If you did and it still doesn't work, try running the server once manually.

Q: Old ActiveForm information displayed when updates have been made: I updated my ActiveForm and I hit my web page in Internet Explorer, but I get the old ActiveForm. What am I doing wrong?

A: Even if you use version info, you'll see this happening in some cases. Usually it goes away if you unregister the OCX (in C:\WINDOWS\OCCACHE\ or C:\WINNT\OCCACHE) using REGSVR32.EXE. The recommended way of doing this is:

  1. Exit Internet Explorer.
  2. Run "regsvr32.exe /u c:\windows\occache\MyActiveX.ocx"
  3. Restart Internet Explorer and hit the web page again.
If you deploy extra files that you ActiveX is using you might be helped by knowing that all the stuff is being pulled down to your OCCACHE directory and if Internet Explorer determines that there is a conflict - it'll put some files in OCCACHE\CONFLICT.x (x is a number) directories.

Q: What directory is the ActiveForm in: I need to know the exact directory that my ActiveForm is located in; How do I get that information?

A: GetModuleFilename(HInstance, ModuleName, SizeOf(ModuleName)). ModuleName can be declared as an array [0..255] of Char for instance. For your information:

If one of the above topics need to be elaborated on, or you do not see a simple question about Delphi 3 that is not answered above, send a short message to the BDE Administrator

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