Visual Form Inheritance problem. “cannot inherit from (parent form). Contains a component that does not support inheritance”

3
votes

I have a base form with descendents. The base form has a TElRelDataSource on it (from ElPack v4.00). When I open the descendent form I get an error "cannot inherit from (parent form). Contains a component that does not support inheritance". This is with the ancestor forms open (there are two), to avoid the "ancestor for (parent form) not found" error.

Delphi 7 doesn't (AFAIK) support finalising classes, but this seems to be a standard error message. It's in the index for "Tomes of Delphi: Troubleshooting" but I don't have a copy of that and I'm reluctant to spend $US60 to read an entry like "this is why you should avoid VFI" (being pessimistic about the contents of the book). Other than that, no references in Google or Bing.

This is in code that I've recently inherited from someone who left a while ago and I'm the first person to change it since they left. Which means I can't ask how he created the offending form.

So, how do I get around this error message?

2
delphidelphi-7

2 Answers

5
votes

You got yourself into an interesting problem. First of all, you say the parent form already includes the offending DataSource, so you've got to ask yourself a question: How did the original developer manage to put the dataset on the parent form and then create child forms, if creating child forms is rejected by the datasource?

The original code didn't have the TElRelDataSource

This implies you're trying to add it your self. Don't add it, find an other solution that doesn't rely on the TElRelDataSource.

The original code did have it!

The original was probably compiled against a different version of TElRelDataSource, one that did allow inheritance. "Inherability" is controlled by the "csInheritable" component style. That is, if the component doesn't include "csInheritable" in ComponentStyle, the Delphi IDE will not allow visual inheritance. You have a number of options:

  • Use a different version of TElRelDataSource, maybe the original was compiled against a NEWER version of TElRelDataSource. If you are using the latest TElRelDataSource, the author probably decided it's code can't handle visual inheritance so he/she removed the csInheritable flag from ComponentStyle!
  • Try making your own derived component and set that style yourself. This is easily done, but if TElRelDataSource's author removed the csInheritable flag for a reason, you'll probably get into trouble.
  • Normally HeartWare's idea would work just fine; Unfortunately it's not that easy with a DATA SOURCE component: I assume you have DB-Aware controls linked to that data source. If you create the DataSource from code, you'll need to re-create the connections to data-aware controls as well, and that's going to take a lot of "if-s" (or clever use of RTTI).
  • You may try moving the DataSource to a TDataModule. This is probably more trouble then it's worth it.
3
votes

One option would be to remove the offending component from the form and allocate it dynamically in the FormCreate event. Depending on how many properties the (presumed) non-visual Data Source component has, this could be fairly simple.