Delete TDataModule but keep TADOConnection intact

Question

I have a lib with some TDataModules who share a TADOConnection. I create and delete datamodules in some applications.

When I delete a datamodule, I get an EAccessViolation error. I think this is due to the fact that the datamodule wants to delete the TADOConnection, which is shared.

I tried setting the tdatamodule->tbquery->Connection property to NULL when the destructor is called, without any luck.

Why do I think the error resides in TADOConnection? Because when I build my application without a lib, I can create and delete datamodules without any problems. And when I create a lib with datamodules who have their own connection, I have no problems either.

Any help? Thanks in advance!

The error: http://oi60.tinypic.com/noyc6x.jpg

The call stack: http://oi61.tinypic.com/sgljx5.jpg

Answer 1

The solution is adding borlndmm.dll to the project folder of the application using the libraries or dll's.

From the dll/lib projects in Borland C++ Builder:

//---------------------------------------------------------------------------
//   Important note about DLL memory management when your DLL uses the
//   static version of the RunTime Library:
//
//   If your DLL exports any functions that pass String objects (or structs/
//   classes containing nested Strings) as parameter or function results,
//   you will need to add the library MEMMGR.LIB to both the DLL project and
//   any other projects that use the DLL.  You will also need to use MEMMGR.LIB
//   if any other projects which use the DLL will be performing new or delete
//   operations on any non-TObject-derived classes which are exported from the
//   DLL. Adding MEMMGR.LIB to your project will change the DLL and its calling
//   EXE's to use the BORLNDMM.DLL as their memory manager.  In these cases,
//   the file BORLNDMM.DLL should be deployed along with your DLL.
//
//   To avoid using BORLNDMM.DLL, pass string information using "char *" or
//   ShortString parameters.
//
//   If your DLL uses the dynamic version of the RTL, you do not need to
//   explicitly add MEMMGR.LIB as this will be done implicitly for you
//---------------------------------------------------------------------------

Answer 2

The TDataModule only frees what it owns. If you are sharing a single TADOConnection in multiple TDataModule instances then they cannot all own it. However, maybe you are freeing the Owner and not informing the other instances that the TADOConnection is being freed. The VCL has a mechanism for that purpose - the TComponent::FreeNotification() method. Your TDataModule objects can override the virtual Notification() method and then call FreeNotification() on the TADOConnection. That way, if the TADOConnection is ever freed, any TDataModule that is using it can act accordingly, such as setting their local pointer to NULL so they know the TADOConnection is gone.