Derive from an Class in a DLL

[Sergey K.]

I've implemented this kind of thing once via FPC. The basic idea is that you patch a VMT entry of the instantiated class. Code goes like this:

Function tObjectsLinker.IntroduceVMT(VMT:Pointer):String;
Var ParentVMT:Pointer;
    ParentClass:tLClass;
Begin
   While Assigned(VMT) Do Begin
      ParentVMT:=tClass(VMT).ClassParent;
      If Not Assigned(ParentVMT) Then Exit;
      If FindClass(tStaticClass(ParentVMT).ClassName,ParentClass) Then Begin
         If ParentClass.PackageID=CORE_PACKAGE Then Begin
            Logger.Log('Class '+tStaticClass(VMT).ClassName+' expands '+ParentClass.ClassName);
            pPointer(VMT+vmtParent)^:=ParentClass.StaticClass;    // do injection
            Exit(ParentClass.ClassName);
         End;
      End;
      VMT:=tClass(VMT).ClassParent;
   End;
   Logger.Fatal('Unable to determine class ancestor');
End;

Procedure tObjectsLinker.IntroduceClass(Var LClass:tLClass);
Var ParentVMT:Pointer;
//    VMT:Pointer;
//    ParentClass:tLClass;
Begin
   If LClass.PackageID=CORE_PACKAGE Then Begin
      // don't touch Core classes
      // FIXME: 
      ParentVMT:=LClass.StaticClass.ClassParent;
      If Assigned(ParentVMT) Then LClass.ExpandsClass:=tStaticClass(ParentVMT).ClassName Else
         LClass.ExpandsClass:='';
      Exit;                       
   End;
   LClass.ExpandsClass:=IntroduceVMT(LClass.StaticClass);
End;

You could find my solution at http://www.linderdaum.com in old downloads