How to get instance of TForm from a Handle?

Question

I'm converting some functions into a DLL which relate to Windows 7 functionality. I can't pass a TForm through DLL, so I need to pass its handle instead. except, once I have that handle on the other side, how do I reconstruct it back into a TForm instance? Also, what's the appropriate way to pass the handle (HWND) through a Delphi DLL to be compatible to call from C# for example?

If not possible, then I at least need to know how to change the color of a window using windows API only, no reference to the TForm. The goal is that from within this DLL, it needs to somehow change the color of a Form. Passing the handle to the DLL is no problem, but how to use that handle to work with the form that the handle represents?

What I'm doing specifically is putting together a single DLL that contains everything needed to make a Delphi7 application compatible with Windows7, for example, drawing glass, properly showing multiple forms in the taskbar (and minimizing forms), showing the green progress in the taskbar's icon, and whatever else might be involved. This type of work though requires modifying the form. I need to be able to do those modifications from within the DLL.

Answer 1

In general, you can convert an HWND to a VCL TWinControl-derived object pointer using the VCL's FindControl() function in the Controls unit. You can then check if the TWinControl is actually a TForm using the is operator.

However, as others have stated, passing VCL objects over the DLL boundary in general is dangerous and can cause problems if both EXE and DLL are not compiled with the exact same VCL version, RTL version, memory manager, etc. To pass VCL objects over the DLL boundary safely, change your DLL project into a BPL Package project, and make sure Dynamic RTL is enabled in both EXE and BPL.