Question

I have an application in Delphi 6, and due to library issues, I need to write a component in C# or C++ that will exist on a Delphi form. The program is currently Delphi and C#, and it calls C# for dialogs. I need something embedded into a TForm that will allow me to draw that component in C#.

I think I can create a component in Delphi and during its Paint routine I just call the function in the DLL. But what do I pass to the C# DLL so that it can paint in that window, and how would I get C# to actually do it?

I guess just being able to paint in some arbitrary DC on demand in C# is all I need to do.

Answer 1

C#

public static void DoIt(IntPtr srcWindow)
{
Graphics g = Graphics.FromHwnd(srcWindow);
g.DrawLine(new Pen(Color.Red), new Point(0, 0), new Point(400, 400));
}

Delphi:

type
    intPtrArgs = procedure(hand : hwnd); StdCall;

procedure T_CS.RePaint;  //extends TWinControl
var
  Hm: HModule;
  ExtDoIt: intptrArgs;  
begin
  Hm := LoadLibrary(pchar('my.dll'));
  try
    @ExtDoIt := GetProcAddress(Hm, 'DoIt');
    ExtDoIt(Handle);
  finally
    FreeLibrary(Hm);
  end;
end;

It’s silly to load an unload on each paint, but for example purposes it works.

For anyone else trying to do this, you will need to google “Reverse P/Invoke” to be able to see the C# DLL procedure in another language.

Example Reverse P/Invoke
http://www.blong.com/Conferences/BorConUK2002/Interop1/Win32AndDotNetInterop.htm