Hooking a DLL function in Delphi

Question

I have a coclass which checks the registry to determine if an application is installed, but it does this poorly and doesn't find newer versions of the application. In cases where a competing application has been installed, it will try to open that one. If the competing application has been uninstalled, the program will crash. This coclass is defined in a DLL file I do not have the source code for the library, so I can't just change that. I have been looking into using hooking to replace the function with one that works, but it seems complicated when I look at the MSDN documentation on using SetWindowsHookEx. Can someone please provide an example of how to use SetWindowsHookEx or another method of hooking into Windows?

Thank You

EDIT: I would like to note that I accepted the answer I did because it worked for me. I could not use the other answer at the time the question was asked, but it looks like its just as good.

Answer 1

Here is a short example from my own codebase which shows the most basic hooking technique:

unit MethodHooker;

interface

implementation

uses
  SysUtils, Windows, Classes;

procedure Patch(Address: Pointer; const NewCode; Size: Integer);
var
  NumberOfBytes: DWORD;
begin
  WriteProcessMemory(GetCurrentProcess, Address, @NewCode, Size, NumberOfBytes);
end;

type
  PInstruction = ^TInstruction;
  TInstruction = packed record
    Opcode: Byte;
    Offset: Integer;
  end;

procedure Redirect(OldAddress, NewAddress: Pointer);
var
  NewCode: TInstruction;
begin
  NewCode.Opcode := $E9;//jump relative
  NewCode.Offset := Integer(NewAddress)-Integer(OldAddress)-SizeOf(NewCode);
  Patch(OldAddress, NewCode, SizeOf(NewCode));
end;

function GetCursorPos(var lpPoint: TPoint): BOOL; stdcall;
(* The GetCursorPos API in user32 fails if it is passed a memory address >2GB which
   breaks LARGEADDRESSAWARE apps.  We counter this by calling GetCursorInfo instead
   which does not suffer from the same problem. *)
var
  CursorInfo: TCursorInfo;
begin
  CursorInfo.cbSize := SizeOf(CursorInfo);
  Result := GetCursorInfo(CursorInfo);
  if Result then begin
    lpPoint := CursorInfo.ptScreenPos;
  end else begin
    lpPoint := Point(0, 0);
  end;
end;

initialization
  if not ModuleIsPackage then begin
    if not CheckWin32Version(6, 1) then begin
      //this bug was fixed in Windows 7
      Redirect(@Windows.GetCursorPos, @MethodHooker.GetCursorPos);
    end;

end.