The Detour library is an amazing way to mock the Delphi routines or apply run-time redirection of a function calls inside of the executable module. The library can be found at the following location: https://github.com/MahdiSafsafi/delphi-detours-library.

Using this library you can hook not only the Delphi routines but also the WinAPI calls! This is one of the possibilities to test the code which uses the system calls.

For example if we have the following code we want to “test”:

function Victim: String; begin Result := GetCurrentDir; end;

we can write following small test project to do that:

program DetourTest; {$APPTYPE CONSOLE} uses DDetours, System.SysUtils; function Victim: String; begin Result := GetCurrentDir; end; function VictimMock: String; begin Result := 'C:\Your\Test\Path' ; end; function TestVictim: String; var P: Pointer; begin P := InterceptCreate(@Victim, @VictimMock); try Result := Victim; finally InterceptRemove(P); end; end; begin Writeln(Victim); Writeln(TestVictim); end.

The output of the program above will be:

C:\Users\Admin\Documents\Embarcadero\Studio\Projects\Win32\Debug C:\Your\Test\Path

The main idea there is not about the thing that we test GetCurrentDir routine, but the thing that we can mock the call to it and give the return value we need for our purposes.

This library works like a charm with Delphi Berlin 10.2 Update 2 – haven’t tested with any version before.

More detailed documentation about the hooking you can find at the author’s documentation page: https://github.com/MahdiSafsafi/delphi-detours-library/blob/wiki/DDetours.md.