type
  IResource = interface
    function GetResType: Integer;

    property iResType: Integer
      read GetResType
  end;

type
  TBaseRes = class(TInterfacedClass)
    function GetInterface: IResource; virtual; abstract;
  end;

// Res1

type
  TRes1 = class(TBaseRes, IRes)
    function GetInterface: IResource; override;
    function GetResType: Integer;
  end;

function TRes1.GetInterface: IResource;
var
  IntRes: IResource;
begin
  IntRes:= Self;
  Result:= IntRes;
end;

function TRes1.GetResType: Integer;
begin
  Result:= 1;
end;

// Res2

type
  TRes2 = class(TBaseRes, IRes)
    function GetInterface: IResource; override;
    function GetResType: Integer;
  end;

function TRes2.GetInterface: IResource;
var
  IntRes: IResource;
begin
  IntRes:= Self;
  Result:= IntRes;
end;

function TRes2.GetResType: Integer;
begin
  Result:= 2;
end;

// problem code

var
  iResItem : Integer;
  ResItem : TBaseRes;
  IntRes : IResource;
  TestId : Integer;
begin
  TestId:= 100;

  for iResItem:= 0 to ResList.Count-1 do
  begin
    ResItem:= ResList[iResItem];

    IntRes:= ResItem.GetInterface;


  // XXX //
    if IntRes.iResType=1 do
    begin
      blaaa


   // TestId wurde verändert !!
  end;