TFileListTh = class(TThread)
  private
    FSource: TStringList;
    FDest: TStringList;
    FSourceDir: string;
    FDestDir: string;
    FMode: Word;
    procedure CreateSourceFileList;
    procedure CreateDestFileList;
  public
    property Source: TStringList read FSource write FSource;
    property Dest: TStringList read FDest write FDest;
    property sSourceDir: string read FSourceDir write FSourceDir;
    property sDestDir: string read FDestDir write FDestDir;
    property nMode: Word read FMode write FMode;

    destructor Destroy; override;
  protected
    procedure Execute; override;
    constructor Create(CreateSuspended: Boolean);
  end;

...

constructor TFileListTh.Create(CreateSuspended: Boolean);
begin
  inherited Create(CreateSuspended);
  FSource := TStringList.Create;
  FDest := TStringList.Create;
end;

procedure TFileListTh.Execute;
begin
  case FMode of
    0: CreateSourceFileList;
    1: CreateDestFileList;
  end;
end;

procedure TFileListTh.CreateSourceFileList;
var
  nCount: Integer;
begin
  if (FSourceDir <> '') then
    if AdvBuildFileList(IncludeTrailingPathDelimiter(FSourceDir) + '*.*',
      faAnyFile or faDirectory, FSource, amAny, [flFullNames, flRecursive]) then
    begin
      for nCount := FSource.Count - 1 downto 0 do
        if (Pos('\.', FSource.Strings[nCount]) > 0) then
          FSource.Delete(nCount);
    end;

  FSource.Sort;
end;

procedure TFileListTh.CreateDestFileList;
var
  nCount: Integer;
begin
  if (FDestDir <> '') then
    if AdvBuildFileList(IncludeTrailingPathDelimiter(FDestDir) + '*.*',
      faAnyFile or faDirectory, FDest, amAny, [flFullNames, flRecursive]) then
    begin
      for nCount := FDest.Count - 1 downto 0 do
        if (Pos('\.', FDest.Strings[nCount]) > 0) then
          FDest.Delete(nCount);
    end;

  FDest.Sort;
end;

destructor TFileListTh.Destroy;
begin
  FSource.Free;
  FDest.Free;
  inherited;
end;