vtkClipPolyData and vtkXMLUnstructuredGridReader

I try to display some filled isolines from a vtu file containing a 2d mesh and a scalar node field. Based on this example, it seems that vtkClipPolyData is not compatible with vtkXMLUnstructuredGridReader.
Moreover i can see what is the best way to display isolines, either vtkContour, vtkClip, vtkMarchingSquares…
Best regards.