writing excel cell values to generic list using c#

Question

Sketchs.Add(new SKETCH(SketchWorkSheet.Cells[i, 3].ToString(),
                                          SketchWorkSheet.Cells[i, 4].ToString(),
                                          SketchWorkSheet.Cells[i, 5].ToString(),
                                          SketchWorkSheet.Cells[i, 6].ToString(),
                                          SketchWorkSheet.Cells[i, 7].ToString(),
                                          SketchWorkSheet.Cells[i, 8].ToString(),   Convert.ToDouble(SketchWorkSheet.Cells[i,9])));

when i run this i get an error as

Input string was not in a correct format

Can any one help me in this