This is based on jiverson's answer, but much more concise for basic needs; just create a range, get its borders and add a border to the bottom of the range:
var platypusRange = _xlSheet.Range[_xlSheet.Cells[1, 1], _xlSheet.Cells[1, 3]]; . . . Borders border = platypusRange.Borders; border[XlBordersIndex.xlEdgeBottom].LineStyle = XlLineStyle.xlContinuous;
Of course, you can add a border at the top and sides using xlEdgeTop, xlEdgeLeft and xlEdgeRight.
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