How to convert xls to xlsb using c#

Hi Guys,
Kindly guide me to do How to convert a Already saved Excel format. For an Example i have in Excel in the name a.xls and b.xlsb in a folder. I have to convert that both excel format to .xlsb format i.e Binary format. I have to refer that path(i.e folder name) as input. It have to convert all the excel files which is contained in that folder. If in that specific folder contains sub folder means also have to check whether that contains Excel or not. Kinldy guide me do that in .net application(either or C#)