New Post: How to format Fractions

With ClosedXML I think you would use:

mCurrWorkSheet.column(1).style.numberformat.format = "# ???/???"
(this would set the format of a whole column)

From what I've seen you can pretty much put any custom format you would use in excel here and it will work.

See the documentation section for a full example of using numberformat.format :
https://closedxml.codeplex.com/wikipage?title=Styles%20-%20NumberFormat&referringTitle=Documentation