How to validate the excel sheet using ASP.NET C#?

i upload one excel document with more than one sheet and stored in dataset as data table.now i want to validate the dataset.
if 'item no' is mandatory.the item number must be mandatory all sheets in excel sheet.
if item number in 7 th row in all sheets but column value is different. how to validate,
*All mandatory fields rows are same in all sheets but column is difference how to validate it.