code for comparing two .xls files(excel documents) in c sharp .net

if you're willing to consider 3rd party, The following tool will do it for you:

http://nobhillsoft.com/Columbo.aspx

Use the Excel object model (also known as Excel Interop) to read each spreadsheet.

Create a DataSet which is an in-memory copy of data; you do not need to write the data to a database. Create two tables in the DataSet, one for the data for each spreadsheet. Use a SQL query to create a union. Use the result of that to create the new spreadsheet.

Alternatively you can read the data of each of the two spreadsheets into collections and then use LINQ to create the union.