Something like this, not tested. See http://dontbreakthebuild.com/2011/01/30/excel-and-c-interop-with-net-4-how-to-read-data-from-excel/
using (StreamWriter sw = File.CreateText("ExtractedText.txt"))
{
var excelApp = new Excel.Application();
var workBook = excelApp.Workbooks.Open(thisFile);
foreach (var sheet in workBook.Worksheets)
{
foreach (var row in sheet.UsedRange.Rows)
{
foreach (var cell in row.Columns)
{
sw.Write(cell.Value + " ");
}
sw.WriteLine();
}
}
}