Попробуйте это
string strFileName = "D:\\test1.xlsx";
Microsoft.Office.Interop.Excel.Application ExcelObj = null;
ExcelObj = new Microsoft.Office.Interop.Excel.Application();
Microsoft.Office.Interop.Excel.Workbook theWorkbook = ExcelObj.Workbooks.Open(strFileName, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing);
Microsoft.Office.Interop.Excel.Sheets sheets = theWorkbook.Worksheets;
Microsoft.Office.Interop.Excel.Worksheet worksheet = (Microsoft.Office.Interop.Excel.Worksheet)sheets.get_Item(1);
Microsoft.Office.Interop.Excel.Range range = worksheet.get_Range("A11", "G21"); // Your requied range here
foreach (Microsoft.Office.Interop.Excel.Range cell in range.Cells)
{
if(cell.TextToString()=="Yes")
{
}
if(cell.TextToString()=="No")
{
}
}
Для получения дополнительной информации проверьте здесь
http://msdn.microsoft.com/en-us/library/4zs9xy29(v=vs.80).aspx