using Microsoft.Office.Interop.Excel;

Microsoft.Office.Interop.Excel.Application excelApp = new Microsoft.Office.Interop.Excel.Application();

Workbook workbook1 = excelApp.Workbooks.Open(filename, 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);
Worksheet sheet1 = (Worksheet)workbook1.Sheets[1];
Range excelRange = sheet1.UsedRange;

for (int i = 1; i <= excelRange.Rows.Count; i++)
{
    //do stuff.
}

workbook1.Close(false, filename, null);
excelApp.Quit();