Count cells in Excel spreadsheet from within C#

I have a directory with thousands of Excel spreadsheets and I need to find the number of non blank cells in each excluding row and column headings.  I'm trying to do it using Excel's CountA function.
I'm running the code below which is giving this error message after looking at the first spreadsheet in the directory.  Please can you help?

"COM object that has been separated from its underlying RCW cannot be used"


using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using System.IO;
using System.Runtime.InteropServices;
using System.Threading;
using Excel = Microsoft.Office.Interop.Excel;

namespace WindowsFormsApplication4
{
    public partial class Form1 : Form
    {

        public Form1()
        {
            InitializeComponent();
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            string OutputFile;
            long NumberOfNonBlankCells;
            Excel.Application xlApp;
            Excel.Workbook xlWorkBook;
            Excel.Worksheet xlWorkSheet;
            object misValue = System.Reflection.Missing.Value;
            xlApp = new Excel.ApplicationClass();
            OutputFile = "C:\\Output\\Test2.csv";
            try
            {
                using (StreamWriter sw = new StreamWriter(OutputFile))
                {

                    foreach (string inputfile in Directory.GetFiles("C:\\Output\\Test\\Sample"))
                    {
                        try
                        {
                            sw.Write(inputfile.ToString() + "\n");


                            xlWorkBook = xlApp.Workbooks.Open(inputfile, 0, true, 5, "", "", true, Microsoft.Office.Interop.Excel.XlPlatform.xlWindows, "\t", false, false, 0, true, 1, 0);
                            //xlApp.WorksheetFunction wsf = xlApp.WorksheetFunction;
                            Excel.WorksheetFunction wsf;

                            xlWorkSheet = (Excel.Worksheet)xlWorkBook.Worksheets.get_Item(1);
                            xlApp.DisplayAlerts = false;

                            int result = (int)xlApp.WorksheetFunction.CountA(xlWorkSheet.Cells.get_Range("b2", "f100"),
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, 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, Type.Missing); //wsf.CountA(xlWorkSheet.Cells ,"S","S","K" );


                            xlWorkBook.Close(true, "C:\\test\\FileName.xls", misValue);
                            xlApp.Quit();
                            xlApp.DisplayAlerts = true;
                            releaseObject(xlWorkSheet);
                            releaseObject(xlWorkBook);
                            releaseObject(xlApp);
                           
                            sw.Write(inputfile.ToString() + "\t" + result  + "\n");


                        }
                        catch (Exception ex)
                        {
                            MessageBox.Show(ex.Message);
                        }
                    }
                }
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message);
            }

        }
        private void releaseObject(object obj)
        {
            try
            {
                System.Runtime.InteropServices.Marshal.ReleaseComObject(obj);
                obj = null;
            }
            catch (Exception ex)
            {
                obj = null;
                MessageBox.Show("Unable to release the Object " + ex.ToString());
            }
            finally
            {
                GC.Collect();
               
            }
        }

    }
}