Problem filtering data row in datagridview between two value in textbox

Hi,

Thank you for your time. I kindly ask your assist to rectify the problem regarding with my code. Basically, my program is to retrieve the value from CSV file and export to DataGridView.

Then i try to retrieve the rows value in column [0] and filter it using two textbox value.

( Textbox1.value <= Columns[0].Value <= Textbox2.value)

The problem is i try to using the rowfilter but its unsuccessfully.

Here is my code:-

using System;
using System.Collections.Generic;
using System.Drawing;
using System.Windows.Forms;
using System.Data;
using System.IO;
using System.Linq;
using System.ComponentModel;

namespace FilterCSV
{
	/// <summary>
	/// Description of MainForm.
	/// </summary>
	public partial class MainForm : Form
	{
		public MainForm()
		{
			//
			// The InitializeComponent() call is required for Windows Forms designer support.
			//
			InitializeComponent();
			DgvFilterManager filterManager = new DgvFilterManager(DataGridValue);
			//
			// TODO: Add constructor code after the InitializeComponent() call.
			//
		}

		
		void BtnSelectClick(object sender, EventArgs e)
		{
			Stream myStream;
            OpenFileDialog openFileDialog1 = new OpenFileDialog();

            openFileDialog1.InitialDirectory = "c:\\";
            openFileDialog1.Filter = "csv files (*.csv)|*.csv|All files (*.*)|*.*";
            openFileDialog1.FilterIndex = 2;
            openFileDialog1.RestoreDirectory = true;

            if (openFileDialog1.ShowDialog() == DialogResult.OK)
            {

                if ((myStream = openFileDialog1.OpenFile()) != null)
                {
                    // Insert code to read the stream here.
                    tbxSelectFile.Text = openFileDialog1.FileName;
                    myStream.Close();

                }

            } 
		}
		
		void BtnCancelClick(object sender, EventArgs e)
		{
			tbxSelectFile.Text = null;
		}

        void BtnGenerateClick(object sender, EventArgs e)
        {
              //  get all lines of csv file
            string[] str = File.ReadAllLines(tbxSelectFile.Text);

            // create new datatable
            DataTable dt = new DataTable();

            
            // get the column header means first line
            string[] temp = str[0].Split(',');

            // creates columns of gridview as per the header name
            foreach (string t in temp)
            {
                dt.Columns.Add(t, typeof(string));
            }

            // now retrive the record from second line and add it to datatable
            for (int i = 1; i < str.Length; i++)
            {
                string[] t = str[i].Split(',');
                dt.Rows.Add(t);
            }
            DataGridValue.DataSource = dt;
            }



        void BtnFilterClick(object sender, EventArgs e)
        {

            (DataGridValue.DataSource as DataTable).DefaultView.RowFilter = string.Format("Northing = '{0}'", tbxX1.Text);

        }
               
		}


}

And this is my sample csv;

Northing,Easting,Result 
645789.900,578778.982,6.78 
645782.892,578767.289,5.54 
645801.435,579213.430,6.78 
645804.156,579445.670,5.79 
645980.188,582544.389,8.90 
645983.456,582667.344,8.79 
646590.253,584788.212,7.60 
646800.789,585690.312,2.50 
646909.452,585780.212,4.30 
647900.323,585890.345,6.89 

Please show me how the right way to do it. Thank you so much.