access gridview column and find control of the textboxes

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ASP.NET

a grid view in that a column for checkbox and second column for textboxes .
when i check the checkbox then textbox shows sequence of the checkbox.when i first time check on the checkbox then textbox shows the 1, and next time check the checkbox then textbox shows the 2.and so on.......
when i unselect the checkbox then sequence are rearranged.for example when i unselect the checkbox sequence is 3 then rearranged the sequence 1,2,4 becomes 3,5 becomes 4 and so on.

protected void grd_comm_grp_SelectedIndexChanged(object sender, EventArgs e)
    {      
        //Find the CheckBox and get the row 
        CheckBox cbSelection = (CheckBox)sender;
        GridViewRow row = (GridViewRow)cbSelection.NamingContainer;
        TextBox CenterSequence = (TextBox)row.FindControl("CenterSequence");
        // If Checked 
        if (cbSelection.Checked)
        {
            // Enable the TextBox 
            CenterSequence.Enabled = true;
            // Find Rows with sequence value 
            var ValidSequences = grd_comm_grp.Rows.Cast<gridviewrow>().Where(a => ((TextBox)a.FindControl("CenterSequence")).Text != "").ToList();
            // If there are any sequences, increment by 1 
            if (ValidSequences.Count > 0)
                CenterSequence.Text = (ValidSequences.Count + 1).ToString();
            // If new, put 1 
            else
                CenterSequence.Text = "1";
        }
        // If unchecked, clear the text, disable it and reset the sequence 
        else
        {
            CenterSequence.Enabled = false;
            CenterSequence.Text = "";
            // Find rows with sequence values 
            var ValidSequences = grd_comm_grp.Rows.Cast<gridviewrow>().Where(a => ((TextBox)a.FindControl("CenterSequence")).Text != "").OrderBy(a => ((TextBox)a.FindControl("CenterSequence")).Text).ToList();
           // If more than one row, reset the sequence 
            if (ValidSequences.Count > 1)
                // loop until last but one 
                for (int i = 0; i < ValidSequences.Count - 1; i++)
                {
                    // check if the difference between the current number and next number is one. 
                    int InitialNumber = Convert.ToInt32(((TextBox)ValidSequences[i].FindControl("CenterSequence")).Text);
                    int NextNumber = Convert.ToInt32(((TextBox)ValidSequences[i + 1].FindControl("CenterSequence")).Text);
                    
                   // First one in the sequence should be 1 
                   // If sequence does not start from 1, make it 1 
                    if (i == 0 && InitialNumber > 1)
                    {
                        ((TextBox)ValidSequences[i].FindControl("CenterSequence")).Text = "1";
                         InitialNumber = 1;
                    }
                    // If the difference is not 1, make the difference 1 
                    // Ex: 1 and 3 make 3 as 2 
                    if (InitialNumber + 1 != NextNumber)
                        ((TextBox)ValidSequences[i + 1].FindControl("CenterSequence")).Text =(InitialNumber + 1).ToString();
                }
            // If only one element, make the sequence 1 
            else if (ValidSequences.Count == 1)
                ((TextBox)ValidSequences.First().FindControl("CenterSequence")).Text = "1";
        }       
    }

problem is that when sequence is above from 9 then NextNumber contain the second element is 10 instead of 2 . The problem in this line

int NextNumber = Convert.ToInt32(((TextBox)ValidSequences[i + 1].FindControl("CenterSequence")).Text);

i think problem in line

var ValidSequences = grd_comm_grp.Rows.Cast<gridviewrow>().Where(a =>(((TextBox)a.FindControl("CenterSequence")).Text) != "").OrderBy(a => ((TextBox)a.FindControl("CenterSequence")).Text).ToList();

because var ValidSequences contain the index of the textbox list .it gives the validSequence[0]=1 ,validSequences[1]=10 instead of ValidSequence[1]=2