Please solve the error

using System;
using System.Configuration;
using System.Data;
using System.Data.SqlClient;
using System.Drawing;
using System.Linq;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.HtmlControls;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Xml.Linq;


public partial class viewfile : System.Web.UI.Page
{
    private SqlConnection conn = new SqlConnection("Data Source=.\\sqlexpress;Initial Catalog=college_education;User ID=sa;Password=system;");

    protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
        {
            gvbind();
        }
    }

    protected void gvbind()
    {
        conn.Open();
        SqlCommand cmd = new SqlCommand("Select * from file_rec", conn);
        SqlDataAdapter da = new SqlDataAdapter(cmd);
        DataSet ds = new DataSet();
        da.Fill(ds);
        conn.Close();
        if (ds.Tables[0].Rows.Count > 0)
        {
            GridView1.DataSource = ds;
            GridView1.DataBind();
        }
        else
        {
            ds.Tables[0].Rows.Add(ds.Tables[0].NewRow());
            GridView1.DataSource = ds;
            GridView1.DataBind();
            int columncount = GridView1.Rows[0].Cells.Count;
            GridView1.Rows[0].Cells.Clear();
            GridView1.Rows[0].Cells.Add(new TableCell());
            GridView1.Rows[0].Cells[0].ColumnSpan = columncount;
            GridView1.Rows[0].Cells[0].Text = "No Records Found";
        }

    }

    protected void GridView1_RowDeleting(object sender, GridViewDeleteEventArgs e)
    {
        GridViewRow row = (GridViewRow)GridView1.Rows[e.RowIndex];
        Label lbldeleteid = (Label)row.FindControl("lblID");
        conn.Open();
        SqlCommand cmd = new SqlCommand("delete FROM file_rec where serial_no='" + Convert.ToInt32(GridView1.DataKeys[e.RowIndex].Value.ToString()) + "'", conn);
        cmd.ExecuteNonQuery();
        conn.Close();
        gvbind();

    }

    protected void GridView1_RowEditing(object sender, GridViewEditEventArgs e)
    {
        GridView1.EditIndex = e.NewEditIndex;
        gvbind();
    }
    protected void GridView1_RowUpdating(object sender, GridViewUpdateEventArgs e)
    {
        int serial_no = Convert.ToInt32(GridView1.DataKeys[e.RowIndex].Value.ToString());
        GridViewRow row = (GridViewRow)GridView1.Rows[e.RowIndex];
        Label lblID = (Label)row.FindControl("lblID");
        //TextBox txtname=(TextBox)gr.cell[].control[];
        TextBox textinternal_receiving = (TextBox)row.Cells[0].Controls[0];
        TextBox textdate = (TextBox)row.Cells[1].Controls[0];
        TextBox textdepartment = (TextBox)row.Cells[2].Controls[0];
        TextBox textdepartment_type = (TextBox)row.Cells[3].Controls[0];
        TextBox textorganization = (TextBox)row.Cells[4].Controls[0];
        TextBox textorganization_type = (TextBox)row.Cells[5].Controls[0];
        TextBox textfile_no = (TextBox)row.Cells[6].Controls[0];
        TextBox textsubject = (TextBox)row.Cells[7].Controls[0];
        TextBox textfile_name = (TextBox)row.Cells[8].Controls[0];
        TextBox textstatus = (TextBox)row.Cells[9].Controls[0];
        //TextBox textadd = (TextBox)row.FindControl("txtadd");
        //TextBox textc = (TextBox)row.FindControl("txtc");
        GridView1.EditIndex = -1;
        conn.Open();
        //SqlCommand cmd = new SqlCommand("SELECT * FROM detail", conn);
        SqlCommand cmd = new SqlCommand("update file_rec set internal_receiving='" + textinternal_receiving + "',date='" + textdate + "',department='" + textdepartment + "',department_type='" + textdepartment_type + "',organization='" + textorganization + "',organization_type='" + textorganization_type + "',file_no='" + textfile_no + "',subject='" + textsubject + "',file_name='" + textfile_name + "',status='" + textstatus + "'where ='" + serial_no + "'", conn);
        cmd.ExecuteNonQuery();
        conn.Close();
        gvbind();
        //GridView1.DataBind();
    }
    protected void GridView1_PageIndexChanging(object sender, GridViewPageEventArgs e)
    {
        GridView1.PageIndex = e.NewPageIndex;
        gvbind();
    }
    protected void GridView1_RowCancelingEdit(object sender, GridViewCancelEditEventArgs e)
    {
        GridView1.EditIndex = -1;
        gvbind();
    }

}