plz solve this error given below

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.Data.SqlClient;
namespace WindowsFormsApplication1
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void btnsubmit_Click(object sender, EventArgs e)
        {

            String connectionString = "....;user id=sa;password=...;database=student";
            SqlConnection thisConnection = new SqlConnection(connectionString);
            SqlCommand sqlCommand = new SqlCommand();


            thisConnection.Open();

            String thisQuery = "INSERT INTO detail (ROLL_NO, S_NAME, AGE) VALUES ('" + txtroll.Text + "', '" + txtname.Text + "', '" + txtage.Text + "')";
            SqlCommand thisCommand = new SqlCommand(thisQuery, thisConnection);

            thisCommand.ExecuteNonQuery();
            thisConnection.Close();

            using (SqlConnection sqlConnection = new SqlConnection(connectionString))
            {
                sqlCommand = sqlConnection.CreateCommand();
                SqlDataAdapter sda = new SqlDataAdapter(sqlCommand.CommandText, connectionString);
                SqlCommandBuilder scb = new SqlCommandBuilder(sda);
                DataTable detail = new DataTable();
                sda.Fill(detail);
                BindingSource bSource = new BindingSource();
                bSource.DataSource = detail;
                dataGridView1.DataSource = bSource;
            }
        }
    }
}