problem when deleting rows in gridview in update panel

protected void Page_Load(object sender, EventArgs e)

    {

        if (!IsPostBack)

        {

            ShowDataInGrid();

        }

    }

    public void ShowDataInGrid()

    {

        SqlConnection con = new SqlConnection(@"Data Source=.\SQLEXPRESS;AttachDbFilename=|DataDirectory|\Database.mdf;Integrated Security=True;User Instance=True");

        con.Open();

        SqlCommand comm = new SqlCommand("SELECT * FROM Users", con);

        GridView1.DataSource = comm.ExecuteReader();

        DataBind();

        con.Close();

    }

    protected void btnDeleteChecked_Click(object sender, EventArgs e)

    {

        SqlConnection con = new SqlConnection(@"Data Source=.\SQLEXPRESS;AttachDbFilename=|DataDirectory|\Database.mdf;Integrated Security=True;User Instance=True");

        con.Open();

        foreach (GridViewRow r in GridView1.Rows)

        {

            if (((CheckBox)r.FindControl("CheckBox1")).Checked == true)

            {

                string UserName = r.Cells[1].Text;

                SqlCommand comm = new SqlCommand("DELETE FROM Users where username=@username", con);

                comm.Parameters.AddWithValue("@username", UserName);

                comm.ExecuteNonQuery();

                ShowDataInGrid();

            }

        }

    }