Asp .Net Bank with Deposit and Withdraw Functionality

using System.Data;
using System.Data.SqlClient;
public class UserInfo
{
  SqlConnection cnn;
 public DataTable GetUsers()
 {
  using (cnn = new SqlConnection(ConfigurationManager.ConnectionStrings["DefaultConnection"].ConnectionString))
  {
   cnn.Open();

   DataSet ds = new DataSet();
   SqlDataAdapter da;
   da = new SqlDataAdapter("select * from UserInfo", cnn);
   da.Fill(ds, "Usr");

   return ds.Tables ["Usr"];
  }
 }

using System.Data.SqlClient;
public partial class Admin_AdminHome : System.Web.UI.Page
{
 UserInfo ui = new UserInfo();
 SqlConnection cnn;

 double dblCurBal = 0;
    protected void Page_Load(object sender, EventArgs e)
    {
     if (!IsPostBack)
     {
      ShowBal();
     }
    }

    private void ShowBal()
    {
     ddlUser.DataSource = ui.GetUsers();
     ddlUser.DataTextField = "UserName";
     DataBind();
    }
    protected void ddlUser_SelectedIndexChanged(object sender, EventArgs e)
    {
     using (cnn = new SqlConnection(ConfigurationManager.ConnectionStrings["DefaultConnection"].ConnectionString))
     {
      ShowBalInLabel();
     }
    }

    private void ShowBalInLabel()
    {
     if (cnn.State == System.Data.ConnectionState.Closed)
     {
      cnn.Open();
     }

     SqlCommand cmd = new SqlCommand("select * from UserInfo where UserName = @UserName", cnn);
     cmd.Parameters.AddWithValue("@UserName", ddlUser.Text);
     SqlDataReader dr = cmd.ExecuteReader();

     while (dr.Read())
     {
      lblCurBal.Text = dr["Balance"].ToString();
     }
    }


    protected void btnDeposit_Click(object sender, EventArgs e)
    {
     using (cnn = new SqlConnection(ConfigurationManager.ConnectionStrings["DefaultConnection"].ConnectionString))
     {
      cnn.Open();
      SqlCommand cmd = new SqlCommand("select * from UserInfo where UserName = @UserName", cnn);
      cmd.Parameters.AddWithValue("@UserName", ddlUser.Text);
      SqlDataReader dr = cmd.ExecuteReader();
     

      while (dr.Read())
      {
       dblCurBal = double.Parse(dr["Balance"].ToString());
      }
      dr.Close();

      double dblNewBal = dblCurBal + double.Parse(txtAmount.Text);

      cmd = new SqlCommand("update UserInfo set Balance = @NewBal where UserName = @UserName", cnn);
      cmd.Parameters.AddWithValue("@NewBal", dblNewBal);
      cmd.Parameters.AddWithValue("@UserName", ddlUser.Text);
      cmd.ExecuteNonQuery();

      ShowBalInLabel();
    
     }
    }
    protected void btnWithdraw_Click(object sender, EventArgs e)
    {
     using (cnn = new SqlConnection(ConfigurationManager.ConnectionStrings["DefaultConnection"].ConnectionString))
     {
      cnn.Open();
      SqlCommand cmd = new SqlCommand("select * from UserInfo where UserName = @UserName", cnn);
      cmd.Parameters.AddWithValue("@UserName", ddlUser.Text);
      SqlDataReader dr = cmd.ExecuteReader();


      while (dr.Read())
      {
       dblCurBal = double.Parse(dr["Balance"].ToString());
      }
      dr.Close();

      double dblNewBal = dblCurBal - double.Parse(txtAmount.Text);

      cmd = new SqlCommand("update UserInfo set Balance = @NewBal where UserName = @UserName", cnn);
      cmd.Parameters.AddWithValue("@NewBal", dblNewBal);
      cmd.Parameters.AddWithValue("@UserName", ddlUser.Text);
      cmd.ExecuteNonQuery();

      ShowBalInLabel();

     }
    }
}