Regarding Exception Error

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C#

I'm doing project in C#.net windows application.I've wriitten somecalculation code in textchanged event of textbox.When Text is changed in that paricular textbox, Exception Error occurs (ie) There is already an open DataReader associated with this Command which must be closed first.But I've closed all reader and Connection. My Code is

private void BL_DueAmttxt_TextChanged(object sender, EventArgs e)
        {

                con_str.Open();
                cmd = new SqlCommand("select isnull(count(BL_BalAmt),0) from Bill where BL_Lno='" + BL_LnoCombo.Text + "'", con_str);
                SqlDataReader dr = cmd.ExecuteReader();
                while (dr.Read())
                {
                    int count = int.Parse(dr[0].ToString());
                    //float count = dr.GetValue(0);

                    if (count == 0)
                    {
                        try
                        {
                                float interest = float.Parse(BL_Inttxt.Text);
                                float DAmt = float.Parse(BL_DueAmttxt.Text);
                                float TotAmt = interest + DAmt;
                                BL_TotAmttxt.Text = TotAmt.ToString();
                                float SAmt = float.Parse(BL_LAmttxt.Text);
                                float BalAmt = SAmt - DAmt;
                                BL_BalAmttxt.Text = BalAmt.ToString();
                        }
                        catch (Exception ex)
                        { }
                    }
                    else
                    {
                        cmd = new SqlCommand("select top 1 BL_BalAmt-'" + BL_DueAmttxt.Text + "' from Bill where BL_Lno='" + BL_LnoCombo.Text + "' order by BL_Date Desc", con_str);
                        dr = cmd.ExecuteReader();
                        while (dr.Read())
                        {
                            try
                            {
                                {
                                    BL_BalAmttxt.Text = dr.GetValue(0).ToString();
                                    float interest = float.Parse(BL_Inttxt.Text);
                                    float DAmt = float.Parse(BL_DueAmttxt.Text);
                                    float TotAmt = interest + DAmt;
                                    BL_TotAmttxt.Text = TotAmt.ToString();
                                }
                            }
                            catch (Exception ex)
                            { }
                        }
                        dr.Close();
                    }
                }
                dr.Close();
                con_str.Close();
            }