我试图从数据库中获取值，但得到的错误是cmd、dr在当前上下文中不存在

可能重复：
C#在获取时出现一些错误

public partial class BioreCottonPurchaseSlip2011 : Form
{
    Library obj = new Library();

    public BioreCottonPurchaseSlip2011()
    {
        InitializeComponent();
    }


    public void enableDisableControls(bool flag)
    {
        TxtExtensionNo.Enabled = flag;
        TxtFarmerCode.Enabled = flag;
        TxtFarmerName.Enabled = flag;
        TxtBasicPrice.Enabled = flag;
        TxtPremium.Enabled = flag;
        TxtWeight.Enabled = flag;
        TxtTotalAmountBasic.Enabled = flag;
        TxtTotalAmountPremium.Enabled = flag;
        TxtBalancePay.Enabled = flag;
        BtnSave.Enabled = flag;
        BtnCancel.Enabled = flag;
    }


    public void clearControls()
    {
        TxtExtensionNo.Text = "";
        TxtFarmerCode.Text = "";
        TxtFarmerName.Text = "";
        TxtBasicPrice.Text = "";
        TxtPremium.Text = "";
        TxtWeight.Text = "";
        TxtTotalAmountBasic.Text = "";
        TxtTotalAmountPremium.Text = "";
        TxtBalancePay.Text = "";

    }
    private void BtnNew_Click(object sender, EventArgs e)
    {
        if (obj.GetConnection() == true)
        {
            lblError.Text = "Connected !!!";
        }
        else
        {
            lblError.Text = "Not connnected !!!";
        }
        enableDisableControls(true);
        BtnNew.Enabled = false;
        // lblError.Text = "";
        string connectionString = "Data Source=TANYA-PC;Initial Catalog=biore1;Integrated Security=True";
        string sql = "SELECT * FROM cottonpurchase";
        SqlConnection connection = new SqlConnection(connectionString);
        SqlDataAdapter da = new SqlDataAdapter(sql, connection);
        DataSet ds = new DataSet();
        connection.Open();
        da.Fill(ds, "cottonpurchase");
        connection.Close();
        dataGridView1.DataSource = ds;
        dataGridView1.DataMember = "cottonpurchase";

    }
    private void BtnSave_Click(object sender, EventArgs e)
    {
        if (obj.GetConnection() == true)
        {
            //dateTimePicker1.Value = DateTime.Today;
            MessageBox.Show("insert into cottonpurchase values(" + TxtExtensionNo.Text + ",'" + monthCalendar1.TodayDate + "'," + TxtFarmerCode.Text + ",'" + TxtFarmerName.Text + "'," + TxtBasicPrice.Text + "," + TxtPremium.Text + "," + TxtWeight.Text + "," + TxtTotalAmountBasic.Text + "," + TxtTotalAmountPremium.Text + "," + TxtBalancePay.Text + ")");
            if (obj.ExecuteSQLStatement("insert into cottonpurchase values(" + TxtExtensionNo.Text + ",'" + monthCalendar1.TodayDate + "'," + TxtFarmerCode.Text + ",'" + TxtFarmerName.Text + "'," + TxtBasicPrice.Text + "," + TxtPremium.Text + "," + TxtWeight.Text + "," + TxtTotalAmountBasic.Text + "," + TxtTotalAmountPremium.Text + "," + TxtBalancePay.Text + " )") == true)
            {
                lblError.Text = "Item(s) Saved";
                clearControls();
                BtnSave.Enabled = false;
                BtnNew.Enabled = true;
                enableDisableControls(false);
            }
            else
            {
                lblError.Text = "Item(s) Not Saved";
            }
        }

        else
        {
            lblError.Text = "Connection Error. Please contact your administrator.";
        }


        enableDisableControls(false);
        clearControls();
        BtnNew.Enabled = true;
        try
        {
            //double getvat = 0;
            //double calculatevat = 0;
            //getvat = Convert.ToDouble(TxtTotalAmountBasic.Text);
            //calculatevat = getvat * 0.18;
            //TxtBasicPrice.Text = calculatevat.ToString();
            //TxtBasicPrice.Enabled = false;
        }
        catch (Exception)
        {
            // lblError.Text = "Please contact your administrator. (Error - TARS0001DATMIS)";
            clearControls();
            enableDisableControls(false);
            BtnNew.Enabled = true;
            //lblInvoiceNo.Text = "INVOIC NO";
        }
    }

    private void BtnCancel_Click(object sender, EventArgs e)
    {
        enableDisableControls(false);
        clearControls();
        BtnNew.Enabled = true;
    }
    private void panel1_Paint(object sender, PaintEventArgs e)
    {
    }
    private void TxtFarmerCode_TextChanged(object sender, EventArgs e)
    {
         try
        {
            SqlConnection conn= new SqlConnection("Data Source=TANYA-PC;Initial Catalog=biore1;Integrated Security=True");
            conn.Open();
            cmd = new SqlCommand("Select farmername, from cottonpurchase where farmercode=@aa", conn);
            cmd.Parameters.Add("@aa", SqlDbType.Int).Value = TxtFarmerCode.Text;
            dr = cmd.ExecuteReader();
            if (dr.HasRows == false)
            {
                throw new Exception();
            }
            if (dr.Read())
            {
                // textBox1.Text = dr[0].ToString(); Since U r going to give the ID and retrieve in textBox1.
                TxtFarmerName.Text = dr[0].ToString();
                //textBox3.Text = dr[1].ToString();
                //textBox4.Text = dr[2].ToString();
                //textBox7.Text = dr[3].ToString();
                //dateTimePicker1.Text = dr[4].ToString();
                //dateTimePicker2.Text = dr[5].ToString();
                //textBox5.Text = dr[6].ToString();
            }
        }
        catch
        {
        //    lblError = "THE GIVEN ID IS UNAVAILABLE";
        }
        finally
        {
            conn.Close();
        }
    }
    }