C# 获取时出现一些错误

 private void TxtFarmerCode_TextChanged(object sender, EventArgs e)
       {
             try
            {
                cmd.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();
            }

您缺少连接块（更换connectionString与您要连接的任何内容一起）：

 using (SqlConnection conn= new SqlConnection(
           connectionString))
   {
      //You have cmd.Open it should be connection
      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();
    }
   }

边注...NEVER do catch {}