There is already an open DataReader associated with this Command which must be closed first.

string Conn="Database=snpsolution; user id=sa; password=chinu111";
        int Flag = 0;
        SqlConnection con = new SqlConnection("Database=snpsolution; user id=sa; password=chinu111");
        
        //SqlCommandBuilder cb;
        //SqlDataReader dr;
        //DataRow rd;
        //DataSet ds;
        public Form3()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            //using (SqlConnection conn = new SqlConnection(Conn))
            //{

            //    using (SqlCommand cmd = new SqlCommand(Quary, conn))
            //    {

            //        //cmd.CommandType = CommandType.StoredProcedure;
            //        con.Open();
            //       id = (int)cmd.ExecuteNonQuery();
            //       con.Close();

            //    }

            //}
            try
            {
                SqlConnection conn = new SqlConnection(Conn);
                conn.Open();



                //For Checking User is Already Registerd.
                string strLoginCheck = "select * from Useraccount";
                SqlCommand cmdLoginCheck = new SqlCommand(strLoginCheck, conn);
                SqlDataReader dr = cmdLoginCheck.ExecuteReader();
                while (dr.Read())
                {
                    if (dr["Userid"].ToString() ==txtusername.Text)
                    {
                        MessageBox.Show("User is Already Exists.", "Snp Solution...    ", MessageBoxButtons.RetryCancel, MessageBoxIcon.Information);
                        Flag = 1;
                    }
                }


                
                if (txtusername. Text.Equals("") || txtPassword. Text.Equals("") || textBox1. Text.Equals(""))
                {
                    MessageBox.Show("Fill the Information.", " Snp Solution...     ", MessageBoxButtons.OK, MessageBoxIcon.Information);
                }
                else if (txtPassword.Text !=textBox1. Text)
                {
                    MessageBox.Show("Password is Not Match.", "Snp Solution...     ", MessageBoxButtons.OK, MessageBoxIcon.Information);
                }
                else if (txtPassword. Text ==textBox1. Text && Flag == 0)
                {
                    string str = "insert into Useraccount values('" + txtusername.Text + "','" + txtPassword.Text + "')";
                    SqlCommand cmd = new SqlCommand(str, conn);
                    cmd.ExecuteNonQuery();

                    MessageBox.Show("User Registerd.....     ", "Snp Solution...      ", MessageBoxButtons.OK, MessageBoxIcon.Information);
                }
                Flag = 0;

                dr.Close();
                conn.Close();
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message.ToString(), "Snp Solution...", MessageBoxButtons.OK, MessageBoxIcon.Error);
            }

cmd1 = new SqlCommand("Select subject,july,augest,september,october,halfyearly from studentmark where sgroup='B' AND SERIALNO='" + Wserialno.Text + "'", cl.con);
               SqlDataReader dr1 = cmd1.ExecuteReader();
               // Create a data row of the created table.
               if (dr1.HasRows)
               {
                   while (dr1.Read())
                   {
                       DataRow row = demoTable.NewRow();
                       // Assign values to each cell. We can assign any type of data to
                       // the cell (object type)
                       row["subject"] = dr1[0].ToString();
                       row["july"] = dr1[1].ToString();
                       row["augest"] = dr1[2].ToString();
                       row["september"] = dr1[3].ToString();
                       row["october"] = dr1[4].ToString();
                       row["halfyearly"] = dr1[5].ToString();
                       demoTable.Rows.Add(row);
                   }
                   dr1.Close();
               }
               else
               {
                   dr1.Close();
               }