Issue From C#

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using System.Data.SqlClient;

namespace IT_Store_Management
{
    public partial class Issue : Form
    {
        protected String ConString = Properties.Settings.Default.DefaultDB; //connection string of default database

        public Issue()
        {
            InitializeComponent();
        }

        private void Received_Load(object sender, EventArgs e)
        {
            ReceivedDate.CustomFormat = "ddd dd MMM yyyy";
            generateSerialNo();
            getVendorList();
            getItemList();
        }

        private void generateSerialNo()
        {
            try
            {
                SqlConnection con = new SqlConnection(this.ConString);
                SqlCommand cmd = con.CreateCommand();
                cmd.CommandText = "SELECT TOP (1) [SL] + 1 AS [Serial No] FROM [it_store].[dbo].[Received_Items] ORDER BY [SL] DESC";

                con.Open();
                SqlDataReader rdr = cmd.ExecuteReader();
                if (rdr.Read())
                {
                    SlNoTextBox.Text = rdr["Serial No"].ToString();

                }
                con.Close();
            }
            catch (SqlException exc)
            {
                MessageBox.Show("Error: " + exc.Message, "Database Error", MessageBoxButtons.OK, MessageBoxIcon.Error);
            }
        }

        private void getItemList()
        {
            try {
                SqlConnection con = new SqlConnection(this.ConString);
                SqlCommand cmd = con.CreateCommand();
                cmd.CommandText = "SELECT [ID],[Name]  FROM [it_store].[dbo].[Items] WHERE [Status] = @status ";
                cmd.Parameters.AddWithValue("@status", 1);
                con.Open();
                SqlDataReader rdr = cmd.ExecuteReader();
                while (rdr.Read())
                {

                    ItemsComboBox.Items.Add(new ComboBoxItem(rdr["Name"].ToString(), rdr["ID"].ToString()));
                }
                con.Close();
            }
            catch(SqlException exc)
            {
                MessageBox.Show("Error: " + exc.Message, "Database Error", MessageBoxButtons.OK, MessageBoxIcon.Error);
            }


        }

        private void getVendorList()
        {
            try {
                SqlConnection con = new SqlConnection(this.ConString);
                SqlCommand cmd = con.CreateCommand();
                cmd.CommandText = "SELECT [ID],[Name]FROM [it_store].[dbo].[Departments] WHERE [Status] = @status ";
                cmd.Parameters.AddWithValue("@status", 1);
                con.Open();
                SqlDataReader rdr = cmd.ExecuteReader();
                while (rdr.Read())
                {

                    VendorComboBox.Items.Add(new ComboBoxItem(rdr["Name"].ToString(), rdr["ID"].ToString()));
                }
                con.Close();
            }
            catch (SqlException exc)
            {
                MessageBox.Show("Error: " + exc.Message, "Database Error", MessageBoxButtons.OK, MessageBoxIcon.Error);
            }

        }



        private void SaveButton_Click(object sender, EventArgs e)
        {
            //if form validation passed
            if (IsValid())
            {
                // form data save successful
                if (SaveReceivedItems())
                {
                    MessageBox.Show("Issue information saved", "Success", MessageBoxButtons.OK, MessageBoxIcon.Information);
                    Form _Issue = new Issue();
                    _Issue.Show();
                    this.Close();
                }
            }
        }

        private bool SaveReceivedItems()
        {
        try
            {

                SqlConnection con = new SqlConnection(this.ConString);
                SqlCommand cmd = con.CreateCommand();
                cmd.CommandText = "INSERT INTO [dbo].[Issued_Items]([Received Date],[Memo No],[DepartmentID],[ItemID],[Quantity],[Remarks],[Balance],[Status]) VALUES( @reeivedate,  @memono, @vendorid, @itemid, @qty, @remarks, @balance , @status )";
                cmd.Parameters.AddWithValue("@reeivedate", ReceivedDate.Value);
                cmd.Parameters.AddWithValue("@memono", MemoNoTextBox.Text);
                cmd.Parameters.AddWithValue("@vendorid", int.Parse(((ComboBoxItem)VendorComboBox.SelectedItem).HiddenValue));
                cmd.Parameters.AddWithValue("@itemid", int.Parse(((ComboBoxItem)ItemsComboBox.SelectedItem).HiddenValue));
                cmd.Parameters.AddWithValue("@qty", QuantityTextBox.Text);
                cmd.Parameters.AddWithValue("@remarks", (RemarksRickTextBox.Text.Trim() != String.Empty) ? RemarksRickTextBox.Text.Trim() : (object) DBNull.Value); //If Remarks empty, assign value to DBNull.value
                cmd.Parameters.AddWithValue("@balance", (int.Parse(BfTextBox.Text.Trim()) - int.Parse(QuantityTextBox.Text.Trim())));
                cmd.Parameters.AddWithValue("@status", 1);


                con.Open();
                int a = cmd.ExecuteNonQuery();

                // check insertion unsuccessful
                if (a < 0)
                {
                    return false;
                }

            }
            catch (SqlException exc)
            {
                MessageBox.Show(exc.Message);
                return false;
            }
            return true;

        }

        private bool IsValid()
        {
            if (ItemsComboBox.SelectedIndex == -1)
            {
                MessageBox.Show("An item must be selected", "Validation Error", MessageBoxButtons.OK, MessageBoxIcon.Error);
                return false;
            }

            if (MemoNoTextBox.Text.Trim() == String.Empty)
            {
                MessageBox.Show("Memo No. field is required", "Validation Error", MessageBoxButtons.OK, MessageBoxIcon.Error);
                return false;
            }

            if (VendorComboBox.SelectedIndex == -1)
            {
                MessageBox.Show("A department must be selected", "Validation Error", MessageBoxButtons.OK, MessageBoxIcon.Error);
                return false;
            }



            if (QuantityTextBox.Text.Trim() == String.Empty)
            {
                MessageBox.Show("Department field is required", "Validation Error", MessageBoxButtons.OK, MessageBoxIcon.Error);
                return false;
            }

        if (int.Parse(QuantityTextBox.Text.Trim()) > int.Parse(BfTextBox.Text.Trim()))
            {
                MessageBox.Show("Quantity should be less than or equal to BF", "Validation Error", MessageBoxButtons.OK, MessageBoxIcon.Error);
                return false;
            }





            return true;
        }

        private void ItemsComboBox_SelectedIndexChanged(object sender, EventArgs e)
        {
            if (ItemsComboBox.SelectedIndex != -1)
            {
                getBf();
                CodeTextBox.Text = ((ComboBoxItem)ItemsComboBox.SelectedItem).HiddenValue;

            }
        }


        private void getBf()
        {
           try
            {
                SqlConnection con = new SqlConnection(this.ConString);
                SqlCommand cmd = con.CreateCommand();
                cmd.CommandText = "SELECT ISNULL((SELECT SUM(Received_Items.Quantity) FROM Received_Items WHERE Received_Items.ItemID = @itemid ),0) - ISNULL((SELECT SUM(Issued_Items.Quantity) FROM Issued_Items WHERE Issued_Items.ItemID = @itemid ),0) AS [Total Item]";
                cmd.Parameters.AddWithValue("@itemid", int.Parse(((ComboBoxItem)ItemsComboBox.SelectedItem).HiddenValue));

                con.Open();
                SqlDataReader rdr = cmd.ExecuteReader();

                if (rdr.Read())
                {
                    BfTextBox.Text = (Convert.IsDBNull(rdr["Total Item"])) ? "0" : rdr["Total Item"].ToString();
                    BalanceTextBox.Text = (Convert.IsDBNull(rdr["Total Item"])) ? "0" : rdr["Total Item"].ToString();

                }
                con.Close();
            }
            catch(SqlException exc)
            {
                MessageBox.Show("Error: " + exc.Message, "Database Error", MessageBoxButtons.OK, MessageBoxIcon.Error);
            }
        }

        private void QuantityTextBox_KeyUp(object sender, KeyEventArgs e)
        {
            //if item selected
            if (ItemsComboBox.SelectedIndex != -1)
            {
                int tmpnum;
                bool isSuccess;
                isSuccess = int.TryParse(QuantityTextBox.Text.Trim(), out tmpnum);

                // if number is valid
                if (isSuccess)
                {
                    BalanceTextBox.Text = ( int.Parse(BfTextBox.Text.Trim()) - int.Parse(QuantityTextBox.Text.Trim()) ).ToString();
                }
                else
                {
                    MessageBox.Show("Invalid number \"" + QuantityTextBox.Text.Trim() + "\"", "Input Error", MessageBoxButtons.OK, MessageBoxIcon.Error);
                    QuantityTextBox.Focus();
                }

            }
            else
            {
                MessageBox.Show("Before entering quantity an item must be selected", "Input Error", MessageBoxButtons.OK, MessageBoxIcon.Error);
                ItemsComboBox.Focus();
            }


        }

        private void pictureBox1_Click(object sender, EventArgs e)
        {

        }

        private void ReceivedDate_ValueChanged(object sender, EventArgs e)
        {

        }
    }
}