Generate UniqueId based on multiple DropDownList selection in Windows Application

Form View

Code Behind - 

namespace WindowsFormsApplication1.Generate_Unique_Id
{
    public partial class Unique_Id_Using_DDL : Form
    {
        SqlConnection cnn = new SqlConnection(Properties.Settings.Default.Con_Win_General_DB);
        //DataTable dt;
        //DataSet ds;
        DataRow dr;
        public Unique_Id_Using_DDL()
        {
            InitializeComponent();
        }

        private void Unique_Id_Using_DDL_Load(object sender, EventArgs e)
        {
            DataTable dt1 = new DataTable();
            dt1.Columns.AddRange(new DataColumn[] { new DataColumn("Value", typeof(string)), new DataColumn("Name", typeof(string)) });
            dt1.Rows.Add("C01", "Chhibramau");
            dt1.Rows.Add("B01", "Bidhuna");
            dt1.Rows.Add("K01", "Kannauj");
            dr = dt1.NewRow();
            dr[0] = 0;
            dr[1] = "-Please Select-";
            dt1.Rows.InsertAt(dr, 0);
            comboBox1.DataSource = dt1;
            comboBox1.DisplayMember = "Name";
            comboBox1.ValueMember = "Value";

            DataTable dt2 = new DataTable();
            dt2.Columns.AddRange(new DataColumn[] { new DataColumn("Value", typeof(string)), new DataColumn("Name", typeof(string)) });
            dt2.Rows.Add("AD", "Adviser");
            dt2.Rows.Add("DO", "Development Officer");
            dt2.Rows.Add("UN", "Unit");
            dr = dt2.NewRow();
            dr[0] = 0;
            dr[1] = "-Please Select-";
            dt2.Rows.InsertAt(dr, 0);
            comboBox2.DataSource = dt2;
            comboBox2.DisplayMember = "Name";
            comboBox2.ValueMember = "Value";

            DataTable dt3 = new DataTable();
            cnn.Open();
            SqlDataAdapter sda = new SqlDataAdapter("Select * From UniqueIdUsingDDL order by SN desc", cnn);
            sda.Fill(dt3);
            dataGridView1.DataSource = dt3;
            dataGridView1.Columns[0].Width = 50;
            cnn.Close();
        }

        private string UniqueId(string connection, string query)
        {
            string NewId = string.Empty;
            string BrCode = Convert.ToString(comboBox1.SelectedValue.ToString());
            string PostCode = Convert.ToString(comboBox2.SelectedValue.ToString());
            using (SqlConnection cnn = new SqlConnection(connection))
            {
                cnn.Open();
                SqlCommand cmd = new SqlCommand(query, cnn);
                SqlDataReader dr = cmd.ExecuteReader();
                while (dr.Read())
                {
                    string i = dr[0].ToString();
                    if (string.IsNullOrEmpty(i))
                    {
                        NewId = BrCode + PostCode +  "000001";   //NEMP ( NEW Employee)
                    }
                    else
                    {
                        i = i.Substring(5);
                        int j = Convert.ToInt32(i);
                        j = j + 1;
                        NewId = BrCode + PostCode + j.ToString().PadLeft(5, '0');
                    }
                }
                cnn.Close();
            }
            return string.Concat(NewId);
        }

        private void button1_Click(object sender, EventArgs e)
        {
            string avoidrepeatid = Properties.Settings.Default.Con_Win_General_DB;
            string previousId = "select Max(EmpCode) EmpCode from UniqueIdUsingDDL";
            string EmpCode = UniqueId(avoidrepeatid, previousId);

            if (comboBox1.Text == "-Please Select-" || comboBox2.Text == "-Please Select-" || textBox1.Text == "")
            {
                MessageBox.Show("Please Enter Name and Select Branch Code and Employee Post.");
            }
            else
            {
                SqlCommand cmd = new SqlCommand("Select * From UniqueIdUsingDDL where EmpName='" + textBox1.Text + "'", cnn);
                SqlDataAdapter da = new SqlDataAdapter(cmd);
                DataSet ds = new DataSet();
                da.Fill(ds);
                int i = ds.Tables[0].Rows.Count;
                if (i > 0)
                {
                    MessageBox.Show("This Employee Name - '" + textBox1.Text + "'\nis Already Axists.");
                    ds.Clear();
                }
                else
                {
                    try
                    {
                        cnn.Open();
                        SqlCommand cmdinsert = new SqlCommand("Insert into UniqueIdUsingDDL Values('" + textBox1.Text + "','" + comboBox1.SelectedValue.ToString() + "','" + comboBox1.Text.Trim() + "','" + comboBox2.SelectedValue.ToString() + "','" + comboBox2.Text.Trim() + "','" + EmpCode + "')", cnn);
                        cmdinsert.ExecuteNonQuery();
                        cnn.Close();
                        SqlDataAdapter sda = new SqlDataAdapter("Select * From UniqueIdUsingDDL order by SN desc", cnn);
                        DataTable dt = new DataTable();
                        sda.Fill(dt);
                        dataGridView1.DataSource = dt;
                        MessageBox.Show("Employee Code has been Created Successfully.\nNew Employee Code is - '" + EmpCode + "'");
                    }
                    catch (Exception ex)
                    {
                        MessageBox.Show(ex.Message);
                    }
                }
            }
        }
    }
}

Output -