Show username belong to selected item value from combobox

Scenario
Fullname of user will be displayed in comboBox and when the user, select Fullname corresponding Username will be displayed in the Label control

Table User:
Firstname:
Lastname:
Username:
Password:
Email:
Notification

Combobox--> show Firstname + lastname

Label: Display username belong to selected item value from combo box.

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.OleDb;
namespace Firstname_lastname_combo_username
{
    public partial class Form1 : Form
    {
        string ConnectionString = System.Configuration.ConfigurationSettings.AppSettings["dsn"];
        OleDbCommand com;
        OleDbDataAdapter oda;
        DataSet ds;
        string str;

        public Form1()
        {
            InitializeComponent();
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            comboBox1.Text = "Select";
            OleDbConnection con = new OleDbConnection(ConnectionString);
            con.Open();
            str = "select * from emp";
            com = new OleDbCommand(str, con);
            oda = new OleDbDataAdapter(com);
            ds = new DataSet();
            oda.Fill(ds, "emp");
            for (int i = 0; i < ds.Tables[0].Rows.Count; i++)
            {
                comboBox1.Items.Add(ds.Tables[0].Rows[i]["Firstname"] + ds.Tables[0].Rows[i]["Lastname"].ToString());
            }
            con.Close();
        }

        private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
        {
            OleDbConnection con = new OleDbConnection(ConnectionString);
            con.Open();
            str = "select * from emp where Firstname+Lastname='" + comboBox1.SelectedItem + "'";
            com = new OleDbCommand(str, con);
            OleDbDataReader reader = com.ExecuteReader();
            while (reader.Read())
            {
                label1.Text = reader["Username"].ToString();
            }
            reader.Close();
            con.Close();
        }
    }
}