how can add items in a combobox?

You can also add items from an array or list rather than from a database:

 private void Form1_Load(object sender, EventArgs e)
 {
 comboBox1.Items.Add("a");
 comboBox1.Items.Add("b");
 comboBox1.Items.Add("c"); 
 }


From database

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 Combobox
{
  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.Items.Add("Choose CourseID");
  OleDbConnection con = new OleDbConnection(ConnectionString);
  con.Open();
  str = "select * from Courses";
  com = new OleDbCommand(str, con);
  OleDbDataReader reader = com.ExecuteReader();
  while (reader.Read())
  {
  comboBox1.Items.Add(reader["CourseID"]); 
  }
 
 
  }
 
 
  }
}