C#

private void comboBox3_SelectedIndexChanged(object sender, EventArgs e)
        {
            try
            {
 
                database = new OleDbConnection(connectionString);
 
                // Create a command object and then a query which accepts a papameter
                System.Data.OleDb.OleDbCommand cmd = new System.Data.OleDb.OleDbCommand("Select * from Products", database);
                database.Open();
                System.Data.OleDb.OleDbDataReader dr;
                dr=cmd.ExecuteReader();
 
                while (dr.Read())
                {
                    //comboBox3.Items.AddRange(new object[] { dr[1].ToString() });  //Change the index according to the values you want to populate in the combobox.
                    comboBox3.Items.Add(dr["ProductName"]);
 
                }
            }
                catch (Exception ex)
            {
 
                MessageBox.Show(ex.Message);
                return;
            }
                
               
               }