BackgroundWorker

The BackgroundWorker class allow you to have a responsive UI while you retrieve your data.
For more information, please refer to the Threading > BackgroundWorker page.

using System.Linq; 
using System.Windows.Forms; 

namespace MyProject.Forms 
{ 
    public partial class MyForm : Form 
    { 
        System.ComponentModel.BackgroundWorker _worker = new System.ComponentModel.BackgroundWorker(); 
        System.Threading.SynchronizationContext _syncContext; 

        public MyForm() 
        { 
            InitializeComponent(); 
            _syncContext = System.Threading.SynchronizationContext.Current; 
        } 

        private void MyForm_Load(object sender, System.EventArgs e) 
        { 
            try 
            { 
                _worker.WorkerReportsProgress = true; 
                _worker.WorkerSupportsCancellation = true; 

                _worker.DoWork += new System.ComponentModel.DoWorkEventHandler(HandleDoWork); 
                _worker.RunWorkerCompleted += new System.ComponentModel.RunWorkerCompletedEventHandler(HandleWorkerCompleted); 
                _worker.ProgressChanged += new System.ComponentModel.ProgressChangedEventHandler(HandleProgressChanged); 

                _worker.RunWorkerAsync(); 

                this.comboBox1.Items.Add(""); 

            } 
            catch (System.Exception ex) 
            { 
                ExceptionHandling.MessageShow(System.Reflection.MethodBase.GetCurrentMethod(), ex); 
            } 
        } 

        private void HandleDoWork( 
            object sender, 
            System.ComponentModel.DoWorkEventArgs e) 
        { 

            System.Collections.Generic.List<string> companieslist; 
            companieslist.Add("one"); 
            companieslist.Add("two"); 
            companieslist.Add("three"); 

            this.comboBox1.Items.Clear(); 
            for (int i = 0; i < companieslist.Count; i++) 
            { 
                this.comboBox1.Items.Add(companieslist[i]); 
            } 

            if (_worker.CancellationPending) 
            { 
                e.Cancel = true; 
            } 
        } 

        private void HandleProgressChanged( 
            object sender, 
            System.ComponentModel.ProgressChangedEventArgs e) 
        { 
            // DO Progress Bar Updates Here 

            System.Threading.SendOrPostCallback callback = 
                new System.Threading.SendOrPostCallback((o) => 
            { 

            }); 
            _syncContext.Send(callback, null); 
        } 

        private void HandleWorkerCompleted( 
            object sender, 
            System.ComponentModel.RunWorkerCompletedEventArgs e) 
        { 
            // DO Any completed step items here 
        } 

    } 
}