using System;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace WindowsFormsApp1
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private async void button1_Click(object sender, EventArgs e)
        {
            progressBar1.Value = 0;
            progressBar1.Maximum = 100;
            var progress = new Progress<int>(s => progressBar1.Value = s);
            await Task.Factory.StartNew(() => Tasker.LongWork(progress),
                TaskCreationOptions.LongRunning);
            progressBar1.Value = 100;
        }
    }

    class Tasker
    {
        public static void LongWork(IProgress<int> progress)
        {
            // Perform a long running work...
            for (var i = 0; i < 100; i++)
            {
                Task.Delay(100).Wait();
                progress.Report(i);
            }
        }
    }
}