BackgroundWorker

This makes threads easy to implement in Windows Forms. Intensive tasks need to be done on another thread so the UI does not freeze.

With BackgroundWorker, it is necessary to post messages and update the user interface when the task is done. There is some complexity to this—but it makes sense with an example.

DoWork

Here we must look at the C# view of your file, where we will see the DoWork method. The backgroundWorker1_DoWork event is generated when you double-click on DoWork.

using System;
using System.ComponentModel;
using System.Windows.Forms;
using System.Threading;

namespace WindowsFormsApplication1
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void backgroundWorker1_DoWork(object sender, DoWorkEventArgs e)
        {
            Thread.Sleep(1000); // One second.
        }
    }
}

Properties

I want to emphasize the Argument, Result, and the RunWorkerAsync methods. These are the properties of BackgroundWorker required to accomplish anything.

DoWorkEventArgs e
Contains e.Argument and e.Result.
It is used to access those properties.

e.Argument
Used to get the parameter reference received by RunWorkerAsync.

e.Result
Check to see what the BackgroundWorker processing did.

backgroundWorker1.RunWorkerAsync
Called to start a process on the worker thread.

RunWorkerAsync

We can add arguments and return values to BackgroundWorker. We need to add arguments, invoke the BackgroundWorker, and then receive the results of the thread.

public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();

        //
        // Example argument object
        //
        TestObject test = new TestObject
        {
            OneValue = 5,
            TwoValue = 4
        };
        //
        // Send argument to our worker thread
        //
        backgroundWorker1.RunWorkerAsync(test);
    }
}

/// <summary>
/// The test class for our example.
/// </summary>
class TestObject
{
    public int OneValue { get; set; }
    public int TwoValue { get; set; }
}

DoWork 2

Place some code in DoWork. Then, use the DoWorkEventArgs in its body and as its result. We hook up the arguments and results from the RunWorkerAsync call.

/// <summary>
/// Where we do the work in the program (the expensive slow stuff).
/// </summary>
private void backgroundWorker1_DoWork(object sender, DoWorkEventArgs e)
{
    //
    // e.Argument contains whatever was sent to the background worker in RunWorkerAsync.
    // We can cast it to its original type.
    //
    TestObject argumentTest = e.Argument as TestObject;

    //
    // Boring....
    //
    Thread.Sleep(10000);

    argumentTest.OneValue = 6;
    argumentTest.TwoValue = 3;

    //
    // Now, return the values we generated in this method.
    // ... Use e.Result.
    //
    e.Result = argumentTest;
}

RunWorkerCompleted

You can use the RunWorkerCompleted event handler by clicking on the backgroundWorker1 icon in the tray. Now double-click in RunWorkerCompleted.

/// <summary>
/// This is on the main thread, so we can update a TextBox or anything.
/// </summary>
private void backgroundWorker1_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
{
    //
    // Receive the result from DoWork, and display it.
    //
    TestObject test = e.Result as TestObject;
    this.Text = test.OneValue.ToString() + " " + test.TwoValue.ToString();

    //
    // Will display "6 3" in title Text (in this example)
    //
}

Review, steps

Call RunWorkerAsync with an argument. You can pass any argument to this method on BackgroundWorker, including null. It simply must inherit from object, which everything does.

ProgressBar

A ProgressBar visually displays progress. And the ProgressBar control in Windows Forms can be used with a BackgroundWorker.

We implemented the code for a BackgroundWorker control. We generally instead prefer ThreadPool when we need many threads. And threads are not always useful.