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Walkthrough: Multithreading with the BackgroundWorker Component

Walkthrough: Multithreading with the BackgroundWorker Component (C# and Visual Basic)

This walkthrough demonstrates how to create a multithreaded application that searches a text file for occurrences of a word. It demonstrates:

To create the user interface

  1. Open a new Visual Basic or C# Windows Application project, and create a form named Form1.

  2. Add two buttons and four text boxes to Form1.

  3. Name the objects as shown in the following table.




    First button

    Name , Text

    Start, Start

    Second button

    Name , Text

    Cancel, Cancel

    First text box

    Name , Text

    SourceFile, ""

    Second text box

    Name , Text

    CompareString, ""

    Third text box

    Name , Text

    WordsCounted, "0"

    Fourth text box

    Name , Text

    LinesCounted, "0"

  4. Add a label next to each text box. Set the Text property for each label as shown in the following table.




    First label


    Source File

    Second label


    Compare String

    Third label


    Matching Words

    Fourth label


    Lines Counted

To create a BackgroundWorker component and subscribe to its events

  1. Add a BackgroundWorker component from the Components section of the ToolBox to the form. It will appear in the form's component tray.

  2. Set the following properties for the BackgroundWorker1 object in Visual Basic or the backgroundWorker1 object in C#.







  3. In C# only, subscribe to the events of the backgroundWorker1 object. At the top of the Properties window, click the Events icon. Double-click the RunWorkerCompleted event to create an event handler method. Do the same for the ProgressChanged and DoWork events.

To define the method that will run on a separate thread

  1. From the Project menu, choose Add Class to add a class to the project. The Add New Item dialog box is displayed.

  2. Select Class from the templates window and type Words.vb or Words.cs in the name field.

  3. Click Add. The Words class is displayed.

  4. Add the following code to the Words class:

    public class Words
        // Object to store the current state, for passing to the caller. 
        public class CurrentState
            public int LinesCounted;
            public int WordsMatched;
        public string SourceFile;
        public string CompareString;
        private int WordCount;
        private int LinesCounted;
        public void CountWords(
            System.ComponentModel.BackgroundWorker worker,
            System.ComponentModel.DoWorkEventArgs e)
            // Initialize the variables.
            CurrentState state = new CurrentState();
            string line = "";
            int elapsedTime = 20;
            DateTime lastReportDateTime = DateTime.Now;
            if (CompareString == null ||
                CompareString == System.String.Empty)
                throw new Exception("CompareString not specified.");
            // Open a new stream. 
            using (System.IO.StreamReader myStream = new System.IO.StreamReader(SourceFile))
                // Process lines while there are lines remaining in the file. 
                while (!myStream.EndOfStream)
                    if (worker.CancellationPending)
                        e.Cancel = true;
                        line = myStream.ReadLine();
                        WordCount += CountInString(line, CompareString);
                        LinesCounted += 1;
                        // Raise an event so the form can monitor progress. 
                        int compare = DateTime.Compare(
                            DateTime.Now, lastReportDateTime.AddMilliseconds(elapsedTime));
                        if (compare > 0)
                            state.LinesCounted = LinesCounted;
                            state.WordsMatched = WordCount;
                            worker.ReportProgress(0, state);
                            lastReportDateTime = DateTime.Now;
                    // Uncomment for testing. 
                // Report the final count values.
                state.LinesCounted = LinesCounted;
                state.WordsMatched = WordCount;
                worker.ReportProgress(0, state);
        private int CountInString(
            string SourceString,
            string CompareString)
            // This function counts the number of times 
            // a word is found in a line. 
            if (SourceString == null)
                return 0;
            string EscapedCompareString =
            System.Text.RegularExpressions.Regex regex;
            regex = new System.Text.RegularExpressions.Regex( 
                // To count all occurrences of the string, even within words, remove 
                // both instances of @"\b" from the following line.
                @"\b" + EscapedCompareString + @"\b",
            System.Text.RegularExpressions.MatchCollection matches;
            matches = regex.Matches(SourceString);
            return matches.Count;

To handle events from the thread

  • Add the following event handlers to your main form:

    private void backgroundWorker1_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
    // This event handler is called when the background thread finishes. 
    // This method runs on the main thread. 
    if (e.Error != null)
        MessageBox.Show("Error: " + e.Error.Message);
    else if (e.Cancelled)
        MessageBox.Show("Word counting canceled.");
        MessageBox.Show("Finished counting words.");
    private void backgroundWorker1_ProgressChanged(object sender, ProgressChangedEventArgs e)
        // This method runs on the main thread.
        Words.CurrentState state =
        this.LinesCounted.Text = state.LinesCounted.ToString();
        this.WordsCounted.Text = state.WordsMatched.ToString();

To start and call a new thread that runs the WordCount method

  1. Add the following procedures to your program:

    private void backgroundWorker1_DoWork(object sender, DoWorkEventArgs e)
        // This event handler is where the actual work is done. 
        // This method runs on the background thread. 
        // Get the BackgroundWorker object that raised this event.
        System.ComponentModel.BackgroundWorker worker;
        worker = (System.ComponentModel.BackgroundWorker)sender;
        // Get the Words object and call the main method.
        Words WC = (Words)e.Argument;
        WC.CountWords(worker, e);
    private void StartThread()
        // This method runs on the main thread. 
        this.WordsCounted.Text = "0";
        // Initialize the object that the background worker calls.
        Words WC = new Words();
        WC.CompareString = this.CompareString.Text;
        WC.SourceFile = this.SourceFile.Text;
        // Start the asynchronous operation.
  2. Call the StartThread method from the Start button on your form:

    private void Start_Click(object sender, EventArgs e)

To implement a Cancel button that stops the thread

  • Call the StopThread procedure from the Click event handler for the Cancel button.

    private void Cancel_Click(object sender, EventArgs e)
        // Cancel the asynchronous operation. 

You can now test the application to make sure it works correctly.

To test the application

  1. Press F5 to run the application.

  2. When the form is displayed, enter the file path for the file you want to test in the sourceFile box. For example, assuming your test file is named Test.txt, enter C:\Test.txt.

  3. In the second text box, enter a word or phrase for the application to search for in the text file.

  4. Click the Start button. The LinesCounted button should begin incrementing immediately. The application displays the message "Finished Counting" when it is done.

To test the Cancel button

  1. Press F5 to start the application, and enter the file name and search word as described in the previous procedure. Make sure that the file you choose is large enough to ensure you will have time to cancel the procedure before it is finished.

  2. Click the Start button to start the application.

  3. Click the Cancel button. The application should stop counting immediately.

This application contains some basic error handling. It detects blank search strings. You can make this program more robust by handling other errors, such as exceeding the maximum number of words or lines that can be counted.

© 2015 Microsoft