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:

• Defining a class with a method that can be called by the BackgroundWorker component.

• Handling events raised by the BackgroundWorker component.

• Starting a BackgroundWorker component to run a method.

• Implementing a Cancel button that stops the BackgroundWorker component.

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.

   Object	Property	Setting
   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.

   Object	Property	Setting
   First label	Text	Source File
   Second label	Text	Compare String
   Third label	Text	Matching Words
   Fourth label	Text	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#.

   Property	Setting
   WorkerReportsProgress	True
   WorkerSupportsCancellation	True

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 LinesCounted As Integer 
           Public WordsMatched As Integer 
       End Class 
   
       Public SourceFile As String 
       Public CompareString As String 
       Private WordCount As Integer = 0
       Private LinesCounted As Integer = 0
   
       Public Sub CountWords(
           ByVal worker As System.ComponentModel.BackgroundWorker,
           ByVal e As System.ComponentModel.DoWorkEventArgs
       )
           ' Initialize the variables. 
           Dim state As New CurrentState
           Dim line = "" 
           Dim elapsedTime = 20
           Dim lastReportDateTime = Now
   
           If CompareString Is Nothing OrElse
              CompareString = System.String.Empty Then 
   
              Throw New Exception("CompareString not specified.")
           End If 
   
           Using myStream As New System.IO.StreamReader(SourceFile)
   
               ' Process lines while there are lines remaining in the file. 
               Do While Not myStream.EndOfStream
                   If worker.CancellationPending Then
                       e.Cancel = True 
                       Exit Do 
                   Else
                       line = myStream.ReadLine
                       WordCount += CountInString(line, CompareString)
                       LinesCounted += 1
   
                       ' Raise an event so the form can monitor progress. 
                       If Now > lastReportDateTime.AddMilliseconds(elapsedTime) Then
                           state.LinesCounted = LinesCounted
                           state.WordsMatched = WordCount
                           worker.ReportProgress(0, state)
                           lastReportDateTime = Now
                       End If 
   
                       ' Uncomment for testing. 
                       'System.Threading.Thread.Sleep(5) 
                   End If 
               Loop 
   
               ' Report the final count values.
               state.LinesCounted = LinesCounted
               state.WordsMatched = WordCount
               worker.ReportProgress(0, state)
           End Using 
       End Sub 
   
       Private Function CountInString(
           ByVal SourceString As String,
           ByVal CompareString As String
       ) As Integer 
           ' This function counts the number of times 
           ' a word is found in a line. 
           If SourceString Is Nothing Then 
               Return 0
           End If 
   
           Dim EscapedCompareString =
               System.Text.RegularExpressions.Regex.Escape(CompareString)
   
           ' To count all occurrences of the string, even within words, remove 
           ' both instances of "\b". 
           Dim regex As New System.Text.RegularExpressions.Regex(
               "\b" + EscapedCompareString + "\b",
               System.Text.RegularExpressions.RegexOptions.IgnoreCase)
   
           Dim matches As System.Text.RegularExpressions.MatchCollection
           matches = regex.Matches(SourceString)
           Return matches.Count
       End Function 
   End 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;
                       break;
                   }
                   else
                   {
                       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. 
                   //System.Threading.Thread.Sleep(5);
               }
   
               // 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.Escape(CompareString);
   
           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.RegexOptions.IgnoreCase);
   
           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 Sub BackgroundWorker1_RunWorkerCompleted( 
      ByVal sender As Object, 
      ByVal e As System.ComponentModel.RunWorkerCompletedEventArgs
    ) Handles BackgroundWorker1.RunWorkerCompleted
  
      ' This event handler is called when the background thread finishes. 
      ' This method runs on the main thread. 
      If e.Error IsNot Nothing Then
          MessageBox.Show("Error: " & e.Error.Message)
      ElseIf e.Cancelled Then
          MessageBox.Show("Word counting canceled.")
      Else
          MessageBox.Show("Finished counting words.")
      End If 
  End Sub 
  
  Private Sub BackgroundWorker1_ProgressChanged( 
      ByVal sender As Object, 
      ByVal e As System.ComponentModel.ProgressChangedEventArgs
    ) Handles BackgroundWorker1.ProgressChanged
  
      ' This method runs on the main thread. 
      Dim state As Words.CurrentState = 
          CType(e.UserState, Words.CurrentState)
      Me.LinesCounted.Text = state.LinesCounted.ToString
      Me.WordsCounted.Text = state.WordsMatched.ToString
  End Sub
  

  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.");
  else
      MessageBox.Show("Finished counting words.");
  }
  
  private void backgroundWorker1_ProgressChanged(object sender, ProgressChangedEventArgs e)
  {
      // This method runs on the main thread.
      Words.CurrentState state =
          (Words.CurrentState)e.UserState;
      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 Sub BackgroundWorker1_DoWork( 
       ByVal sender As Object, 
       ByVal e As System.ComponentModel.DoWorkEventArgs
     ) Handles BackgroundWorker1.DoWork
   
       ' 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. 
       Dim worker As System.ComponentModel.BackgroundWorker
       worker = CType(sender, System.ComponentModel.BackgroundWorker)
   
       ' Get the Words object and call the main method. 
       Dim WC As Words = CType(e.Argument, Words)
       WC.CountWords(worker, e)
   End Sub 
   
   Sub StartThread()
       ' This method runs on the main thread. 
       Me.WordsCounted.Text = "0" 
   
       ' Initialize the object that the background worker calls. 
       Dim WC As New Words
       WC.CompareString = Me.CompareString.Text
       WC.SourceFile = Me.SourceFile.Text
   
       ' Start the asynchronous operation.
       BackgroundWorker1.RunWorkerAsync(WC)
   End Sub
   

   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.
       backgroundWorker1.RunWorkerAsync(WC);
   }
   

2. Call the StartThread method from the Start button on your form:

   Private Sub Start_Click() Handles Start.Click
       StartThread()
   End Sub
   

   private void Start_Click(object sender, EventArgs e)
   {
       StartThread();
   }
   

To implement a Cancel button that stops the thread

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

  Private Sub Cancel_Click() Handles Cancel.Click
      ' Cancel the asynchronous operation. 
      Me.BackgroundWorker1.CancelAsync()
  End Sub
  

  private void Cancel_Click(object sender, EventArgs e)
  {
      // Cancel the asynchronous operation. 
      this.backgroundWorker1.CancelAsync();
  }
  

Testing

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.

Next Steps

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.

See Also

Tasks

Walkthrough: Authoring a Simple Multithreaded Component with Visual Basic

How to: Subscribe to and Unsubscribe from Events (C# Programming Guide)

Other Resources

Threading (C# and Visual Basic)