Walkthrough: Multithreading

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 build the code example for this topic

1. Open a new Visual Basic 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

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

6. Set the following properties for the BackgroundWorker1 object.

   Property	Setting
   WorkerReportsProgress	True
   WorkerSupportsCancellation	True

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 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)
   
           Dim regex As New System.Text.RegularExpressions.Regex( _
               EscapedCompareString, _
               System.Text.RegularExpressions.RegexOptions.IgnoreCase)
   
           Dim matches As System.Text.RegularExpressions.MatchCollection
           matches = regex.Matches(SourceString)
           Return matches.Count
       End Function 
   End Class
   

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
          MsgBox("Error: " & e.Error.Message)
      ElseIf e.Cancelled Then
          MsgBox("Word counting canceled.")
      Else
          MsgBox("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
  

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 Works 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
   

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

   Private Sub Start_Click( _
       ByVal sender As System.Object, _
       ByVal e As System.EventArgs) _
       Handles Start.Click
   
       StartThread()
   End Sub
   

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( _
      ByVal sender As System.Object, _
      ByVal e As System.EventArgs) _
      Handles Cancel.Click
  
      ' Cancel the asynchronous operation. 
      Me.BackgroundWorker1.CancelAsync()
  End Sub
  

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

Other Resources

Multithreading in Visual Basic

Change History

Date	History	Reason
January 2010	Modified example.	Content bug fix.
July 2008	Fixed errors in the WordCount method of the example.	Customer feedback.