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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


    Source File

    Second label


    Compare String

    Third label


    Matching Words

    Fourth label


    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





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 an Option Compare statement to the top of the Words class, above the Class statement:

    Option Compare Text ' Case insensitive search.
    ' Use Option Compare Binary for case sensitive search.
  5. 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 mystream As System.IO.StreamReader = Nothing
            Dim line As String = ""
            If CompareString Is Nothing Or _
                    CompareString = System.String.Empty Then
                Throw New Exception("CompareString not specified.")
            End If
                ' Open a new stream.
                mystream = My.Computer.FileSystem.OpenTextFileReader(SourceFile)
                ' Do until the stream returns Nothing at end of file.
                Do Until line Is Nothing
                    If worker.CancellationPending Then
                        e.Cancel = True
                        Exit Do
                        line = mystream.ReadLine
                        WordCount += CountInString(line, CompareString)
                        LinesCounted += 1 ' Increment line count.
                        ' Raise an event so the form can monitor progress.
                        Dim state As New CurrentState
                        state.LinesCounted = LinesCounted
                        state.WordsMatched = WordCount
                        worker.ReportProgress(0, state)
                    End If
                If mystream IsNot Nothing Then
                    ' Close the file.
                End If
            End Try
        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 regex As New System.Text.RegularExpressions.Regex( _
            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.")
            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 event handler is called after the background thread
        ' reads a line from the source file.
        ' 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.
    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
    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.
    End Sub

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.