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Walkthrough: Multithreading

Visual Studio .NET 2003

This walkthrough demonstrates how to create a multithreaded application that searches a text file for occurrences of a word. The walkthrough consists of four main parts:

Defining a class with a method that can be run on a thread independently
Handling events raised by other threads
Starting a new thread to run a method
Implementing a Cancel button that stops the thread

To build the code example for this topic

Open a new Visual Basic .NET Windows Application project, and create a form named Form1.
Add two buttons and four text boxes to Form1.

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"

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

Object	Property	Setting
First label	Text	Source File
Second label	Text	Compare String
Third label	Text	Matching Words
Fourth label	Text	Lines Counted

To define the method that will run on a separate thread

From the Project menu, choose Add Class to add a class to the project. The Add New Item dialog is displayed.
Select Class from the templates window and type Words.vb in the name field.
Click Open. The Words class is displayed.

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

Option Compare Text ' Case insensitive search.
' Use Option Compare Binary for case sensitive search.

Add the following code to the Words class:

Public Class Words
   Private strloc As Integer
   Private RemainingString As String
   Private SourceString As String
   Public SourceFile As String
   Public CompareString As String
   Public WordCount As Integer
   Private LinesCounted As Integer = 0
   Public Event Status(ByVal LinesCounted As Integer, _
                       ByVal WordsMatched As Integer)
   Public Event FinishedCounting(ByVal NumberOfMatches As Integer)
       
   Sub CountWords()
      Dim f As System.IO.File
      Dim mystream As System.IO.StreamReader
      Dim mystr As String = " " ' Initialize to not empty.

      If SourceFile = System.String.Empty _
         Or CompareString = System.String.Empty Then
         MsgBox("You must set the SourceFile " & _
                " and CompareString fields before calling CountWords.")
         Exit Sub
      End If

      If f.Exists(SourceFile) = False Then
         MsgBox("Error; Unable to open the input file.")
         Exit Sub
      End If

      Try
         mystream = f.OpenText(SourceFile) ' Open a new stream.
         ' Do until the stream returns Nothing at end of file.
         Do Until IsNothing(mystr)
            mystr = mystream.ReadLine
            WordCount += CountInString(1, mystr, CompareString)
            LinesCounted += 1 ' Increment line count.
            ' Raise an event so the form can monitor progress.
            RaiseEvent Status(LinesCounted, WordCount)
         Loop
      Catch eof As IO.EndOfStreamException
      ' No action is necessary, the end of the stream has been reached.
      Catch IOExcep As IO.IOException
         ' Some kind of error occurred.
         MsgBox(IOExcep.Message)
      Finally
         mystream.Close() ' Close the file.
      End Try
      RaiseEvent FinishedCounting(WordCount)
   End Sub       

   Private Function CountInString(ByVal StartingPoint As Integer, _
                                  ByVal SourceString As String, _
                                  ByVal CompareString As String) As Integer
      ' This function counts the number of times
      ' a word is found in a sentence.
      ' If the word is found, the function is called recursively on
      ' the remainder of the string.
      strloc = Strings.InStr(StartingPoint, SourceString, CompareString)
      If strloc <> 0 Then
         CountInString += 1
         CountInString += CountInString(strloc + Strings.Len(CompareString), _
                                        SourceString, CompareString)
      End If
   End Function
End Class

To handle events from the thread

Add the following event handlers to your main form:

Sub FinishedCountingEventHandler(ByVal WordCount As Integer)
   ' This event handler is called when the word-counting thread
   ' raises a FinishedCounting event indicating that
   ' the whole file has been checked. The WordCount parameter
   ' is the number of matches found.
   Me.WordsCounted.Text = CStr(WordCount)
   MsgBox("Finished counting words.")
End Sub
Sub LineCountEventHandler(ByVal LinesCounted As Integer, _
                          ByVal WordsMatched As Integer)
   ' This event handler is called when the word-counting thread
   ' raises a status event after reading each line
   ' from the source file.
   Me.LinesCounted.Text = CStr(LinesCounted)
   Me.WordsCounted.Text = CStr(WordsMatched)
End Sub

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

Add the following code to the Declarations section of Form1:
Copy
Dim Thread As System.Threading.Thread

Add the following procedure to your program:

Sub StartThread()
   Dim WC As New words()
   Me.WordsCounted.Text = "0"
   ' Set the values of some object fields, because
   ' you cannot create a thread and pass it a method
   ' signature that contains parameters.
   WC.CompareString = Me.CompareString.Text ' The string to look for.
   WC.SourceFile = Me.SourceFile.Text       ' The file to search.
   ' Associate an event handler with the event that
   ' is raised when the new thread is done.
   AddHandler WC.FinishedCounting, AddressOf _ 
                                   FinishedCountingEventHandler
   ' Associate an event handler with the event that
      ' is raised after each line is read.
   AddHandler WC.Status, AddressOf LineCountEventHandler
   ' Create the new thread.
   Thread = New System.Threading.Thread(AddressOf WC.CountWords)
   ' Start the new thread.
   Thread.Start()
End Sub

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

Add the following procedure to your main form:

Protected Sub StopThread(Thread as System.Threading.Thread)
'   Call StopThread from a Cancel button procedure.
    Thread.Abort()
End Sub

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
   StopThread(Thread)
End Sub

Testing

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

To test the application

Press F5 to run the application.
When the form is displayed, enter the file path in the sourceFile textbox for a file you want to test. For example, assuming your test file is named Test.txt, enter C:\Test.txt.
In the second text box, enter a word or phrase for the application to search for in the text file.
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

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 so you will have time to cancel the procedure before it is finished.
Click the Start button to start the application.
Click the Cancel button. The application should stop counting immediately.

Error Handling

This application contains some basic error handling. It detects and handles source file problems and 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.

Walkthrough: Multithreading

Testing

Error Handling

See Also