Question

I have been creating multiple background threads to parse xml files and recreate new xml files. Now the problem I am having is that even though I use synclock on global variables, I will still at times get errors and I am sure that this is just the crude way of coding I am doing, but I was wondering if someone had a better option.
program flow =

• access local folder and upload all files into list
• strip each file into xml entries and put these entries into an arraylist
• parse for specific values and enter these values into a database table
• now create a thread and take the arraylist of entries and the thread will reparse
• thread parses and creates a new xml file
• main thread continues with another function and then goes and get a file from list

I will add some code to show problem areas but if I have declared global variable in use does the different threads overwrite that value in the variable causing contamination.

    For Each g In resultsList
        gXmlList.Add(g)
    Next

    Dim bgw As New BackgroundWorker
    bgw.WorkerSupportsCancellation = True
    AddHandler bgw.DoWork, New DoWorkEventHandler(AddressOf createXML)
    AddHandler bgw.RunWorkerCompleted, AddressOf WorkComplete
    threadlist.Add(bgw)
    bgw.RunWorkerAsync()

  Private Sub createXML()
     num += 1
     Dim file As String = Module1.infile
     xmlfile = directoryPath & "\New" & dateTime.Now.ToUniversalTime.ToString("yyyyMMddhhmmss") & endExtension
     Thread.Sleep(2000)
     Dim doc As XmlDocument = New XmlDocument
     **xwriter = New XmlTextWriter(xmlfile, Encoding.UTF8)** this is where ioexception error
     xwriter.Formatting = Formatting.Indented
     xwriter.Indentation = 2
     xwriter.WriteStartDocument(True)
     xwriter.WriteStartElement("Posts")

I have global variables through out the app and should I be locking each one and does this not make using threads then useless.

    Dim j As Integer = 0

Answer 1

I believe your biggest problem is not knowing what features in .Net are thread safe. A list for example is not (a dictionary is). While you may get away with it you will eventually run into problems with locking, etc.

Your using classes and variables that are not thread safe. Any time you are working with threads you have to be Extremely careful with locking. To answer your question, yes, you have to lock and unlock everything you are working with unless the type / method specifically handles it for you.

There are a lot of multi threading (PLINQ for example) in .Net 4.0 which handle a lot of the “grunt work” for you. While you should learn and understand how to do thread safe code yourself it will give you a head start.

Try passing the data into the createXML() method. That may help isolate the code from other data being accessed. I would suggest reading up on threading and learning how to do it without a background worker.

Global variables are generally a bad idea. Given your VB code I’m guessing this is a carry over from the VB6 world for you. That’s not in any way intended to be insulting, just trying to help advance your skills forward. Variable scope should be as confined as possible.

Another thought looking at your code is to learn how to use String.Format() when building strings / paths.

Simple manual thread in VB to get you started:

Dim bThread As New Threading.Thread(AddressOf createXML)
bThread.IsBackground = True
bThread.Start()