C# RSS feed reader with thread, feeds are written to listView multiple times

Question

I have a RSS news reader which reads a rss feed and writes the news with links in a ListView. when I'm executing my program I'm starting a new thread like this:

    Thread myThread = new Thread(getNews);
    myThread.Start();

My method to read the feed looks like this:

    public void getNews()
    {  
        //Creates a XmlTextReader which reads from the url entered in input field
        rssReader = new XmlTextReader(txtUrl.Text);

        //Creates an xml doc to save the content of the entered path
        rssDoc = new XmlDocument();

        //Loads the xml content from the reader into a XmlDocument
        rssDoc.Load(rssReader);

        //Make a loop to search for the <rss> tag
        for (int i = 0; i < rssDoc.ChildNodes.Count; i++)
        {
            //If the childenode is the rss tag
            if (rssDoc.ChildNodes[i].Name == "rss")
            {
                //the <rss> tag is found, and we know where it is
                nodeRss = rssDoc.ChildNodes[i];
            }
        }

        //Make a loop to search for the <channel> tag
        for (int i = 0; i < nodeRss.ChildNodes.Count; i++)
        {
            //If the childnode is the channel tag
            if (nodeRss.ChildNodes[i].Name == "channel")
            {
                //The channel tag is found and we know where it is
                nodeChannel = nodeRss.ChildNodes[i];
            }
        }

        //Make a loop to search for the <item> tag
        for (int i = 0; i < nodeChannel.ChildNodes.Count; i++)
        {
            //If the childnode is the item tag
            if (nodeChannel.ChildNodes[i].Name == "item")
            {
                //the item tag is found, and we know where it is
                nodeItem = nodeChannel.ChildNodes[i];

                //Creates a new row in the LstView which contains information from inside the nodes
                rowNews = new ListViewItem();
                rowNews.Text = nodeItem["title"].InnerText;
                rowNews.SubItems.Add(nodeItem["link"].InnerText);

                if (this.lstView.InvokeRequired)
                {
                    AddItemCallback d = new AddItemCallback(getNews);
                    this.Invoke(d);
                }
                else
                {
                    lstView.Items.Add(rowNews);
                }
            }

        }

My problem is, after I started to run my code in a new thread and use the delegate to check if the listView requres an invoked all the news feeds are written in my listView mulitple times. if I run method without starting a new thread and using the delegate its only written once, why is that? It's probably a very simple question but just can't seeme to figure out why

Thanks in advance, code examples are appreciated :)

Answer 1

If invoke is required, you're calling the getNews method again. Instead of using this code:

if (this.lstView.InvokeRequired)
{
     AddItemCallback d = new AddItemCallback(getNews); // STOP CALLING getNews
     this.Invoke(d);
}

You need to call another method to update your UI. It's a giant loop.

Answer 2

You put your invocation check in the wrong place - thus your getNews function is being called many many more times than you intended.

at the very top of getNews try this:

if (this.lstView.InvokeRequired) {
AddItemCallback d = new AddItemCallback(getNews);
this.Invoke(d);
return;
}

Then replace the if statement at the end with lstView.Items.Add(rowNews);

Of course, doing it this way effectively marshalls your getNews to the UI thread. What you really need is to build a collection of ListViewItems and pass that collection to an Updater method.. and in THAT method you check if invokation is required.