Retrieve from RSS Feed using XmlDocument

Jay A. Allen — Thu, 03 Aug 2006 01:50:00 GMT

So I'm trying to wrap up this white paper I'm writing on consuming RSS in Windows Forms. I wrote a whole section on how to apply a custom XSLT transform to a feed, and display the resulting HTML in the WebBrowser. (I gave a less-complete version of a solution in an earlier blog post.) Then it hit me: "Jay, you fool - what if the RSS feed already has a stylesheet associated with it?"

Fortunately, factoring this into your logic is simple. You can use XmlDocument, SelectSingleNode(), and a rather funky-looking XPath command to test whether the feed has a stylesheet. If it does, you can pass it straight to the WebBrowser control for rendering, with no further processing required.

            XmlDocument doc = new XmlDocument();
            try
            {
                doc.Load("http://blogs.msdn.com/winformsue/rss.xml");
            }
            catch (Exception ex)
            {
                MessageBox.Show("Could not load XML file. Error: " + ex.Message);
                return;
            }

            XmlProcessingInstruction node = (XmlProcessingInstruction)doc.SelectSingleNode("/processing-instruction(\"xml-stylesheet\")");
            if (node != null)
            {
                webBrowser1.Url = new Uri("http://blogs.msdn.com/winformsue/rss.xml");
            }
            else
            {
                // Apply custom style.
            }

Using the Microsoft Feeds (RSS) API from Windows Forms

Jay A. Allen — Fri, 21 Jul 2006 22:11:00 GMT

Wim De Cleen has a great two-part walkthrough up on how to use the new Microsoft Feeds API to display a folder-style list of RSS feeds in Windows Forms. The Feeds API is new with Internet Explorer 7, and is the programmatic interface to the new Windows RSS Platform.

I'm working on a white paper that details several ways to consume RSS inside of a Windows Forms application, which will include a section on the Windows RSS Platform. Building a feed reader app well is difficult, as you have to account for users (like, um, me) who have dozens or hundreds of feeds. Updating these in near-real-time without pegging your processor and absorbing all of your client's bandwidth is a challenge. The RSS Platform makes it easier for developers to build RSS-powered apps by handling all of the scheduling, throttling, and storage issues on their behalf.

De Cleen's example is what is traditionally called a "folder" approach to viewing feeds: view one feed and its contents at a time. I'm working on a sample that takes a "river of news" approach, displaying all unread items sorted by date. I'll post it here when it's complete, and include a pointer to the published white paper when it's ready.

How to Data Bind to an RSS Feed (Blog Posts) in Windows Forms

Jay A. Allen — Thu, 15 Jun 2006 00:48:00 GMT

RSS feeds are XML files that represent the current posts on a syndicated content channel, such as a blog or news feed. Since RSS feeds are XML, they can be loaded into an ADO.NET DataSet and bound to the elements of a form.

Picture 1: Simple RSS Feed Reader

The following C# code shows the basics of wiring up a feed to a form so that the title element is bound to a Label and the description (blog post content) is bound to the DocumentText property of the WebBrowser control. The method GetBlogPost() is called in my code from a list of entries contained on a ToolStrip control; I use the Padding property of ToolStripItem to make the ToolStrip list appear as a categorized hierarchy of max one depth level. The feed URI is saved in the Tag property of the ToolStripButton, which is what the user clicks to call GetBlogPost(). The button controls at the bottom of the form allow you to flip back and forward through the currently available posts in the latest version of the feed.

The form for this sample requires these controls:

postTitleLink: LinkLabel
webBrowser1: WebBrowser
firstbutton, lastButton, previousButton, nextButton: Button

The trickiest part of this code occurs partway down the GetBlogPosts() method, where I need to obtain the list of item elements in the RSS feed to which I data bind. The item elements are consider children of the channel element. When I load the XML file into an ADO.NET DataSet, ADO.NET makes the item elements available as a DataRelation whose name is a conjunction of the channel and item elements. In other words, to get at the item elements, I have to use this line:

           bindingTable = dt.ChildRelations["channel_item"].ChildTable;

- where dt is the name of my DataTable containing the RSS data.

Here's the full sample. (NOTE: This is a preview of a full sample I'll be publishing as the subject of a white paper in July. The final sample will be more fleshed out than this skeleton sample.)

        private void GetBlogPost(string feed)
        {
            DataSet ds = new DataSet();
            XmlReader reader = null;

            try
            {
                reader = XmlReader.Create(feed);
            }
            catch (FileNotFoundException fileEx)
            {
                MessageBox.Show("Cannot file blog feed. Error returned: " + fileEx.Message);
                return;
            }
            catch (SecurityException secEx)
            {
                MessageBox.Show("You do not have permission to view this feed. Error returned: " + secEx.Message);
                return;
            }

            // Cruise forward to the <item> elements.
            reader.ReadToDescendant("channel");

            ds.ReadXml(reader);
            DataTable dt = ds.Tables[0];

            // Set up data binding.
            bindingTable = dt.ChildRelations["channel_item"].ChildTable;
            postTitleLink.DataBindings.Add(new Binding("Text", bindingTable, "title"));
            //dateLabel.DataBindings.Add(new Binding("Text", bindingTable, "dc:date"));
            webBrowser1.DataBindings.Add(new Binding("DocumentText", bindingTable, "description"));

            // Set the Previous and Next buttons as applicable.
            previousButton.Enabled = false;
            BindingManagerBase myContext = this.BindingContext[bindingTable];
            if (myContext.Count == 1)
            {
                nextButton.Enabled = false;
                lastButton.Enabled = false;
            }
            else
            {
                nextButton.Enabled = true;
                lastButton.Enabled = true;
            }

            firstButton.Enabled = false;

            recordLabel.Text = (myContext.Position + 1).ToString();

            myContext.CurrentChanged += new EventHandler(myContext_CurrentChanged);
        }

        void myContext_CurrentChanged(object sender, EventArgs e)
        {
            recordLabel.Text = (this.BindingContext[bindingTable].Position + 1).ToString();
        }

        private void nextButton_Click(object sender, EventArgs e)
        {
            BindingManagerBase myContext = this.BindingContext[bindingTable];
            myContext.Position += 1;
            previousButton.Enabled = true;
            if (myContext.Position + 1 == myContext.Count)
            {
                nextButton.Enabled = false;
                lastButton.Enabled = false;
            }
            if (myContext.Position == 1)
            {
                firstButton.Enabled = true;
            }
        }

        private void previousButton_Click(object sender, EventArgs e)
        {
            BindingManagerBase myContext = this.BindingContext[bindingTable];
            myContext.Position -= 1;
            nextButton.Enabled = true;
            if (myContext.Position == 0)
            {
                previousButton.Enabled = false;
                firstButton.Enabled = false;
            }
            if (myContext.Position + 2 == myContext.Count)
            {
                lastButton.Enabled = true;
            }
        }

        private void firstButton_Click(object sender, EventArgs e)
        {
            BindingManagerBase myContext = this.BindingContext[bindingTable];
            myContext.Position = 0;
            previousButton.Enabled = false;
            firstButton.Enabled = false;
            if (lastButton.Enabled == false)
            {
                lastButton.Enabled = true;
                nextButton.Enabled = true;
            }
        }

        private void lastButton_Click(object sender, EventArgs e)
        {
            BindingManagerBase myContext = this.BindingContext[bindingTable];
            myContext.Position = myContext.Count - 1;
            nextButton.Enabled = false;
            lastButton.Enabled = false;
            if (firstButton.Enabled == false)
            {
                firstButton.Enabled = true;
                previousButton.Enabled = true;
            }
        }

        private void postTitleLink_LinkClicked(object sender, LinkLabelLinkClickedEventArgs e)
        {
            BindingManagerBase myContext = this.BindingContext[bindingTable];
            DataRowView rowView = (DataRowView)myContext.Current;
            string permalink = rowView["link"].ToString();
            Process.Start(permalink);
        }

Windows Forms Documentation Updates : XML, XSL & RSS

Retrieve from RSS Feed using XmlDocument

Using the Microsoft Feeds (RSS) API from Windows Forms

How to Data Bind to an RSS Feed (Blog Posts) in Windows Forms