Introduction
After seeing numerous applications available on the Internet to download and view RSS Feeds off the Internet, I wondered what would be needed to develop a .NET component to Read, Parse and Display RSS Feeds in aspx pages.
The Component development is beyond the scope of this article, which will cover the base class that is used by the component to render the detail you see in the screenshot above.
RSS feeds are designed to be parsed simply using a standard XML Parser. To this end, I made extensive use of the System.XML libraries which I encapsulated in a separate class to handle loading of XML documents via a HTTP Stream.
On to the code!!
Deriving the RSS Version
It is always prudent to derive the version of the document being parsed. All RSS Feeds contain a version number. The class currently supports RSS feeds from v09.1 to v2.00. Since the XML Parser offers a simple way of selecting a specific node in the document, it was subsequently simple to return the RSS Version using the function below:
Public Enum RSSVersion
version_Unknown = 0
version_0_91 = 1
version_0_92 = 2
version_2_00 = 3
End Enum
Private Function getRSSVersion() As RSSVersion
Dim myrootNode As XmlNode
Dim versionAttribute As XmlAttribute
Dim myRSSVersion As RSSVersion
myRSSVersion = RSSVersion.version_Unknown
myrootNode = fetchNode(Document.ChildNodes, "rss")
versionAttribute = fetchAttribute(myrootNode, "version")
Select Case versionAttribute.Value
Case "0.91"
myRSSVersion = RSSVersion.version_0_91
Case "0.92"
myRSSVersion = RSSVersion.version_0_92
Case "2.00"
myRSSVersion = RSSVersion.version_2_00
End Select
Return myRSSVersion
End Function
RSS documents follow a standard structure with only a few differences between versions. This class only concentrates on the standard items which are the same across all versions.
Loading the RSS Header
The RSS Header contains important information such as Author, Title, Source Link and Description. This is all loaded into the structure below and is accessible from the class.
Public Structure RSSHeader
Dim Title As String
Dim Link As Uri
Dim Description As String
Dim Language As String
Dim Copyright As String
Dim managingEditor As String
Dim webMaster As String
Dim lastBuildDate As DateTime
End Structure
This structure is populated in much the same way using the parsing functionality in the csxmlparser class.
Me.myRSSHeader.Copyright = ""
Me.myRSSHeader.Description = ""
Me.myRSSHeader.Language = ""
Me.myRSSHeader.Title = ""
Me.myRSSHeader.webMaster = ""
Me.myRSSHeader.managingEditor = ""
Me.myRSSHeader.lastBuildDate = Nothing
Mybase.DocumentRoot = Me.fetchNode(Document.ChildNodes, "rss")
Mybase.DocumentRoot = Me.fetchNode(Mybase.DocumentRoot.ChildNodes, "channel")
Me.myRSSVersion = Me.getRSSVersion
myNode = fetchNode(Mybase.DocumentRoot.ChildNodes, "title")
If Not IsNothing(myNode) Then Me.myRSSHeader.Title = myNode.InnerText
myNode = fetchNode(Mybase.DocumentRoot.ChildNodes, "link")
If Not IsNothing(myNode) Then Me.myRSSHeader.Link = New Uri(myNode.InnerText)
myNode = fetchNode(Mybase.DocumentRoot.ChildNodes, "description")
If Not IsNothing(myNode) Then Me.myRSSHeader.Description = myNode.InnerText
myNode = fetchNode(Mybase.DocumentRoot.ChildNodes, "language")
If Not IsNothing(myNode) Then Me.myRSSHeader.Language = myNode.InnerText
myNode = fetchNode(Mybase.DocumentRoot.ChildNodes, "copyright")
If Not IsNothing(myNode) Then Me.myRSSHeader.Copyright = myNode.InnerText
myNode = fetchNode(Mybase.DocumentRoot.ChildNodes, "managingEditor")
If Not IsNothing(myNode) Then Me.myRSSHeader.managingEditor = myNode.InnerText
myNode = fetchNode(Mybase.DocumentRoot.ChildNodes, "webMaster")
If Not IsNothing(myNode) Then Me.myRSSHeader.webMaster = myNode.InnerText
myNode = fetchNode(Mybase.DocumentRoot.ChildNodes, "lastBuildDate")
If Not IsNothing(myNode) Then
Me.myRSSHeader.lastBuildDate = DateTime.Parse(myNode.InnerText)
End If
Again, it's a simple case of finding the correct nodes within the RSS Feed document.
Loading the RSS Items
I choose to store the RSS news items stored in the feed in a custom collection class inherited from CollectionBase. This collection stores the base information for a RSS Item in a structure shown below:
Public Structure RSSItem
Dim Title As String
Dim Description As String
Dim pubDate As DateTime
Dim Link As Uri
End Structure
This is enough information to render the chosen items in our control. And since the class has already derived the root node of the RSS Document (The Channel tag), then it is simply a case of stepping through all Item nodes within the Document and populating the collection as follows:
Me.myRSSItems.Clear()
For i = 0 To Mybase.DocumentRoot.ChildNodes.Count - 1
If Mybase.DocumentRoot.ChildNodes(i).Name = "item" Then
myItemNode = Mybase.DocumentRoot.ChildNodes(i)
Dim myItem As RSSItem
myItem.pubDate = Nothing
myNode = fetchNode(myItemNode.ChildNodes, "description")
If Not IsNothing(myNode) Then myItem.Description = myNode.InnerText
myNode = fetchNode(myItemNode.ChildNodes, "title")
If Not IsNothing(myNode) Then myItem.Title = myNode.InnerText
myNode = fetchNode(myItemNode.ChildNodes, "link")
If Not IsNothing(myNode) Then myItem.Link = New Uri(myNode.InnerText)
myNode = fetchNode(myItemNode.ChildNodes, "pubDate")
If Not IsNothing(myNode) Then
myItem.pubDate = DateTime.Parse(myNode.InnerText)
Select Case Me.myRSSVersion
Case RSSVersion.version_0_91
Case RSSVersion.version_0_92
Case RSSVersion.version_2_00
End Select
Me.myRSSItems.Add(myItem)
End If
Next
Using the class
Two functions are exposed to allow loading a RSS Feed from a file or from a HTTP Stream. These can be used as follows:
Rem Load my RSS Feed from a File
Dim myRSSFeed as New csRSSFeed()
myRSSFeed.Load("C:\feeds\MSDN.rss")
Rem Load my RSS Feed from a HTTP Server
Dim myRSSFeed as New csRSSFeed()
myRSSFeed.LoadFromHTTP("http://www.theregister.co.uk/headlines.rss")
Once you have executed the relevant load function, information about the document can be found in the Header, Version and items properties, which return the RSS Header, Document Version and RSS Items respectively.
