How to sort an object in a list on a non-unique value? - vb.net

Question

I'm trying to categorize articles by stored keywords. I have a list of keywords for a category, and I want an article to get assigned a category that has the most keyword count.
For Each keyword As String In category.Keywords
category.tempCount += Regex.Matches(article.Item("title").InnerXml, Regex.Escape(keyword)).Count
category.tempCount += Regex.Matches(article.Item("description").InnerXml, Regex.Escape(keyword)).Count
Next
And this is done for each category, ran for each article. I'm trying to sort the list in order to tell which category is the best one for this article. However it is possible more than one category is the best, and that none of the categories fit. So running this did not help me:
Categories.Sort(
Function(article1 As ArticleCategory, article2 As ArticleCategory)
Return article1.tempCount.CompareTo(article2.tempCount)
End Function)
Maybe I'm doing this all wrong, but so far I think I'm on the right path. (I also have a default compare in the Category class, it just wasn't working either.)
I get an exception on the sorting most likely caused because they are not unique.
The exception I get is an InvalidOperationException: Failed to compare two elements in the array. That's with using the comparer I built in the ArticleClass
Imports System.Xml
Class ArticleCategory
Implements IComparer(Of ArticleCategory)
Public ReadOnly key As Int32
Public ReadOnly Name As String
Public ReadOnly Keywords As List(Of String)
Public tempCount As Integer = 0
Public Sub New(ByVal category As XmlElement)
key = System.Web.HttpUtility.UrlDecode(category.Item("ckey").InnerXml)
Name = System.Web.HttpUtility.UrlDecode(category.Item("name").InnerXml)
Dim tKeywords As Array = System.Web.HttpUtility.UrlDecode(category.Item("keywords").InnerXml).Split(",")
Dim nKeywords As New List(Of String)
For Each keyword As String In tKeywords
If Not keyword.Trim = "" Then
nKeywords.Add(keyword.Trim)
End If
Next
Keywords = nKeywords
End Sub
'This should be removed if your using my solution.
Public Function Compare(ByVal x As ArticleCategory, ByVal y As ArticleCategory) As Integer Implements System.Collections.Generic.IComparer(Of ArticleCategory).Compare
Return String.Compare(x.tempCount, y.tempCount)
End Function
End Class

Answer 1

You need to implement IComparable instead of IComparer.
IComparer would be implemented by the class performing the sorting (such as a List class) while IComparable would be implemented by the class being sorted.
For example:
Public Function CompareTo(other As ArticleCategory) As Integer Implements System.IComparable(Of ArticleCategory).CompareTo
Return Me.tempCount.CompareTo(other.tempCount)
End Function

Answer 2

The best solution I found was using the Microsoft LINQ (a query language for objects) it works very well and quickly produces the right result.
Dim bestCat As ArticleCategory
bestCat = (From cat In Categories
Order By cat.tempCount Descending, cat.Name
Select cat).First
Completing my solution:
For Each category As ArticleCategory In Categories
category.tempCount = 0
For Each keyword As String In category.Keywords
category.tempCount += Regex.Matches(System.Web.HttpUtility.UrlDecode(article.Item("title").InnerXml), Regex.Escape(keyword)).Count
category.tempCount += Regex.Matches(System.Web.HttpUtility.UrlDecode(article.Item("description").InnerXml), Regex.Escape(keyword)).Count
Next
Next
Dim bestCat As ArticleCategory
Try
bestCat = (From cat In Categories
Order By cat.tempCount Descending, cat.Name
Select cat).First
Catch ex As Exception
ReportStatus(ex.Message)
End Try
So this is my preferred method to do a sort or a query on a list object or an array. It produces the best result, in the fastest time without having to add the IComparer implementations to your class.
Check it out at Microsoft.com