Adding WHERE clause in LINQ query dynamically

Private Sub Test()
   Dim items As New List(Of Item)
   items.Add(New Item("this is a test"))
   items.Add(New Item("this is not a test"))
   items.Add(New Item("this is maybe a test"))
   items.Add(New Item("something else"))
   items.Add(New Item("something else test"))

   Dim lookfor As String = "this is a test"

   Console.WriteLine("Exact Match: " & lookfor)
   For Each itm As Item In GetMatchedItems(items, lookfor, MatchType.Exact)
      Console.WriteLine(itm.Title)
   Next itm

   Console.WriteLine("----------------------")
   Console.WriteLine("Contains all: " & lookfor)
   For Each itm As Item In GetMatchedItems(items, lookfor, MatchType.ContainsAll)
      Console.WriteLine(itm.Title)
   Next itm

   Console.WriteLine("----------------------")
   lookfor = "else not"
   Console.WriteLine("Contains one or more: " & lookfor)
   For Each itm As Item In GetMatchedItems(items, lookfor, MatchType.ContainsOneOrMore)
      Console.WriteLine(itm.Title)
   Next itm
End Sub

Private Function GetMatchedItems(ByVal source As List(Of Item), ByVal lookfor As String, ByVal mt As MatchType) As IEnumerable(Of Item)
   Return From el In source _
          Let itm As Item = DirectCast(el, Item) _
          Where MatchTester(itm.Title, lookfor, mt) _
          Select itm
End Function

Private Function MatchTester(ByVal teststring As String, ByVal lookfor As String, ByVal match As MatchType) As Boolean
   If Not [Enum].IsDefined(GetType(MatchType), match) Then
      Throw New ArgumentException("Invalid MatchType")
   End If

   Select Case match
      Case MatchType.Exact
         MatchTester = String.Equals(teststring, lookfor)

      Case MatchType.ContainsAll, MatchType.ContainsOneOrMore
         Dim source As New List(Of String)(teststring.Split(New Char() {" "c}, StringSplitOptions.RemoveEmptyEntries))
         Dim findthese As New List(Of String)(lookfor.Split(New Char() {" "c}, StringSplitOptions.RemoveEmptyEntries))

         If match = MatchType.ContainsAll Then
            MatchTester = True
            For Each word As String In findthese
               If Not source.Contains(word) Then
                  MatchTester = False
                  Exit For
               End If
            Next word
         Else
            MatchTester = False
            For Each word As String In findthese
               If source.Contains(word) Then
                  MatchTester = True
                  Exit For
               End If
            Next
         End If
   End Select
End Function

Private Enum MatchType
   Exact
   ContainsAll
   ContainsOneOrMore
End Enum 'MatchType

Private Class Item
   Public Sub New(ByVal Title As String)
      Me._Title = Title
   End Sub

   Private _Title As String
   Public Property Title() As String
      Get
         Return _Title
      End Get
      Set(ByVal value As String)
         _Title = value
      End Set
   End Property 'Title
End Class ' Item