Things to consider – Sitecore and LINQ

The Linq to Items layer does not implement all of IQueryable. Below are list of supported and not supported LINQ items.

• Supported:
   All
   Any
   Between — with an extra overload for including or excluding the start and end ranges.
   Boost — makes this part of the query more important than the other parts.
   Cast — you can use this instead of Select.
   Contains
   Count
   ElementAt
   EndsWith
   Equal
   Facets — an extension that fetches the facets of predicates.
   First
   FirstOrDefault
   Last
   LastOrDefault
   Min
   Max
   Match — an extension for running regular expression queries.
   OrderBy
   OrderByDescending
   Select
   Single
   SingleOrDefault
   Skip
   Reverse
   Take
   ToList()
   ToLookUp()
   ToDictionary()
   Page — an extension that does Skip and Take automatically for you.
   StartsWith

• Not supported:
   Join
   GroupBy
   GroupByJoin
   Intersect
   Sum
   Average
   Concat
   TakeWhile
   SkipWhile
   Union