.net - Requirements for collection class to be used with LINQ -

i thought enough requirement class should satisfy able use where() implement ienumerable.

but today friend of mine asked me question, why cannot apply where() object of spusercollection class (it sharepoint). since class derived spbasecollection implements ienumerable - expected should fine. not.

any ideas, why?

the linq extension methods defined on ienumerable<t>, not ienumerable. example, see where<t> signature:

public static ienumerable<tsource> where<tsource>(     ienumerable<tsource> source,     func<tsource, bool> predicate ) 

to mitigate issue, linq cast<t> extension method turns ienumerable ienumerable<t> can used normal linq functions.

in example below, can't e.where(...), can cast , use where.

int[] xs = new[] { 1, 2, 3, 4 }; ienumerable e = xs; var odds = e.cast<int>().where(x => x % 2 == 1); 

unfortunately, needs used lot when dealing pre-generics apis in .net bcl.