There are a few main uses.
-
As mentioned in other answers, you can use it to mock a queryable data source using an in-memory data source so that you can more easily test methods that will eventually be used on a non-enumerable based
IQueryable
. -
You can write helper methods for manipulating collections that can apply to either in-memory sequences or external data sources. If you write your help methods to use
IQueryable
entirely you can just useAsQueryable
on all enumerables to use them. This allows you to avoid writing two separate versions of very generalized helper methods. -
It allows you to change the compile time type of a queryable to be an
IQueryable
, rather than some more derived type. In effect; you’d use it on anIQueryable
at the same times that you’d useAsEnumerable
on anIEnumerable
. You might have an object that implementsIQueryable
but that also has an instanceSelect
method. If that were the case, and you wanted to use the LINQSelect
method, you’d need to change the compile time type of the object toIQueryable
. You could just cast it, but by having anAsQueryable
method you can take advantage of type inference. This is simply more convenient if the generic argument list is complex, and it is actually necessary if any of the generic arguments are anonymous types.