When testing if a collection contains a specific item by simple equality, both ICollection.Contains(T item) and
IEnumerable.Any(x ⇒ x == item) can be used. However, Any searches the data structure in a linear manner using a foreach
loop, whereas Contains is considerably faster in some collection types, because of the underlying implementation. More specifically:
-
HashSet<T> is a hashtable, and therefore has an O(1) lookup
-
SortedSet<T> is a red-black tree, and therefore has a O(logN) lookup
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List<T> is a linear search, and therefore has an O(N) lookup, but the EqualityComparer is optimized for the T
type, which is not the case for Any
For small collections, the performance difference may be negligible, but for large collections, it can be noticeable.
What is the potential impact?
We measured a significant improvement both in execution time and memory allocation. For more details see the Benchmarks section from
the More info tab.
Exceptions
Since LINQ to
Entities relies a lot on System.Linq for query conversion,
this rule won’t raise when used within LINQ to Entities syntaxes.
