Passing a collection as an argument to the collection’s own method is a code defect. Doing so might either have unexpected side effects or always
have the same result.
Another case is using set-like operations. For example, using Union between a list and itself will always return the same list.
Conversely, using Except between a list and itself will
always return an empty list.
var list = new List<int>();
list.AddRange(list); // Noncompliant
list.Concat(list); // Noncompliant
list.Union(list); // Noncompliant: always returns list
list.Intersect(list); // Noncompliant: always returns list
list.Except(list); // Noncompliant: always returns empty
list.SequenceEqual(list); // Noncompliant: always returns true
var set = new HashSet<int>();
set.UnionWith(set); // Noncompliant: no changes
set.IntersectWith(set); // Noncompliant: no changes
set.ExceptWith(set); // Noncompliant: always returns empty
set.SymmetricExceptWith(set); // Noncompliant: always returns empty
set.IsProperSubsetOf(set); // Noncompliant: always returns false
set.IsProperSupersetOf(set); // Noncompliant: always returns false
set.IsSubsetOf(set); // Noncompliant: always returns true
set.IsSupersetOf(set); // Noncompliant: always returns true
set.Overlaps(set); // Noncompliant: always returns true
set.SetEquals(set); // Noncompliant: always returns true
