This is a proposal for a new analyzer/codefix for:

// given
Dictionary<int, string> dic = ...;

dic.Keys.Contains(1);
dic.Values.Contains("");

// Could instead be
dic.ContainsKey(1);
dic.ContainsValue("");

I found this code in some design-time build targets today.

EDIT I'd previously incorrectly stated that the former form was O(N), but System.Linq.Enumerable.Contains does type test for ICollection<> and delegate to ICollection<>.Contains, which is also O(1). There is some overhead associated with that however, which could be avoided.