Here is an example based on your comment and original question
Dictionary<string, int> mapping =
new Dictionary<string, int>();
mapping.Add("cat", 2);
mapping.Add("dog", 1);
mapping.Add("llama", 0);
mapping.Add("iguana", -1);
List<String> x = mapping.Where(w=> w.Key.Contains("a"))
.Select(n => n.Key).ToList();