Enumerable

    public class EnumerableLookupSample
    {
        private static readonly Dictionary<string, string> FullNamesDb =
            new Dictionary<string, string>
                {
                    { "Bill Gates", "[email protected]" },
                    { "Bill Clinton", "[email protected]" },
                    { "Michael Jackson", "[email protected]" }
                };

        private static readonly Dictionary<string, string> NickNamesDb =
            new Dictionary<string, string>
                {
                    { "billy", "[email protected]" },
                    { "slick willy", "[email protected]" },
                    { "jacko", "[email protected]" }
                };

        private static readonly Dictionary<string, string> PrefsDb =
            new Dictionary<string, string>
                {
                    { "[email protected]", "HTML" },
                    { "[email protected]", "Plain" },
                    { "[email protected]", "HTML" }
                };

        [Fact]
        public void DictionaryLookup()
        {
            Assert.Equal("HTML", LookUp("billy").First());
            Assert.Equal("HTML", LookUp("Bill Gates").First());
            Assert.Equal(0, LookUp("Steffen").Count());
        }

        private static IEnumerable<string> LookUp(string text)
        {
            return
                FullNamesDb.TryFind(text)
                    .Concat(NickNamesDb.TryFind(text))
                    .SelectMany(y => PrefsDb.TryFind(y));

        }
    }