Hi, consider the following example: public IEnumerable<String> Test () { IEnumerable<String> lexicalStrings = new List<String> { "test", "t" }; IEnumerable<String> allLexicals = new List<String> { "test", "Test", "T", "t" }; IEnumerable<String> lexicals = new List<String> (); foreach (String s in lexicalStrings) lexicals = lexicals.Union (allLexicals.Where (lexical => lexical == s)); return lexicals; } I'd hoped for it to produce "test", "t" as output, but it does not (The output is only "t"). I'm not sure, but may have to do something with the deferred processing. Any ideas how to get this to work or for a good alternative?