public void Concat() { var a = new System.Collections.Specialized.NameValueCollection { { "1", "one" } }; var b = new System.Collections.Specialized.NameValueCollection { { "2", "two" } }; var c = a.Concat(b); b.Add("2", "dos"); Assert.AreEqual("one", c["1"]); Assert.AreEqual("two", c["2"]); Assert.AreEqual("two,dos", b["2"]); }