public void ReadonlyDictionaryCantChange() { var underTest = new ReadonlyDictionary <string, string>(new Dictionary <string, string> { { "a", "a" } }); Assert.Throws <NotSupportedException>(() => underTest.Remove("b")); Assert.Throws <NotSupportedException>(() => underTest.Remove(new KeyValuePair <string, string>("b", "b"))); Assert.Throws <NotSupportedException>(() => underTest.Add("b", "b")); Assert.Throws <NotSupportedException>(() => underTest.Add(new KeyValuePair <string, string>("b", "b"))); Assert.Throws <NotSupportedException>(() => underTest.Clear()); Assert.Throws <NotSupportedException>(() => underTest["a"] = "a"); }