public void ShouldRejectDuplicateKeys() { var dictionary = new XmlDictionary { { "key1", "value1" }, { "key2", "value2" } }; // Check for duplicate keys when adding new entries dictionary.Invoking(x => x.Add("key1", "newValue1")).ShouldThrow <ArgumentException>(); // Check for duplicate keys when modifying existing entries var entry = new XmlDictionaryEntry("key3", "value3"); dictionary.Add(entry); entry.Invoking(x => x.Key = "key1").ShouldThrow <InvalidOperationException>(); }