public void GetKey()
{
Assert.Throws <ArgumentOutOfRangeException>(() => _dictionary.GetKey(-1));
Assert.Throws <ArgumentOutOfRangeException>(() => _dictionary.GetKey(3));
Assert.AreEqual("one", _dictionary.GetKey(0));
Assert.AreEqual("three", _dictionary.GetKey(1));
Assert.AreEqual("two", _dictionary.GetKey(2));
}