public void OnceOperator_CalledOnce()
{
int index = 0;
ObservableProperty <int> property = new ObservableProperty <int>();
property.Once().Subscribe(i => index = i);
property.Value = 1;
property.Value = 3;
property.Value = 5;
property.Value = 7;
property.Value = 9;
Assert.AreEqual(0, index);
}