public void ContinuousSum_ByAge_SumsAge() { var readOnlySource = new ReadOnlyObservableCollection <Person>(_source); var sumOfAge = readOnlySource.Sum(x => x.Age); var continuousSum = readOnlySource.ContinuousSum(x => x.Age); Assert.AreEqual(sumOfAge, continuousSum.CurrentValue); _source.Add(new Person("Peter", 12)); Assert.AreEqual(sumOfAge + 12, continuousSum.CurrentValue); }