How to check if a list is ordered?

Question

I am doing some unit tests and I want to know if there's any way to test if a list is ordered by a property of the objects it contains.

Right now I am doing it this way but I don't like it, I want a better way. Can somebody help me please?

// (fill the list) List<StudyFeedItem> studyFeeds =      Feeds.GetStudyFeeds(2120, DateTime.Today.AddDays(-200), 20);     StudyFeedItem previous = studyFeeds.First();  foreach (StudyFeedItem item in studyFeeds) {     if (item != previous)     {         Assert.IsTrue(previous.Date > item.Date);     }      previous = item; } 

Answer 1

If you are using MSTest, you may want to take a look at CollectionAssert.AreEqual.

Enumerable.SequenceEqual may be another useful API to use in an assertion.

In both cases you should prepare a list that holds the expected list in the expected order, and then compare that list to the result.

Here's an example:

var studyFeeds = Feeds.GetStudyFeeds(2120, DateTime.Today.AddDays(-200), 20);    var expectedList = studyFeeds.OrderByDescending(x => x.Date); Assert.IsTrue(expectedList.SequenceEqual(studyFeeds)); 

Answer 2

A .NET 4.0 way would be to use the Enumerable.Zip method to zip the list with itself offset by one, which pairs each item with the subsequent item in the list. You can then check that the condition holds true for each pair, e.g.

var ordered = studyFeeds.Zip(studyFeeds.Skip(1), (a, b) => new { a, b })                         .All(p => p.a.Date < p.b.Date); 

If you're on an earlier version of the framework you can write your own Zip method without too much trouble, something like the following (argument validation and disposal of the enumerators if applicable is left to the reader):

public static IEnumerable<TResult> Zip<TFirst, TSecond, TResult>(     this IEnumerable<TFirst> first,     IEnumerable<TSecond> second,     Func<TFirst, TSecond, TResult> selector) {     var e1 = first.GetEnumerator();     var e2 = second.GetEnumerator();     while (e1.MoveNext() & e2.MoveNext()) // one & is important         yield return selector(e1.Current, e2.Current); }