EqualConstraint provides inadequate failure information for IEnumerables

[Charlie Poole]

When an EqualConstraint compares two IEnumerables and one of them is _not_ a collection, the failure reported inadequately. This was called to our attention by the following post on nunit-discuss by Gishu Pillai but applies more generally than the post indicates.

Given any CustomType containing a Data (int) property and Name
(string) property. Equals and ToString overridden

           var someArray = new[] { "A001", "B002", "C003" };
           CollectionAssert.AreEqual( new[] { "x", "Y", "Z" },
someArray);

 Expected and actual are both <System.String[3]>
 Values differ at index [0]
 Expected string length 1 but was 4. Strings differ at index 0.
 Expected: "x"
 But was: "A001"

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Which is perfect. But now if any of the arguments to AreEqual are a
result of a Linq query, the failure message drops in readability
           var someArray = new[] { "A001", "B002", "C003" };
           CollectionAssert.AreEqual( new[] { "x", "Y", "Z" },

someArray.Select(item => item));

 Expected: < "x", "Y", "Z" >
 But was: <System.Linq.Enumerable
+WhereSelectArrayIterator`2[System.String,System.String]>

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Even worse
              CollectionAssert.AreEqual( new []{'A', 'B',
'C'}.Select(item=>item.ToString()),
                                       new [] {"Apple", "Ball",
"Dog"}.Select(item=> item.Substring(0,1)));

 Expected: <System.Linq.Enumerable
+WhereSelectArrayIterator`2[System.Char,System.String]>
 But was: <System.Linq.Enumerable
+WhereSelectArrayIterator`2[System.String,System.String]>

if you change the 'C' to 'D' - the test passes as expected.

I've been trying to write something similar for my custom-adaptation
of Hamcrest and I think I see what is happening (an are-types-
compatible check failing for IEnumerable<T> and IEnumerable<K>) .
However if CollectionAssert.AreEqual internally uses a ToList() or
ToArray() before processing its arguments, I think the error messages
would be much more helpful.

[Charlie Poole (charlie.poole)]

The fix in NUnit 2.6 is somewhat adhoc. NUnit 3.0 needs a better fix after the Constraint class refactoring is done.

Note that an IEnumerable may not be repeatable, so it's important to save the failing value rather than re-fetching it as we do in the case of a collection.