AssertFailedException: CollectionAssert.AreEqual failed (Element at index 0 do not match)

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AssertFailedException: CollectionAssert.AreEqual failed (Element at index 0 do not match)

Артур Кузнецов 21

Why this method fails with message: "CollectionAssert.AreEqual failed. (Element at index 0 do not match.)"?

[TestMethod]
        public void Listest()
        {
            List<int[]> first = new List<int[]>()
            {
                new int[] { 1, 3, 5 }
            };

            List<int[]> second = new List<int[]>()
            {
                new int[] { 1, 3, 5 }
            };
            CollectionAssert.AreEqual(first, second);
        }

But code below works fine:

[TestMethod]
        public void Listest()
        {
            int[] arr = new int[] { 1, 3, 5 };
            List<int[]> first = new List<int[]>()
            {
                arr
            };

            List<int[]> second = new List<int[]>()
            {
                arr
            };
            CollectionAssert.AreEqual(first, second);
        }

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Answer 1

Michael Taylor 60,326

Because the first test is a list of arrays. Arrays are reference types and the only way for first[0] to be equal to second[0] is if they are the same object. Reference types follow reference semantics meaning 2 objects are equal only if they are the same object.

In the second test you are using the same array in both lists. Since first[0] is the same array/object as second[0] then they are equal. You can confirm they are the same object by changing an element (e.g. arr[1] to 10). Both lists will see the change because they both have the same object in their list.

Refer to the docs here on how equality works in .NET.

Артур Кузнецов 21

Thanks for the answer! In this case then the following test should fail, but it doesn't:

[TestMethod]
public void TestArr()
{
    int[] arr1 = new int[] { 1, 3, 5 };
    int[] arr2 = new int[] { 1, 3, 5 };
    CollectionAssert.AreEqual(arr1, arr2);
}

And List separately also passed test:

[TestMethod]
public void TestList()
{
    List<int> first = new List<int>() { 1, 3, 5 };
    List<int> second = new List<int>() { 1, 3, 5 };
    CollectionAssert.AreEqual(first, second);
}

Michael Taylor 60,326 Reputation points

2022-04-26T00:56:05.847+00:00
No that is not correct. Let's ignore CollectionAssert for a minute and just go back to the basics.

arr1 == arr2; //False, Arrays are ref types and therefore 2 array variables are only equal if they refer to the same object for (var index = 0; index < arr1.Length; ++index) arr1[index] == arr2[index]; //True for all values because the elements are value types

Hence the array's are not equal but the elements within the arrays are.

first == second; // False. Lists are ref types and are only equal if both variables refer to same instance for (var index = 0; index < first.Count; ++index) first[index] == second[index]; //True for all values because the items in the list are value types

Again, the lists are not equal but the elements within each list are.

CollectionAssert is basically a foreach loop that compares each of the elements in the first collection to that in the second. The actual objects passed aren't relevant. Hence in your 2 examples they are equal because you are dealing with value types in an arra/list.

Back to your original example though you had a list of arrays. So the foreach would have done this, which is not true.

for (var index = 0; index < first.Count; ++index) { int[] arr1 = first[index]; int[] arr2 = second[index]; arr1 == arr2; //Not true }
Michael Taylor 60,326 Reputation points

2022-04-26T01:08:53.517+00:00
Another way of looking at this is that CollectionAssert verifies 2 sequences of values have the same values, it doesn't care about the sequence object itself. If you cared about the object then you'd use Assert.AreEqual instead.

Assert.AreEqual(first, second); //False Assert.AreEqual(arr1, arr2); //False Assert.AreEqual(arr1, arr1); //True
Артур Кузнецов 21 Reputation points

2022-04-27T03:23:10.44+00:00

Unfortunately I got exception for the first Assert.AreEqual example: 'Assert.AreEqual failed. Expected:<System.Collections.Generic.List1[System.Int32]>. Actual:<System.Collections.Generic.List1[System.Int32]>. '

But all other things are clear for me now. Thank you, cooldadtx!
Michael Taylor 60,326 Reputation points

2022-04-27T14:01:32.337+00:00

Yes the first 2 Assert calls will fail (throw an exception in your unit test) because the objects are not the same object. The last call would be successful (not throw) because they are the same object. The -Assert types throw an exception when the condition fails so the test is aborted and fails.

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AssertFailedException: CollectionAssert.AreEqual failed (Element at index 0 do not match)

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