Assert that all items in a collection have the value for one of the items property using NUnit

Question

I have to convert MS unit tests to NUnit and came across this assertion.

Assert.AreEqual(collection.Select(item => item.location.id).Distinct().Count(), 1);

I was hoping the there would be an elegant way to write that with constraints but I have not been able to find one. My solution is this, but Im not that happy with it:

Expect(collection.Select(item => item.location.id).Distinct().Count(), Is.EqualTo(1));

Is there a better way of writing that assertion where the intent is clearer readable? (Using Has. or Map(collection). )

---

Edit 2:

I just realized it may be helpful to clearly state what the intent is:

all items in the collection have the same location ID (without knowing what that ID is)

---

Edit 1:

This is what the collection may look like as JSON:

[{itemId=1, location={name="A", id=1}},
 {itemId=2, location={name="A", id=1}},
 {itemId=3, location={name="A", id=1}}]

distinct.count = 1 => pass

[{itemId=1, location={name="A", id=1}},
 {itemId=2, location={name="A", id=1}},
 {itemId=4, location={name="B", id=2}}]

distinct.count = 2 => fail

Edit 3: my final solution, based on Fabio's answer

IEnumerable<long?> locationIds = collection.Select(item => item.location.id);
Expect(locationIds, Has.All.EqualTo(locationIds.FirstOrDefault()));

Answer 1

Readable version

int expectedCount = 1;
int actualCount = collection.Select(item => item.location.id)
                            .Distinct()
                            .Count()

Assert.AreEqual(expectedCount, actualCount);

I am not sure but you can try this version, where phrase " Is all equal to... " must help to "non-programmers" and your code get rid of "magic" numbers

var value = collection.Select(item => item.location.id).FirstOrDefault();
Assert.That(collection.Select(item => item.location.id), Is.All.EqualTo(value));

Answer 2

If I've understood what you want to do... this should do it...

Assert.That(collection.Select(() => item.location.id), Is.Unique);

Leaving this wrong answer here... somebody may want to test uniqueness, but that's not what this guy wanted!!!

Answer 3

Based upon the data in your example, noting that name is equal where Id is equal, you could use:

var distinctLocations = collection.select(item => item.location).Distinct();

Assert.That(distinctLocations, Has.Exactly(1).Items);

If that assumption is incorrect, you would of course need to extract just the id .