Why does IEnumerable.ToList produce new objects

Question

public class TestClass
{
   public int Num { get; set; }
   public string Name { get; set; }
}

IEnumerable<TestClass> originalEnumerable
   = Enumerable.Range(1, 1).
                Select(x => new TestClass() { Num = x, Name = x.ToString() });

List<TestClass> listFromEnumerable = originalEnumerable.ToList();

// false
bool test = ReferenceEquals(listFromEnumerable[0], originalEnumerable.ElementAt(0)); 

If I modify the copied object the original does not change.

I understand from this that System.Linq.IEnumerable<T> collections will always produce new objects even though they are reference types when calling ToList() / ToArray() ? Am I missing something as other SO questions are insistent only references are copied.

Answer 1

Actually, the reason this is false is because your enumerable actually gets enumerated twice , once in the call originalEnumerable.ToList() , and a second time when you invoke originalEnumerable.ElementAt(0) .

Since it enumerates twice, it ends up creating new objects each time it is enumerated.

Answer 2

You're missing the fact, that Enumerable.Range is lazy, so this line actually produce nothing but in-memory query definition:

IEnumerable<TestClass> originalEnumerable
   = Enumerable.Range(1, 1).
                Select(x => new TestClass() { Num = x, Name = x.ToString() });

Calling ToList() fires the query and creates list with new items:

List<TestClass> listFromEnumerable = originalEnumerable.ToList();

Calling originalEnumerable.ElementAt(0) here

bool test = ReferenceEquals(listFromEnumerable[0], originalEnumerable.ElementAt(0)); 

fires the query again, and produces yet another new items.

Update

So you should say, that Enumerable.Range produces new items, not ToList() .

If your source collection would be already evaluated (eg TestClass[] array) ToList() won't create new items:

IEnumerable<TestClass> originalEnumerable
   = Enumerable.Range(1, 1).
                Select(x => new TestClass() { Num = x, Name = x.ToString() })
                .ToArray();

List<TestClass> listFromEnumerable = originalEnumerable.ToList();

// true
bool test = ReferenceEquals(listFromEnumerable[0], originalEnumerable[0]);