Why is LINQ ToList not used here?

Question

I see the following code:

using(var iterator = source.GetEnumerator()) {...}

Where source is a IEnumerable<T> .

What is the advantage of doing the above versus converting source into a List<T> and then iterating over it?

Answer 1

Converting it to a list will iterate the enumerable once and copy all the references (or even values for value types) into a new List<> . Then, you would iterate over the list. That means you would iterate twice.

Using the IEnumerable<> as a source for enumeration iterates over the sequence only once.

Why someone decided to do the iteration manually using the enumerator instead of leaving the details to a foreach is unclear from the small scope you posted.

Answer 2

1. Converting to a List<T> would require additional memory and CPU cycles to perform the conversion not to mention you'd be iterating over the data twice.

2. There's no need to convert to a List<T> before iterating. foreach can iterate over anything that implements IEnumerable<T> .