High memory consumption with Enumerable.Range?

Question

Originally i wanted to know whether ToList allocates more memory than using the constructor of List which takes an IEnumerable

Answer 1

This probably relates to the doubling algorithm used to resize the backing buffer when adding to a list. When you allocate as an array, the length of that is known, and can be queried by checking for IList[] and/or ICollection[]; thus it can allocate a single array, right-sized the first time, and then just block-copy the contents.

With the sequence this is not possible (the sequence does not expose the length in any accessible way); thus it must instead fall back to "keep filling up the buffer; if full, double it and copy".

Obviously this needs approx double the memory.

An interesting test would be:

var list = new List(10000000);
list.AddRange(Enumerable.Range(1, 10000000));

This will allocate the right size initially, while still using the sequence.

tl;dr; the constructor, when passed a sequence, first checks to see if it can obtain the length by casting to a well-known interface.