Is the performance of ToList
/ToArray
an O(n) operation which copies the IEnumerable
to a new array/List ?
Yes. ToList
is slightly more efficient, as it doesn't need to trim the internal buffer to the right length first.
If I called a linq extention method on a list, it has an O(1) performance if I call ToList but O(n) if call ToArray (and the opposite if my original list was an array) ?
No. For both calls, a new collection is always created; that's a shallow copy of the original collection. It's more efficient to call ToList
or ToArray
on any ICollection<T>
than on a simple IEnumerable<T>
which doesn't implement ICollection<T>
though, as with a collection the length is known to start with. (This is detected at execution time though; you don't need to worry about the compile-time type.)
Probably to Dictionary is O(n), right ?
Assuming the hash is sensible, it's O(N), yes. Basically it creates a new dictionary in exactly the way you'd probably expect it to.
You might want to read the corresponding posts in my Edulinq blog series:
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…