Performance implications of calling ToArray inside a LINQ selector

Question

If I have the following statement:

whatever.Select(x => collection.ToArray()[index]).ToList();

Is LINQ smart enough to perform the ToArray cast only once (I'm not really aware of how this closure is transformed and evaluated)?

I understand that this code is bad, just interested.

ToArray() is a method call, not a cast.

Albin Sunnanbo
– Albin Sunnanbo

2011-03-31 09:58:31 +00:00
Commented Mar 31, 2011 at 9:58 — Albin Sunnanbo
– Albin Sunnanbo, Commented Mar 31, 2011 at 9:58

cbp · Accepted Answer · 2011-03-31 00:39:55Z

7

No, it will be performed once for every item in whatever.

answered Mar 31, 2011 at 0:39

cbp

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Albin Sunnanbo · Accepted Answer · 2011-03-31 10:10:59Z

You can have a peek at the code for LINQBridge, especially the Select method (that ends up calling SelectYield.

The essence of SelectYield is a simple for-loop:

foreach (var item in source) yield return selector(item, i++);

Where selector is the lambda expression you pass in, in your case x => collection.ToArray()[index]. From here it is obvious that the whole lambda expression will be evaluated for every element in whatever.

Note that LINQBridge is a stand alone reimplementation of LINQ2Objects and thus not necessarily identical (but to a very large extent at least behaving exactly like LINQ2Objects, including side effects).

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