Question

I am running a test.

It looks like:

method 1)

List<int> = new List<int>{1,2,4, .....} //assume 1000k
var  result ErrorCodes.Where(x => ReturnedErrorCodes.Contains(x)).First();

method 2)

List<int> = new List<int>{1,2,4, .....} //assume 1000k
var  result = ErrorCodes.Where(x => ReturnedErrorCodes.Contains(x)).ToArray()[0];

Why is method 2 is so slow compared to method 1?

Answer 1

Erm… because you are creating an extra array (rather than just using the iterator). The first approach stops after the first match (Where is a non-buffered streaming API). The second loads all the matches into an array (presumably with several re-sizes), then takes the first item.

As a side note; you can create infinite sequences; the first approach would still work, the second would run forever (or explode).

It could also be:

var  result ErrorCodes.First(x => ReturnedErrorCodes.Contains(x));

(that won’t make it any faster, but is perhaps easier to read)