I think the answer is that the garbage collector is running and changing your timings.
Disclaimer: I can't see the entire context of the OP code because you didn't post a compilable example; I'm assuming you are reallocating the array rather than reusing it. If not, then this is not the correct answer!
Consider this code:
using System;
using System.Diagnostics;
namespace Demo
{
internal class Program
{
private static void Main(string[] args)
{
var ar = new int[500000000];
test1(ar);
//ar = new int[500000000]; // Uncomment this line.
test2(ar);
}
private static void test1(int[] ar)
{
var sw = new Stopwatch();
sw.Start();
var length = ar.Length;
for (var i = 0; i < length; i++)
{
if (ar[i] == 0);
}
sw.Stop();
Console.WriteLine("test1 took " + sw.Elapsed);
}
private static void test2(int[] ar)
{
var sw = new Stopwatch();
sw.Start();
for (var i = 0; i < ar.Length; i++)
{
if (ar[i] == 0);
}
sw.Stop();
Console.WriteLine("test2 took " + sw.Elapsed);
}
}
}
On my system it prints:
test1 took 00:00:00.6643788
test2 took 00:00:00.3516378
If I uncomment the line marked // Uncomment this line.
then the timings change to:
test1 took 00:00:00.6615819
test2 took 00:00:00.6806489
This is because of the GC collecting the previous array.
[EDIT] To avoid JIT startup costs, I put the entire test into a loop:
for (int i = 0; i < 8; ++i)
{
test1(ar);
ar = new int[500000000]; // Uncomment this line.
test2(ar);
}
And then my results with the second array allocation commented out are:
test1 took 00:00:00.6437912
test2 took 00:00:00.3534027
test1 took 00:00:00.3401437
test2 took 00:00:00.3486296
test1 took 00:00:00.3470775
test2 took 00:00:00.3675475
test1 took 00:00:00.3501221
test2 took 00:00:00.3549338
test1 took 00:00:00.3427057
test2 took 00:00:00.3574063
test1 took 00:00:00.3566458
test2 took 00:00:00.3462722
test1 took 00:00:00.3430952
test2 took 00:00:00.3464017
test1 took 00:00:00.3449196
test2 took 00:00:00.3438316
And with the second array allocation enabled:
test1 took 00:00:00.6572665
test2 took 00:00:00.6565778
test1 took 00:00:00.3576911
test2 took 00:00:00.6910897
test1 took 00:00:00.3464013
test2 took 00:00:00.6638542
test1 took 00:00:00.3548638
test2 took 00:00:00.6897472
test1 took 00:00:00.4464020
test2 took 00:00:00.7739877
test1 took 00:00:00.3835624
test2 took 00:00:00.8432918
test1 took 00:00:00.3496910
test2 took 00:00:00.6471341
test1 took 00:00:00.3486505
test2 took 00:00:00.6527160
Note that test2 consistently takes longer due to the GC.
Unfortunately, the GC makes the timing results pretty meaningless.
For example, if I change the test code to this:
for (int i = 0; i < 8; ++i)
{
var ar = new int[500000000];
GC.Collect();
test1(ar);
//ar = new int[500000000]; // Uncomment this line.
test2(ar);
}
With the line commented out I get:
test1 took 00:00:00.6354278
test2 took 00:00:00.3464486
test1 took 00:00:00.6672933
test2 took 00:00:00.3413958
test1 took 00:00:00.6724916
test2 took 00:00:00.3530412
test1 took 00:00:00.6606178
test2 took 00:00:00.3413083
test1 took 00:00:00.6439316
test2 took 00:00:00.3404499
test1 took 00:00:00.6559153
test2 took 00:00:00.3413563
test1 took 00:00:00.6955377
test2 took 00:00:00.3364670
test1 took 00:00:00.6580798
test2 took 00:00:00.3378203
And with it uncommented:
test1 took 00:00:00.6340203
test2 took 00:00:00.6276153
test1 took 00:00:00.6813719
test2 took 00:00:00.6264782
test1 took 00:00:00.6927222
test2 took 00:00:00.6269447
test1 took 00:00:00.7010559
test2 took 00:00:00.6262000
test1 took 00:00:00.6975080
test2 took 00:00:00.6457846
test1 took 00:00:00.6796235
test2 took 00:00:00.6341214
test1 took 00:00:00.6823508
test2 took 00:00:00.6455403
test1 took 00:00:00.6856985
test2 took 00:00:00.6430923
I think the moral of this test is: The GC for this particular test is such a large overhead compared to the rest of the code that it is completely skewing the timing results, and they can't be trusted to mean anything.