Say I want to return a list of 20 items to process. The fastest way to allocate such a list should be a single allocation of a fixed size array, but it turns out that allocation, copying, and processing takes about 20% longer than creating an IEnumerable to directly iterate through the items. The advantage of the second case of course is there’s no need to copy the elements. You just allocate and access.

// elapsed: 00:00:03.4531250
static IEnumerable GetNumbers()
{
for(int i = 0; i < 20; i++)
{
yield return i;
}
}

// elapsed: 00:00:04.2500000
static IEnumerable GetNumbersArray()
{
int[] retval = new int[20];

for(int i = 0; i < 20; i++)
{
retval[i] = i;
}

return retval;
}

static void Main(string[] args)
{
int sum = 0;
DateTime begin = DateTime.Now;
for(int i = 0; i < (1 << 24); i++)
{
foreach(int j in
/*
GetNumbers()
*/
GetNumbersArray()
)
{
sum += j;
}
}
DateTime end = DateTime.Now;
Console.WriteLine(
“elapsed: ” +
(end – begin).ToString()
);
Console.WriteLine(sum);
}