Question

I iterate through an IEnumerable as the result of a Linq query using (ElementAt,Count) and (foreach). To my surprise, the performance difference is 25-30 fold! Why is that?

IEnumerable<double> result =
     ... simple Linq query that joins two tables
     ... returns about 600 items

double total = 0;

// Method 1: iterate with Count and ElementAt
for( int i = 0; i < result.Count(); i++ )
{
    total += result.ElementAt(i);
}

// Method 2: iterate with foreach
foreach( double value in result )
{
    total += value;
}

Answer 1

The ElementAt() method is O(n), unless the actual concrete class that the IEnumerable represents optimizes it. That means that every time you call it, it has to loop through the entire Enumerable to find the element at n. Not to mention that since you have i < result.Count() in the condition part of your for loop, it’s gotta loop through the entire enumerable every single time to get that count.

The second way, you loop through result exactly once.