[SOLVED] Performance of Enumerable Count method in a loop

Issue

I have a sequence of items represented by IEnumerable

I need to loop thru these items, and It should be a for-loop because the index is important.

My question is, is there a difference in performance between the following 2 options?

1.

for (int i = 0; i < items.Count(); i++)
{
    //Do something
}

var itemsLength = items.Count();

for (int i = 0; i < itemsLength; i++)
{
    //Do something
}

In other words, does the method items.Count() run again and again on each iteration in option 1?

Solution

Calling Count() on an enumerable source will exhaust the enumerable until all elements are enumerated. Calling Count() on an enumerable in the condition-block in a for loop will therefore exhaust the enumerable at every iteration. For example, calling

var numbers = VerboseRange(1, 5);
for (var index = 0; index < numbers.Count(); index++)
{
    Console.WriteLine($"For-loop is at index {index}...");
}

IEnumerable<int> VerboseRange(int start, int count)
{
    foreach (var number in Enumerable.Range(start, count))
    {
        Console.WriteLine($"Yielded number {number}.");
        yield return number;
    }
}

will output

Yielded number 1.
Yielded number 2.
Yielded number 3.
Yielded number 4.
Yielded number 5.
For-loop is at index 0...
Yielded number 1.
Yielded number 2.
Yielded number 3.
Yielded number 4.
Yielded number 5.
For-loop is at index 1...
Yielded number 1.
Yielded number 2.
Yielded number 3.
Yielded number 4.
Yielded number 5.
For-loop is at index 2...
Yielded number 1.
Yielded number 2.
Yielded number 3.
Yielded number 4.
Yielded number 5.
For-loop is at index 3...
Yielded number 1.
Yielded number 2.
Yielded number 3.
Yielded number 4.
Yielded number 5.
For-loop is at index 4...
Yielded number 1.
Yielded number 2.
Yielded number 3.
Yielded number 4.
Yielded number 5.

Therefore, counting before is better.

However, I would recommend you to use a counter and a foreach-loop

var count = 0;
foreach (var item in items)
{
    // do something
    count++;
}

In C# 7.0, you can finally do

foreach (var (item, index) in items.WithIndex())
{
    // do something
}

Answered By – Thomas Angeland

Answer Checked By – Jay B. (BugsFixing Admin)