In .NET, which loop runs faster, 'for' or 'foreach'?

Question

Patrick Smacchia blogged about this last month, with the following conclusions:

• for loops on List are a bit more than 2 times cheaper than foreach loops on List.
• Looping on array is around 2 times cheaper than looping on List.
• As a consequence, looping on array using for is 5 times cheaper than looping on List using foreach (which I believe, is what we all do).

First, a counter-claim to Dmitry's (now deleted) answer. For arrays, the C# compiler emits largely the same code for foreach as it would for an equivalent for loop. That explains why for this benchmark, the results are basically the same:

using System;
using System.Diagnostics;
using System.Linq;

class Test
{
    const int Size = 1000000;
    const int Iterations = 10000;

    static void Main()
    {
        double[] data = new double[Size];
        Random rng = new Random();
        for (int i=0; i < data.Length; i++)
        {
            data[i] = rng.NextDouble();
        }

        double correctSum = data.Sum();

        Stopwatch sw = Stopwatch.StartNew();
        for (int i=0; i < Iterations; i++)
        {
            double sum = 0;
            for (int j=0; j < data.Length; j++)
            {
                sum += data[j];
            }
            if (Math.Abs(sum-correctSum) > 0.1)
            {
                Console.WriteLine("Summation failed");
                return;
            }
        }
        sw.Stop();
        Console.WriteLine("For loop: {0}", sw.ElapsedMilliseconds);

        sw = Stopwatch.StartNew();
        for (int i=0; i < Iterations; i++)
        {
            double sum = 0;
            foreach (double d in data)
            {
                sum += d;
            }
            if (Math.Abs(sum-correctSum) > 0.1)
            {
                Console.WriteLine("Summation failed");
                return;
            }
        }
        sw.Stop();
        Console.WriteLine("Foreach loop: {0}", sw.ElapsedMilliseconds);
    }
}

Results:

For loop: 16638
Foreach loop: 16529

Next, validation that Greg's point about the collection type being important - change the array to a List<double> in the above, and you get radically different results. Not only is it significantly slower in general, but foreach becomes significantly slower than accessing by index. Having said that, I would still almost always prefer foreach to a for loop where it makes the code simpler - because readability is almost always important, whereas micro-optimisation rarely is.

foreach loops demonstrate more specific intent than for loops.

Using a foreach loop demonstrates to anyone using your code that you are planning to do something to each member of a collection irrespective of its place in the collection. It also shows you aren't modifying the original collection (and throws an exception if you try to).

The other advantage of foreach is that it works on any IEnumerable, where as for only makes sense for IList, where each element actually has an index.

However, if you need to use the index of an element, then of course you should be allowed to use a for loop. But if you don't need to use an index, having one is just cluttering your code.

There are no significant performance implications as far as I'm aware. At some stage in the future it might be easier to adapt code using foreach to run on multiple cores, but that's not something to worry about right now.

Any time there's arguments over performance, you just need to write a small test so that you can use quantitative results to support your case.

Use the StopWatch class and repeat something a few million times, for accuracy. (This might be hard without a for loop):

using System.Diagnostics;
//...
Stopwatch sw = new Stopwatch()
sw.Start()
for(int i = 0; i < 1000000;i ++)
{
    //do whatever it is you need to time
}
sw.Stop();
//print out sw.ElapsedMilliseconds

Fingers crossed the results of this show that the difference is negligible, and you might as well just do whatever results in the most maintainable code

It will always be close. For an array, sometimes for is slightly quicker, but foreach is more expressive, and offers LINQ, etc. In general, stick with foreach.

Additionally, foreach may be optimised in some scenarios. For example, a linked list might be terrible by indexer, but it might be quick by foreach. Actually, the standard LinkedList<T> doesn't even offer an indexer for this reason.

My guess is that it will probably not be significant in 99% of the cases, so why would you choose the faster instead of the most appropriate (as in easiest to understand/maintain)?

There are very good reasons to prefer foreach loops over for loops. If you can use a foreach loop, your boss is right that you should.

However, not every iteration is simply going through a list in order one by one. If he is forbidding for, yes that is wrong.

If I were you, what I would do is turn all of your natural for loops into recursion. That'd teach him, and it's also a good mental exercise for you.