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

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.

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.

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

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.

There is unlikely to be a huge performance difference between the two. As always, when faced with a "which is faster?" question, you should always think "I can measure this."

Write two loops that do the same thing in the body of the loop, execute and time them both, and see what the difference in speed is. Do this with both an almost-empty body, and a loop body similar to what you'll actually be doing. Also try it with the collection type that you're using, because different types of collections can have different performance characteristics.

This is ridiculous. There's no compelling reason to ban the for-loop, performance-wise or other.

See Jon Skeet's blog for a performance benchmark and other arguments.

you can read about it in Deep .NET - part 1 Iteration

it's cover the results (without the first initialization) from .NET source code all the way to the disassembly.

for example - Array Iteration with a foreach loop:

and - list iteration with foreach loop:

and the end results:

In cases where you work with a collection of objects, foreach is better, but if you increment a number, a for loop is better.

Note that in the last case, you could do something like:

foreach (int i in Enumerable.Range(1, 10))...

But it certainly doesn't perform better, it actually has worse performance compared to a for.

This should save you:

public IEnumerator<int> For(int start, int end, int step) {
    int n = start;
    while (n <= end) {
        yield n;
        n += step;
    }
}

Use:

foreach (int n in For(1, 200, 4)) {
    Console.WriteLine(n);
}

For greater win, you may take three delegates as parameters.

The differences in speed in a for- and a foreach-loop are tiny when you're looping through common structures like arrays, lists, etc, and doing a LINQ query over the collection is almost always slightly slower, although it's nicer to write! As the other posters said, go for expressiveness rather than a millisecond of extra performance.

What hasn't been said so far is that when a foreach loop is compiled, it is optimised by the compiler based on the collection it is iterating over. That means that when you're not sure which loop to use, you should use the foreach loop - it will generate the best loop for you when it gets compiled. It's more readable too.

Another key advantage with the foreach loop is that if your collection implementation changes (from an int array to a List<int> for example) then your foreach loop won't require any code changes:

foreach (int i in myCollection)

The above is the same no matter what type your collection is, whereas in your for loop, the following will not build if you changed myCollection from an array to a List:

for (int i = 0; i < myCollection.Length, i++)

"Are there any arguments I could use to help me convince him the for loop is acceptable to use?"

No, if your boss is micromanaging to the level of telling you what programming language constructs to use, there's really nothing you can say. Sorry.

It probably depends on the type of collection you are enumerating and the implementation of its indexer. In general though, using foreach is likely to be a better approach.

Also, it'll work with any IEnumerable - not just things with indexers.

Every language construct has an appropriate time and place for usage. There is a reason the C# language has a four separate iteration statements - each is there for a specific purpose, and has an appropriate use.

I recommend sitting down with your boss and trying to rationally explain why a for loop has a purpose. There are times when a for iteration block more clearly describes an algorithm than a foreach iteration. When this is true, it is appropriate to use them.

I'd also point out to your boss - Performance is not, and should not be an issue in any practical way - it's more a matter of expression the algorithm in a succinct, meaningful, maintainable manner. Micro-optimizations like this miss the point of performance optimization completely, since any real performance benefit will come from algorithmic redesign and refactoring, not loop restructuring.

If, after a rational discussion, there is still this authoritarian view, it is up to you as to how to proceed. Personally, I would not be happy working in an environment where rational thought is discouraged, and would consider moving to another position under a different employer. However, I strongly recommend discussion prior to getting upset - there may just be a simple misunderstanding in place.

Whether for is faster than foreach is really besides the point. I seriously doubt that choosing one over the other will make a significant impact on your performance.

The best way to optimize your application is through profiling of the actual code. That will pinpoint the methods that account for the most work/time. Optimize those first. If performance is still not acceptable, repeat the procedure.

As a general rule I would recommend to stay away from micro optimizations as they will rarely yield any significant gains. Only exception is when optimizing identified hot paths (i.e. if your profiling identifies a few highly used methods, it may make sense to optimize these extensively).

It is what you do inside the loop that affects perfomance, not the actual looping construct (assuming your case is non-trivial).

The two will run almost exactly the same way. Write some code to use both, then show him the IL. It should show comparable computations, meaning no difference in performance.

It seems a bit strange to totally forbid the use of something like a for loop.

There's an interesting article here that covers a lot of the performance differences between the two loops.

I would say personally I find foreach a bit more readable over for loops but you should use the best for the job at hand and not have to write extra long code to include a foreach loop if a for loop is more appropriate.

In most cases there's really no difference.

Typically you always have to use foreach when you don't have an explicit numerical index, and you always have to use for when you don't actually have an iterable collection (e.g. iterating over a two-dimensional array grid in an upper triangle). There are some cases where you have a choice.

One could argue that for loops can be a little more difficult to maintain if magic numbers start to appear in the code. You should be right to be annoyed at not being able to use a for loop and have to build a collection or use a lambda to build a subcollection instead just because for loops have been banned.

You can really screw with his head and go for an IQueryable .foreach closure instead:

myList.ForEach(c => Console.WriteLine(c.ToString());

I did test it a while ago, with the result that a for loop is much faster than a foreach loop. The cause is simple, the foreach loop first needs to instantiate an IEnumerator for the collection.

for has more simple logic to implement so it's faster than foreach.

Unless you're in a specific speed optimization process, I would say use whichever method produces the easiest to read and maintain code.

If an iterator is already setup, like with one of the collection classes, then the foreach is a good easy option. And if it's an integer range you're iterating, then for is probably cleaner.

Jeffrey Richter talked the performance difference between for and foreach on a recent podcast: http://pixel8.infragistics.com/shows/everything.aspx#Episode:9317

I wouldn't expect anyone to find a "huge" performance difference between the two.

I guess the answer depends on the whether the collection you are trying to access has a faster indexer access implementation or a faster IEnumerator access implementation. Since IEnumerator often uses the indexer and just holds a copy of the current index position, I would expect enumerator access to be at least as slow or slower than direct index access, but not by much.

Of course this answer doesn't account for any optimizations the compiler may implement.

Keep in mind that the for-loop and foreach-loop are not always equivalent. List enumerators will throw an exception if the list changes, but you won't always get that warning with a normal for loop. You might even get a different exception if the list changes at just the wrong time.

I think for is marginally faster than foreach in most cases, but this is really missing the point. One thing I haven't seen mentioned is that, in the scenario you're talking about (i.e., a high volume web app), the difference in performance between for and foreach is going to have no bearing on the site's performance. You're going to be limited by request/response time and DB time, not for v. foreach.

That said, I don't understand your aversion to foreach. In my opinion, foreach is usually clearer in situation where either could be used. I usually reserve for for situations where I need to traverse a collection in some ugly, non-standard way.

    internal static void Test()
    {
        int LOOP_LENGTH = 10000000;
        Random random = new Random((int)DateTime.Now.ToFileTime());

        {
            Dictionary<int, int> dict = new Dictionary<int, int>();
            long first_memory = GC.GetTotalMemory(true);
            var stopWatch = Stopwatch.StartNew();
            for (int i = 0; i < 64; i++)
            {
                dict.Add(i, i);
            }

            for (int i = 0; i < LOOP_LENGTH; i++)
            {
                for (int k = 0; k < dict.Count; k++)
                {
                    if (dict[k] > 1000000) Console.WriteLine("Test");
                }
            }
            stopWatch.Stop();
            var last_memory = GC.GetTotalMemory(true);
            Console.WriteLine($"Dictionary for T:{stopWatch.Elapsed.TotalSeconds}s\t M:{last_memory - first_memory}");

            GC.Collect();
        }


        {
            Dictionary<int, int> dict = new Dictionary<int, int>();
            long first_memory = GC.GetTotalMemory(true);
            var stopWatch = Stopwatch.StartNew();
            for (int i = 0; i < 64; i++)
            {
                dict.Add(i, i);
            }

            for (int i = 0; i < LOOP_LENGTH; i++)
            {
                foreach (var item in dict)
                {
                    if (item.Value > 1000000) Console.WriteLine("Test");
                }
            }
            stopWatch.Stop();
            var last_memory = GC.GetTotalMemory(true);
            Console.WriteLine($"Dictionary foreach T:{stopWatch.Elapsed.TotalSeconds}s\t M:{last_memory - first_memory}");

            GC.Collect();
        }

        {
            Dictionary<int, int> dict = new Dictionary<int, int>();
            long first_memory = GC.GetTotalMemory(true);
            var stopWatch = Stopwatch.StartNew();
            for (int i = 0; i < 64; i++)
            {
                dict.Add(i, i);
            }

            for (int i = 0; i < LOOP_LENGTH; i++)
            {
                foreach (var item in dict.Values)
                {
                    if (item > 1000000) Console.WriteLine("Test");
                }
            }
            stopWatch.Stop();
            var last_memory = GC.GetTotalMemory(true);
            Console.WriteLine($"Dictionary foreach values T:{stopWatch.Elapsed.TotalSeconds}s\t M:{last_memory - first_memory}");

            GC.Collect();
        }


        {
            List<int> dict = new List<int>();
            long first_memory = GC.GetTotalMemory(true);
            var stopWatch = Stopwatch.StartNew();
            for (int i = 0; i < 64; i++)
            {
                dict.Add(i);
            }

            for (int i = 0; i < LOOP_LENGTH; i++)
            {
                for (int k = 0; k < dict.Count; k++)
                {
                    if (dict[k] > 1000000) Console.WriteLine("Test");
                }
            }
            stopWatch.Stop();
            var last_memory = GC.GetTotalMemory(true);
            Console.WriteLine($"list for T:{stopWatch.Elapsed.TotalSeconds}s\t M:{last_memory - first_memory}");

            GC.Collect();
        }


        {
            List<int> dict = new List<int>();
            long first_memory = GC.GetTotalMemory(true);
            var stopWatch = Stopwatch.StartNew();
            for (int i = 0; i < 64; i++)
            {
                dict.Add(i);
            }

            for (int i = 0; i < LOOP_LENGTH; i++)
            {
                foreach (var item in dict)
                {
                    if (item > 1000000) Console.WriteLine("Test");
                }
            }
            stopWatch.Stop();
            var last_memory = GC.GetTotalMemory(true);
            Console.WriteLine($"list foreach T:{stopWatch.Elapsed.TotalSeconds}s\t M:{last_memory - first_memory}");

            GC.Collect();
        }
    }

Dictionary for T:10.1957728s M:2080
Dictionary foreach T:10.5900586s M:1952
Dictionary foreach values T:3.8294776s M:2088
list for T:3.7981471s M:320
list foreach T:4.4861377s M:648

I would suggest reading this for a specific answer. The conclusion of the article is that using for loop is generally better and faster than the foreach loop.

At least I haven't seen any of my collegues or higher ups saying that, that's just ridiculous considering the fact that there is no significant speed difference between for and foreach. The same applies if he is asking to use it in all cases!