In NET which loop runs faster for or foreach

Question

In C  VB NET  NET  which loop runs faster  for or foreach   Ever since I read that a for loop works faster than a foreach loop a long time ago I assumed it stood true for all collections  generic collections  all arrays  etc   I scoured Google and found a few articles  but most of them are inconclusive  read comments on the articles  and open ended   What would be ideal is to have each scenario listed and the best solution for the same   For example  just an example of how it should be     for iterating an array of 1000  strings - for is better than foreach for iterating over IList  non generic  strings - foreach is better than for   A few references found on the web for the same       Original grand old article by  Emmanuel Schanzer CodeProject FOREACH Vs  FOR Blog - To foreach or not to foreach  that is the question ASP NET forum - NET 1 1 C  for vs foreach       Edit   Apart from the readability aspect of it  I am really interested in facts and figures  There are applications where the last mile of performance optimization squeezed do matter

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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

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

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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

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You can really screw with his head and go for an IQueryable  foreach closure instead   myList ForEach c   gt  Console WriteLine c ToString

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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  lt  data Length  i                          data i    rng NextDouble                       double correctSum   data Sum             Stopwatch sw   Stopwatch StartNew            for  int i 0  i  lt  Iterations  i                          double sum   0              for  int j 0  j  lt  data Length  j                                  sum    data j                             if  Math Abs sum-correctSum   gt  0 1                                Console WriteLine  Summation failed                    return                                  sw Stop            Console WriteLine  For loop   0    sw ElapsedMilliseconds            sw   Stopwatch StartNew            for  int i 0  i  lt  Iterations  i                          double sum   0              foreach  double d in data                                sum    d                            if  Math Abs sum-correctSum   gt  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 lt double gt  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

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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

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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

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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

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I mentioned this details based on speed of collection on for and foreach   List -For Loop is slightly faster than Foreach Loop  ArrayList -  For Loop is about more than 2 times faster speed than Foreach Loop    Array - Both are in equal speed But Foreach Loop seems to be a bit faster

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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

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

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Jeffrey Richter on TechEd 2005       I have come to learn over the years the C  compiler is basically a liar to me       It lies about many things       Like when you do a foreach loop            that is one little line of code that you write  but what the C  compiler spits out in order to do that it s phenomenal  It puts out a try finally block in there  inside the finally block it casts your variable to an IDisposable interface  and if the cast suceeds it calls the Dispose method on it  inside the loop it calls the Current property and the MoveNext method repeatedly inside the loop  objects are being created underneath the covers  A lot of people use foreach because it s very easy coding  very easy to do        foreach is not very good in terms of performance  if you iterated over a collection instead by using square bracket notation  just doing index  that s just much faster  and it doesn t create any objects on the heap       On-Demand Webcast  http   msevents microsoft com CUI WebCastEventDetails aspx EventID 1032292286 amp EventCategory 3 amp culture en-US amp CountryCode US

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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

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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

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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

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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

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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

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I ran into a case where foreach way WAY faster than For  why foreach is faster than for loop while reading richtextbox lines  I had a case similar to the OP in that question     A textbox reading about 72K lines  and I was accessling the Lines property which is actually a getter method     And apparently often in winforms there are getter methods that aren t O 1   I suppose it s O n   so the larger the textbox the longer it takes to get a value from that  property   And in the for loop I had as the OP there had for int i 0 i lt textBox1 lines length i    str textBox1 Lines i     and it was really quite slow as it was reading the entire textbox each time it read a line plus it was reading the entire textbox each time it checked the condition   Jon Skeet shows that you can do it accessing the Lines property just one single time not even once per iteration  just once   Rather than twice on each iteration  which is a ton of times   Do string   strarrlines   textBox1 Lines  and loop through the strarrlines   But certainly a for loop in a fairly intuitive form and accessing the Lines property  is very inefficient  for  int i   0  i  lt  richTextBox Lines Length  i          s   richTextBox Lines i       for a textbox  or a rich textbox  it s super slow   The OP  testing that loop on a rich textbox  found that   with 15000 lines for loop took 8 minutes to just loop down to 15000 lines while foreach took fraction of a second to enumerate it    The OP at that link found this foreach to be far more efficient than his The same OP s  for loop mentioned above  As I did      String s String Empty     foreach string str in txtText Lines               s str

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

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In my project in Windows Mobile I used a for loop for a control collection  It took 100 nbsp ms for 20 controls  A foreach loop used only 4 nbsp ms  That was a performance problem

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This should save you   public IEnumerator lt int gt  For int start  int end  int step        int n   start      while  n  lt   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

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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

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I found the foreach loop which iterating through a List faster  See my test results below  In the code below I iterate an array of size 100  10000 and 100000 separately using for and foreach loop to measure the time     private static void MeasureTime                 var array   new int 10000           var list   array ToList            Console WriteLine  Array size   0    array Length            Console WriteLine  Array For loop                   var stopWatch   Stopwatch StartNew            for  int i   0  i  lt  array Length  i                          Thread Sleep 1                     stopWatch Stop            Console WriteLine  Time take to run the for loop is  0  millisecond   stopWatch ElapsedMilliseconds            Console WriteLine               Console WriteLine  Array Foreach loop                   var stopWatch1   Stopwatch StartNew            foreach  var item in array                        Thread Sleep 1                     stopWatch1 Stop            Console WriteLine  Time take to run the foreach loop is  0  millisecond   stopWatch1 ElapsedMilliseconds            Console WriteLine               Console WriteLine  List For loop                   var stopWatch2   Stopwatch StartNew            for  int i   0  i  lt  list Count  i                          Thread Sleep 1                     stopWatch2 Stop            Console WriteLine  Time take to run the for loop is  0  millisecond   stopWatch2 ElapsedMilliseconds            Console WriteLine               Console WriteLine  List Foreach loop                   var stopWatch3   Stopwatch StartNew            foreach  var item in list                        Thread Sleep 1                     stopWatch3 Stop            Console WriteLine  Time take to run the foreach loop is  0  millisecond   stopWatch3 ElapsedMilliseconds           UPDATED  After  jgauffin suggestion I used  johnskeet code and found that the for loop with array is faster than following    Foreach loop with array  For loop with list  Foreach loop with list    See my test results and code below     private static void MeasureNewTime                 var data   new double Size           var rng   new Random            for  int i   0  i  lt  data Length  i                          data i    rng NextDouble                      Console WriteLine  Lenght of array   0    data Length           Console WriteLine  No  of iteration   0    Iterations           Console WriteLine               double correctSum   data Sum             Stopwatch sw   Stopwatch StartNew            for  int i   0  i  lt  Iterations  i                          double sum   0              for  int j   0  j  lt  data Length  j                                  sum    data j                             if  Math Abs sum - correctSum   gt  0 1                                Console WriteLine  Summation failed                    return                                  sw Stop            Console WriteLine  For loop with Array   0    sw ElapsedMilliseconds            sw   Stopwatch StartNew            for  var i   0  i  lt  Iterations  i                          double sum   0              foreach  double d in data                                sum    d                            if  Math Abs sum - correctSum   gt  0 1                                Console WriteLine  Summation failed                    return                                  sw Stop            Console WriteLine  Foreach loop with Array   0    sw ElapsedMilliseconds           Console WriteLine                var dataList   data ToList            sw   Stopwatch StartNew            for  int i   0  i  lt  Iterations  i                          double sum   0              for  int j   0  j  lt  dataList Count  j                                  sum    data j                             if  Math Abs sum - correctSum   gt  0 1                                Console WriteLine  Summation failed                    return                                  sw Stop            Console WriteLine  For loop with List   0    sw ElapsedMilliseconds            sw   Stopwatch StartNew            for  int i   0  i  lt  Iterations  i                          double sum   0              foreach  double d in dataList                                sum    d                            if  Math Abs sum - correctSum   gt  0 1                                Console WriteLine  Summation failed                    return                                  sw Stop            Console WriteLine  Foreach loop with List   0    sw ElapsedMilliseconds

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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

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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

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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

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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

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I mentioned this details based on speed of collection on for and foreach   List -For Loop is slightly faster than Foreach Loop  ArrayList -  For Loop is about more than 2 times faster speed than Foreach Loop    Array - Both are in equal speed But Foreach Loop seems to be a bit faster

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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 lt T gt  doesn t even offer an indexer for this reason

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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  lt  data Length  i                          data i    rng NextDouble                       double correctSum   data Sum             Stopwatch sw   Stopwatch StartNew            for  int i 0  i  lt  Iterations  i                          double sum   0              for  int j 0  j  lt  data Length  j                                  sum    data j                             if  Math Abs sum-correctSum   gt  0 1                                Console WriteLine  Summation failed                    return                                  sw Stop            Console WriteLine  For loop   0    sw ElapsedMilliseconds            sw   Stopwatch StartNew            for  int i 0  i  lt  Iterations  i                          double sum   0              foreach  double d in data                                sum    d                            if  Math Abs sum-correctSum   gt  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 lt double gt  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

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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

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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

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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

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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

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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

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In my project in Windows Mobile I used a for loop for a control collection  It took 100 nbsp ms for 20 controls  A foreach loop used only 4 nbsp ms  That was a performance problem

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

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internal static void Test                 int LOOP LENGTH   10000000          Random random   new Random  int DateTime Now ToFileTime                            Dictionary lt int  int gt  dict   new Dictionary lt int  int gt                 long first memory   GC GetTotalMemory true               var stopWatch   Stopwatch StartNew                for  int i   0  i  lt  64  i                                  dict Add i  i                              for  int i   0  i  lt  LOOP LENGTH  i                                  for  int k   0  k  lt  dict Count  k                                          if  dict k   gt  1000000  Console WriteLine  quot Test quot                                                stopWatch Stop                var last memory   GC GetTotalMemory true               Console WriteLine   quot Dictionary for T  stopWatch Elapsed TotalSeconds s t M  last memory - first memory  quot                 GC Collect                                      Dictionary lt int  int gt  dict   new Dictionary lt int  int gt                 long first memory   GC GetTotalMemory true               var stopWatch   Stopwatch StartNew                for  int i   0  i  lt  64  i                                  dict Add i  i                              for  int i   0  i  lt  LOOP LENGTH  i                                  foreach  var item in dict                                        if  item Value  gt  1000000  Console WriteLine  quot Test quot                                                stopWatch Stop                var last memory   GC GetTotalMemory true               Console WriteLine   quot Dictionary foreach T  stopWatch Elapsed TotalSeconds s t M  last memory - first memory  quot                 GC Collect                                     Dictionary lt int  int gt  dict   new Dictionary lt int  int gt                 long first memory   GC GetTotalMemory true               var stopWatch   Stopwatch StartNew                for  int i   0  i  lt  64  i                                  dict Add i  i                              for  int i   0  i  lt  LOOP LENGTH  i                                  foreach  var item in dict Values                                        if  item  gt  1000000  Console WriteLine  quot Test quot                                                stopWatch Stop                var last memory   GC GetTotalMemory true               Console WriteLine   quot Dictionary foreach values T  stopWatch Elapsed TotalSeconds s t M  last memory - first memory  quot                 GC Collect                                      List lt int gt  dict   new List lt int gt                 long first memory   GC GetTotalMemory true               var stopWatch   Stopwatch StartNew                for  int i   0  i  lt  64  i                                  dict Add i                              for  int i   0  i  lt  LOOP LENGTH  i                                  for  int k   0  k  lt  dict Count  k                                          if  dict k   gt  1000000  Console WriteLine  quot Test quot                                                stopWatch Stop                var last memory   GC GetTotalMemory true               Console WriteLine   quot list for T  stopWatch Elapsed TotalSeconds s t M  last memory - first memory  quot                 GC Collect                                      List lt int gt  dict   new List lt int gt                 long first memory   GC GetTotalMemory true               var stopWatch   Stopwatch StartNew                for  int i   0  i  lt  64  i                                  dict Add i                              for  int i   0  i  lt  LOOP LENGTH  i                                  foreach  var item in dict                                        if  item  gt  1000000  Console WriteLine  quot Test quot                                                stopWatch Stop                var last memory   GC GetTotalMemory true               Console WriteLine   quot list foreach T  stopWatch Elapsed TotalSeconds s t M  last memory - first memory  quot                 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

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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

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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

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Ben Watson  the author of  Writing High-Performance  NET Code        Will these optimization matter to your program  Only if your program   is CPU bound and collection iteration is a core part of your   processing  As you ll see  there are ways you can hurt your   performance if you re not careful  but that only matters if it was a   significant part of your program to begin with  My philosophy is this    most people never need to know this  but if you do  then understanding   every layer of the system is important so that you can make   intelligent choices     The most exaustive explanation can be found here  http   www codeproject com Articles 844781 Digging-Into-NET-Loop-Performance-Bounds-checking

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

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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

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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  lt  data Length  i                          data i    rng NextDouble                       double correctSum   data Sum             Stopwatch sw   Stopwatch StartNew            for  int i 0  i  lt  Iterations  i                          double sum   0              for  int j 0  j  lt  data Length  j                                  sum    data j                             if  Math Abs sum-correctSum   gt  0 1                                Console WriteLine  Summation failed                    return                                  sw Stop            Console WriteLine  For loop   0    sw ElapsedMilliseconds            sw   Stopwatch StartNew            for  int i 0  i  lt  Iterations  i                          double sum   0              foreach  double d in data                                sum    d                            if  Math Abs sum-correctSum   gt  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 lt double gt  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

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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

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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

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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

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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

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This should save you   public IEnumerator lt int gt  For int start  int end  int step        int n   start      while  n  lt   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

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I needed to do parsing of some large data using three nested loops  on List lt MyCustomType gt    I thought  using Rob Fonseca-Ensor s post above  it d be interesting to time and compare the difference using for and foreach   The difference was  The foreach  three foreach nested like foreach foreach forech     did the job in 171 441 seconds where as for  for for for      did it in 158 616 seconds   Now 13 seconds is about 13  reduction in time to do this - which is somewhat significant to me  However  foreach is definitely far more readable than using three indexed-fors

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

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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 lt T gt  doesn t even offer an indexer for this reason

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This has the same two answers as most  which is faster  questions   1  If you don t measure  you don t know   2   Because     It depends   It depends on how expensive the  MoveNext    method is  relative to how expensive the  this int index   method is  for the type  or types  of IEnumerable that you will be iterating over   The  foreach  keyword is shorthand for a series of operations - it calls GetEnumerator   once on the IEnumerable  it calls MoveNext   once per iteration  it does some type checking  and so on  The thing most likely to impact performance measurements is the cost of MoveNext   since that gets invoked O N  times  Maybe it s cheap  but maybe it s not   The  for  keyword looks more predictable  but inside most  for  loops you ll find something like  collection index    This looks like a simple array indexing operation  but it s actually a method call  whose cost depends entirely on the nature of the collection that you re iterating over  Probably it s cheap  but maybe it s not   If the collection s underlying structure is essentially a linked list  MoveNext is dirt-cheap  but the indexer might have O N  cost  making the true cost of a  for  loop O N N

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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

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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

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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 lt T gt  doesn t even offer an indexer for this reason

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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

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

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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  lt  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

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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

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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 lt int gt  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  lt  myCollection Length  i

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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

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Ben Watson  the author of  Writing High-Performance  NET Code        Will these optimization matter to your program  Only if your program   is CPU bound and collection iteration is a core part of your   processing  As you ll see  there are ways you can hurt your   performance if you re not careful  but that only matters if it was a   significant part of your program to begin with  My philosophy is this    most people never need to know this  but if you do  then understanding   every layer of the system is important so that you can make   intelligent choices     The most exaustive explanation can be found here  http   www codeproject com Articles 844781 Digging-Into-NET-Loop-Performance-Bounds-checking

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

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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  lt  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

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You can really screw with his head and go for an IQueryable  foreach closure instead   myList ForEach c   gt  Console WriteLine c ToString

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I needed to do parsing of some large data using three nested loops  on List lt MyCustomType gt    I thought  using Rob Fonseca-Ensor s post above  it d be interesting to time and compare the difference using for and foreach   The difference was  The foreach  three foreach nested like foreach foreach forech     did the job in 171 441 seconds where as for  for for for      did it in 158 616 seconds   Now 13 seconds is about 13  reduction in time to do this - which is somewhat significant to me  However  foreach is definitely far more readable than using three indexed-fors

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Starting from  NET Framework 4 you can also use Parallel For and Parallel ForEach like described here  C  Multithreading Loop with Parallel For or Parallel ForEach

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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

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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

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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 lt int gt  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  lt  myCollection Length  i

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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

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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

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Jeffrey Richter on TechEd 2005       I have come to learn over the years the C  compiler is basically a liar to me       It lies about many things       Like when you do a foreach loop            that is one little line of code that you write  but what the C  compiler spits out in order to do that it s phenomenal  It puts out a try finally block in there  inside the finally block it casts your variable to an IDisposable interface  and if the cast suceeds it calls the Dispose method on it  inside the loop it calls the Current property and the MoveNext method repeatedly inside the loop  objects are being created underneath the covers  A lot of people use foreach because it s very easy coding  very easy to do        foreach is not very good in terms of performance  if you iterated over a collection instead by using square bracket notation  just doing index  that s just much faster  and it doesn t create any objects on the heap       On-Demand Webcast  http   msevents microsoft com CUI WebCastEventDetails aspx EventID 1032292286 amp EventCategory 3 amp culture en-US amp CountryCode US

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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

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

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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

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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 lt int gt  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  lt  myCollection Length  i

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Starting from  NET Framework 4 you can also use Parallel For and Parallel ForEach like described here  C  Multithreading Loop with Parallel For or Parallel ForEach

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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

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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

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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  lt  data Length  i                          data i    rng NextDouble                       double correctSum   data Sum             Stopwatch sw   Stopwatch StartNew            for  int i 0  i  lt  Iterations  i                          double sum   0              for  int j 0  j  lt  data Length  j                                  sum    data j                             if  Math Abs sum-correctSum   gt  0 1                                Console WriteLine  Summation failed                    return                                  sw Stop            Console WriteLine  For loop   0    sw ElapsedMilliseconds            sw   Stopwatch StartNew            for  int i 0  i  lt  Iterations  i                          double sum   0              foreach  double d in data                                sum    d                            if  Math Abs sum-correctSum   gt  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 lt double gt  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

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internal static void Test                 int LOOP LENGTH   10000000          Random random   new Random  int DateTime Now ToFileTime                            Dictionary lt int  int gt  dict   new Dictionary lt int  int gt                 long first memory   GC GetTotalMemory true               var stopWatch   Stopwatch StartNew                for  int i   0  i  lt  64  i                                  dict Add i  i                              for  int i   0  i  lt  LOOP LENGTH  i                                  for  int k   0  k  lt  dict Count  k                                          if  dict k   gt  1000000  Console WriteLine  quot Test quot                                                stopWatch Stop                var last memory   GC GetTotalMemory true               Console WriteLine   quot Dictionary for T  stopWatch Elapsed TotalSeconds s t M  last memory - first memory  quot                 GC Collect                                      Dictionary lt int  int gt  dict   new Dictionary lt int  int gt                 long first memory   GC GetTotalMemory true               var stopWatch   Stopwatch StartNew                for  int i   0  i  lt  64  i                                  dict Add i  i                              for  int i   0  i  lt  LOOP LENGTH  i                                  foreach  var item in dict                                        if  item Value  gt  1000000  Console WriteLine  quot Test quot                                                stopWatch Stop                var last memory   GC GetTotalMemory true               Console WriteLine   quot Dictionary foreach T  stopWatch Elapsed TotalSeconds s t M  last memory - first memory  quot                 GC Collect                                     Dictionary lt int  int gt  dict   new Dictionary lt int  int gt                 long first memory   GC GetTotalMemory true               var stopWatch   Stopwatch StartNew                for  int i   0  i  lt  64  i                                  dict Add i  i                              for  int i   0  i  lt  LOOP LENGTH  i                                  foreach  var item in dict Values                                        if  item  gt  1000000  Console WriteLine  quot Test quot                                                stopWatch Stop                var last memory   GC GetTotalMemory true               Console WriteLine   quot Dictionary foreach values T  stopWatch Elapsed TotalSeconds s t M  last memory - first memory  quot                 GC Collect                                      List lt int gt  dict   new List lt int gt                 long first memory   GC GetTotalMemory true               var stopWatch   Stopwatch StartNew                for  int i   0  i  lt  64  i                                  dict Add i                              for  int i   0  i  lt  LOOP LENGTH  i                                  for  int k   0  k  lt  dict Count  k                                          if  dict k   gt  1000000  Console WriteLine  quot Test quot                                                stopWatch Stop                var last memory   GC GetTotalMemory true               Console WriteLine   quot list for T  stopWatch Elapsed TotalSeconds s t M  last memory - first memory  quot                 GC Collect                                      List lt int gt  dict   new List lt int gt                 long first memory   GC GetTotalMemory true               var stopWatch   Stopwatch StartNew                for  int i   0  i  lt  64  i                                  dict Add i                              for  int i   0  i  lt  LOOP LENGTH  i                                  foreach  var item in dict                                        if  item  gt  1000000  Console WriteLine  quot Test quot                                                stopWatch Stop                var last memory   GC GetTotalMemory true               Console WriteLine   quot list foreach T  stopWatch Elapsed TotalSeconds s t M  last memory - first memory  quot                 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

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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 lt T gt  doesn t even offer an indexer for this reason

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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

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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

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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

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

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I ran into a case where foreach way WAY faster than For  why foreach is faster than for loop while reading richtextbox lines  I had a case similar to the OP in that question     A textbox reading about 72K lines  and I was accessling the Lines property which is actually a getter method     And apparently often in winforms there are getter methods that aren t O 1   I suppose it s O n   so the larger the textbox the longer it takes to get a value from that  property   And in the for loop I had as the OP there had for int i 0 i lt textBox1 lines length i    str textBox1 Lines i     and it was really quite slow as it was reading the entire textbox each time it read a line plus it was reading the entire textbox each time it checked the condition   Jon Skeet shows that you can do it accessing the Lines property just one single time not even once per iteration  just once   Rather than twice on each iteration  which is a ton of times   Do string   strarrlines   textBox1 Lines  and loop through the strarrlines   But certainly a for loop in a fairly intuitive form and accessing the Lines property  is very inefficient  for  int i   0  i  lt  richTextBox Lines Length  i          s   richTextBox Lines i       for a textbox  or a rich textbox  it s super slow   The OP  testing that loop on a rich textbox  found that   with 15000 lines for loop took 8 minutes to just loop down to 15000 lines while foreach took fraction of a second to enumerate it    The OP at that link found this foreach to be far more efficient than his The same OP s  for loop mentioned above  As I did      String s String Empty     foreach string str in txtText Lines               s str

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This has the same two answers as most  which is faster  questions   1  If you don t measure  you don t know   2   Because     It depends   It depends on how expensive the  MoveNext    method is  relative to how expensive the  this int index   method is  for the type  or types  of IEnumerable that you will be iterating over   The  foreach  keyword is shorthand for a series of operations - it calls GetEnumerator   once on the IEnumerable  it calls MoveNext   once per iteration  it does some type checking  and so on  The thing most likely to impact performance measurements is the cost of MoveNext   since that gets invoked O N  times  Maybe it s cheap  but maybe it s not   The  for  keyword looks more predictable  but inside most  for  loops you ll find something like  collection index    This looks like a simple array indexing operation  but it s actually a method call  whose cost depends entirely on the nature of the collection that you re iterating over  Probably it s cheap  but maybe it s not   If the collection s underlying structure is essentially a linked list  MoveNext is dirt-cheap  but the indexer might have O N  cost  making the true cost of a  for  loop O N N

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I found the foreach loop which iterating through a List faster  See my test results below  In the code below I iterate an array of size 100  10000 and 100000 separately using for and foreach loop to measure the time     private static void MeasureTime                 var array   new int 10000           var list   array ToList            Console WriteLine  Array size   0    array Length            Console WriteLine  Array For loop                   var stopWatch   Stopwatch StartNew            for  int i   0  i  lt  array Length  i                          Thread Sleep 1                     stopWatch Stop            Console WriteLine  Time take to run the for loop is  0  millisecond   stopWatch ElapsedMilliseconds            Console WriteLine               Console WriteLine  Array Foreach loop                   var stopWatch1   Stopwatch StartNew            foreach  var item in array                        Thread Sleep 1                     stopWatch1 Stop            Console WriteLine  Time take to run the foreach loop is  0  millisecond   stopWatch1 ElapsedMilliseconds            Console WriteLine               Console WriteLine  List For loop                   var stopWatch2   Stopwatch StartNew            for  int i   0  i  lt  list Count  i                          Thread Sleep 1                     stopWatch2 Stop            Console WriteLine  Time take to run the for loop is  0  millisecond   stopWatch2 ElapsedMilliseconds            Console WriteLine               Console WriteLine  List Foreach loop                   var stopWatch3   Stopwatch StartNew            foreach  var item in list                        Thread Sleep 1                     stopWatch3 Stop            Console WriteLine  Time take to run the foreach loop is  0  millisecond   stopWatch3 ElapsedMilliseconds           UPDATED  After  jgauffin suggestion I used  johnskeet code and found that the for loop with array is faster than following    Foreach loop with array  For loop with list  Foreach loop with list    See my test results and code below     private static void MeasureNewTime                 var data   new double Size           var rng   new Random            for  int i   0  i  lt  data Length  i                          data i    rng NextDouble                      Console WriteLine  Lenght of array   0    data Length           Console WriteLine  No  of iteration   0    Iterations           Console WriteLine               double correctSum   data Sum             Stopwatch sw   Stopwatch StartNew            for  int i   0  i  lt  Iterations  i                          double sum   0              for  int j   0  j  lt  data Length  j                                  sum    data j                             if  Math Abs sum - correctSum   gt  0 1                                Console WriteLine  Summation failed                    return                                  sw Stop            Console WriteLine  For loop with Array   0    sw ElapsedMilliseconds            sw   Stopwatch StartNew            for  var i   0  i  lt  Iterations  i                          double sum   0              foreach  double d in data                                sum    d                            if  Math Abs sum - correctSum   gt  0 1                                Console WriteLine  Summation failed                    return                                  sw Stop            Console WriteLine  Foreach loop with Array   0    sw ElapsedMilliseconds           Console WriteLine                var dataList   data ToList            sw   Stopwatch StartNew            for  int i   0  i  lt  Iterations  i                          double sum   0              for  int j   0  j  lt  dataList Count  j                                  sum    data j                             if  Math Abs sum - correctSum   gt  0 1                                Console WriteLine  Summation failed                    return                                  sw Stop            Console WriteLine  For loop with List   0    sw ElapsedMilliseconds            sw   Stopwatch StartNew            for  int i   0  i  lt  Iterations  i                          double sum   0              foreach  double d in dataList                                sum    d                            if  Math Abs sum - correctSum   gt  0 1                                Console WriteLine  Summation failed                    return                                  sw Stop            Console WriteLine  Foreach loop with List   0    sw ElapsedMilliseconds

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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 lt int gt  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  lt  myCollection Length  i

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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

[c#] In .NET, which loop runs faster, 'for' or 'foreach'?

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