foreach vs someList ForEach

Question

There are apparently many ways to iterate over a collection  Curious if there are any differences  or why you d use one way over the other   First type   List lt string gt  someList    lt some way to init gt  foreach string s in someList        lt process the string gt      Other Way   List lt string gt  someList    lt some way to init gt  someList ForEach delegate string s         lt process the string gt        I suppose off the top of my head  that instead of the anonymous delegate I use above  you d have a reusable delegate you could specify

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Behind the scenes  the anonymous delegate gets turned into an actual method so you could have some overhead with the second choice if the compiler didn t choose to inline the function  Additionally  any local variables referenced by the body of the anonymous delegate example would change in nature because of compiler tricks to hide the fact that it gets compiled to a new method  More info here on how C  does this magic   http   blogs msdn com oldnewthing archive 2006 08 04 688527 aspx

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The ForEach function is member of the generic class List   I have created the following extension to reproduce the internal code   public static class MyExtension lt T gt                public static void MyForEach this IEnumerable lt T gt  collection  Action lt T gt  action                        foreach  T item in collection                  action Invoke item                     So a the end we are using a normal foreach  or a loop for if you want    On the other hand  using a delegate function is just another way to define a function  this code   delegate string s         lt process the string gt      is equivalent to   private static void myFunction string s   lt other variables    gt         lt process the string gt      or using labda expressions    s    gt   lt process the string gt

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Behind the scenes  the anonymous delegate gets turned into an actual method so you could have some overhead with the second choice if the compiler didn t choose to inline the function  Additionally  any local variables referenced by the body of the anonymous delegate example would change in nature because of compiler tricks to hide the fact that it gets compiled to a new method  More info here on how C  does this magic   http   blogs msdn com oldnewthing archive 2006 08 04 688527 aspx

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I guess the someList ForEach   call could be easily parallelized whereas the normal foreach is not that easy to run parallel  You could easily run several different delegates on different cores  which is not that easy to do with a normal foreach  Just my 2 cents

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You could name the anonymous delegate  -   And you can write the second as   someList ForEach s   gt  s ToUpper      Which I prefer  and saves a lot of typing   As Joachim says  parallelism is easier to apply to the second form

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There is one important  and useful  distinction between the two   Because  ForEach uses a for loop to iterate the collection  this is valid  edit  prior to  net 4 5 - the implementation changed and they both throw    someList ForEach x   gt    if x RemoveMe  someList Remove x          whereas foreach uses an enumerator  so this is not valid   foreach var item in someList    if item RemoveMe  someList Remove item       tl dr   Do NOT copypaste this code into your application    These examples aren t best practice  they are just to demonstrate the differences between ForEach   and foreach   Removing items from a list within a for loop can have side effects   The most common one is described in the comments to this question   Generally  if you are looking to remove multiple items from a list  you would want to separate the determination of which items to remove from the actual removal   It doesn t keep your code compact  but it guarantees that you do not miss any items

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I know two obscure-ish things that make them different  Go me   Firstly  there s the classic bug of making a delegate for each item in the list  If you use the foreach keyword  all your delegates can end up referring to the last item of the list          A list of actions to execute later     List lt Action gt  actions   new List lt Action gt             Numbers 0 to 9     List lt int gt  numbers   Enumerable Range 0  10  ToList            Store an action that prints each number  WRONG       foreach  int number in numbers          actions Add      gt  Console WriteLine number            Run the actions  we actually print 10 copies of  9      foreach  Action action in actions          action            So try again     actions Clear            Store an action that prints each number  RIGHT       numbers ForEach number   gt          actions Add      gt  Console WriteLine number             Run the actions     foreach  Action action in actions          action      The List ForEach method doesn t have this problem  The current item of the iteration is passed by value as an argument to the outer lambda  and then the inner lambda correctly captures that argument in its own closure  Problem solved    Sadly I believe ForEach is a member of List  rather than an extension method  though it s easy to define it yourself so you have this facility on any enumerable type    Secondly  the ForEach method approach has a limitation  If you are implementing IEnumerable by using yield return  you can t do a yield return inside the lambda  So looping through the items in a collection in order to yield return things is not possible by this method  You ll have to use the foreach keyword and work around the closure problem by manually making a copy of the current loop value inside the loop   More here

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We had some code here  in VS2005 and C 2 0  where the previous engineers went out of their way to use list ForEach  delegate item    foo     instead of foreach item in list   foo     for all the code that they wrote  e g  a block of code for reading rows from a dataReader   I still don t know exactly why they did this   The drawbacks of list ForEach   are    It is more verbose in C  2 0  However  in C  3 onwards  you can use the    gt   syntax to make some nicely terse expressions  It is less familiar  People who have to maintain this code will wonder why you did it that way  It took me awhile to decide that there wasn t any reason  except maybe to make the writer seem clever  the quality of the rest of the code undermined that   It was also less readable  with the      at the end of the delegate code block  See also Bill Wagner s book  Effective C   50 Specific Ways to Improve Your C   where he talks about why foreach is preferred to other loops like for or while loops - the main point is that you are letting the compiler decide the best way to construct the loop  If a future version of the compiler manages to use a faster technique  then you will get this for free by using foreach and rebuilding  rather than changing your code  a foreach item in list  construct allows you to use break or continue if you need to exit the iteration or the loop   But you cannot alter the list inside a foreach loop    I m surprised to see that list ForEach is slightly faster  But that s probably not a valid reason to use it throughout   that would be premature optimisation  If your application uses a database or web service that  not loop control  is almost always going to be be where the time goes  And have you benchmarked it against a for loop too  The list ForEach could be faster due to using that internally and a for loop without the wrapper would be even faster    I disagree that the list ForEach delegate  version is  more functional  in any significant way  It does pass a function to a function  but there s no big difference in the outcome or program organisation    I don t think that foreach item in list   says exactly how you want it done  - a for int 1   0  i  lt  count  i    loop does that  a foreach loop leaves the choice of control up to the compiler   My feeling is  on a new project  to use foreach item in list  for most loops in order to adhere to the common usage and for readability  and use list Foreach   only for short blocks  when you can do something more elegantly or compactly with the C  3    gt   operator  In cases like that  there may already be a LINQ extension method that is more specific than ForEach    See if Where    Select    Any    All    Max   or one of the many other LINQ methods doesn t already do what you want from the loop

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You could name the anonymous delegate  -   And you can write the second as   someList ForEach s   gt  s ToUpper      Which I prefer  and saves a lot of typing   As Joachim says  parallelism is easier to apply to the second form

User · Answer

List ForEach   is considered to be more functional   List ForEach   says what you want done   foreach item in list  also says exactly how you want it done  This leaves List ForEach free to change the implementation of the how part in the future  For example  a hypothetical future version of  Net might always run List ForEach in parallel  under the assumption that at this point everyone has a number of cpu cores that are generally sitting idle     On the other hand  foreach  item in list  gives you a little more control over the loop  For example  you know that the items will be iterated in some kind of sequential order  and you could easily break in the middle if an item meets some condition     Some more recent remarks on this issue are available here      https   stackoverflow com a 529197 3043

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The entire ForEach scope  delegate function  is treated as a single line of code  calling the function   and you cannot set breakpoints or step into the code  If an unhandled exception occurs the entire block is marked

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List ForEach   is considered to be more functional   List ForEach   says what you want done   foreach item in list  also says exactly how you want it done  This leaves List ForEach free to change the implementation of the how part in the future  For example  a hypothetical future version of  Net might always run List ForEach in parallel  under the assumption that at this point everyone has a number of cpu cores that are generally sitting idle     On the other hand  foreach  item in list  gives you a little more control over the loop  For example  you know that the items will be iterated in some kind of sequential order  and you could easily break in the middle if an item meets some condition     Some more recent remarks on this issue are available here      https   stackoverflow com a 529197 3043

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List ForEach   is considered to be more functional   List ForEach   says what you want done   foreach item in list  also says exactly how you want it done  This leaves List ForEach free to change the implementation of the how part in the future  For example  a hypothetical future version of  Net might always run List ForEach in parallel  under the assumption that at this point everyone has a number of cpu cores that are generally sitting idle     On the other hand  foreach  item in list  gives you a little more control over the loop  For example  you know that the items will be iterated in some kind of sequential order  and you could easily break in the middle if an item meets some condition     Some more recent remarks on this issue are available here      https   stackoverflow com a 529197 3043

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The second way you showed uses an extension method to execute the delegate method for each of the elements in the list   This way  you have another delegate   method  call   Additionally  there is the possibility to iterate the list with a for loop

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For fun  I popped List into reflector and this is the resulting C    public void ForEach Action lt T gt  action        if  action    null                ThrowHelper ThrowArgumentNullException ExceptionArgument match             for  int i   0  i  lt  this  size  i                  action this  items i              Similarly  the MoveNext in Enumerator which is what is used by foreach is this   public bool MoveNext         if  this version    this list  version                ThrowHelper ThrowInvalidOperationException ExceptionResource InvalidOperation EnumFailedVersion             if  this index  lt  this list  size                this current   this list  items this index           this index            return true            this index   this list  size   1      this current   default T       return false      The List ForEach is much more trimmed down than MoveNext - far less processing - will more likely JIT into something efficient    In addition  foreach   will allocate a new Enumerator no matter what   The GC is your friend  but if you re doing the same foreach repeatedly  this will make more throwaway objects  as opposed to reusing the same delegate - BUT - this is really a fringe case   In typical usage you will see little or no difference

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I guess the someList ForEach   call could be easily parallelized whereas the normal foreach is not that easy to run parallel  You could easily run several different delegates on different cores  which is not that easy to do with a normal foreach  Just my 2 cents

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One thing to be wary of is how to exit from the Generic  ForEach method - see this discussion  Although the link seems to say that this way is the fastest  Not sure why - you d think they would be equivalent once compiled

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Behind the scenes  the anonymous delegate gets turned into an actual method so you could have some overhead with the second choice if the compiler didn t choose to inline the function  Additionally  any local variables referenced by the body of the anonymous delegate example would change in nature because of compiler tricks to hide the fact that it gets compiled to a new method  More info here on how C  does this magic   http   blogs msdn com oldnewthing archive 2006 08 04 688527 aspx

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I know two obscure-ish things that make them different  Go me   Firstly  there s the classic bug of making a delegate for each item in the list  If you use the foreach keyword  all your delegates can end up referring to the last item of the list          A list of actions to execute later     List lt Action gt  actions   new List lt Action gt             Numbers 0 to 9     List lt int gt  numbers   Enumerable Range 0  10  ToList            Store an action that prints each number  WRONG       foreach  int number in numbers          actions Add      gt  Console WriteLine number            Run the actions  we actually print 10 copies of  9      foreach  Action action in actions          action            So try again     actions Clear            Store an action that prints each number  RIGHT       numbers ForEach number   gt          actions Add      gt  Console WriteLine number             Run the actions     foreach  Action action in actions          action      The List ForEach method doesn t have this problem  The current item of the iteration is passed by value as an argument to the outer lambda  and then the inner lambda correctly captures that argument in its own closure  Problem solved    Sadly I believe ForEach is a member of List  rather than an extension method  though it s easy to define it yourself so you have this facility on any enumerable type    Secondly  the ForEach method approach has a limitation  If you are implementing IEnumerable by using yield return  you can t do a yield return inside the lambda  So looping through the items in a collection in order to yield return things is not possible by this method  You ll have to use the foreach keyword and work around the closure problem by manually making a copy of the current loop value inside the loop   More here

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I know two obscure-ish things that make them different  Go me   Firstly  there s the classic bug of making a delegate for each item in the list  If you use the foreach keyword  all your delegates can end up referring to the last item of the list          A list of actions to execute later     List lt Action gt  actions   new List lt Action gt             Numbers 0 to 9     List lt int gt  numbers   Enumerable Range 0  10  ToList            Store an action that prints each number  WRONG       foreach  int number in numbers          actions Add      gt  Console WriteLine number            Run the actions  we actually print 10 copies of  9      foreach  Action action in actions          action            So try again     actions Clear            Store an action that prints each number  RIGHT       numbers ForEach number   gt          actions Add      gt  Console WriteLine number             Run the actions     foreach  Action action in actions          action      The List ForEach method doesn t have this problem  The current item of the iteration is passed by value as an argument to the outer lambda  and then the inner lambda correctly captures that argument in its own closure  Problem solved    Sadly I believe ForEach is a member of List  rather than an extension method  though it s easy to define it yourself so you have this facility on any enumerable type    Secondly  the ForEach method approach has a limitation  If you are implementing IEnumerable by using yield return  you can t do a yield return inside the lambda  So looping through the items in a collection in order to yield return things is not possible by this method  You ll have to use the foreach keyword and work around the closure problem by manually making a copy of the current loop value inside the loop   More here

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You could name the anonymous delegate  -   And you can write the second as   someList ForEach s   gt  s ToUpper      Which I prefer  and saves a lot of typing   As Joachim says  parallelism is easier to apply to the second form

User · Answer

You could name the anonymous delegate  -   And you can write the second as   someList ForEach s   gt  s ToUpper      Which I prefer  and saves a lot of typing   As Joachim says  parallelism is easier to apply to the second form

User · Answer

As they say  the devil is in the details     The biggest difference between the two methods of collection enumeration is that foreach carries state  whereas ForEach x   gt       does not    But lets dig a little deeper  because there are some things you should be aware of that can influence your decision  and there are some caveats you should be aware of when coding for either case   Lets use List lt T gt  in our little experiment to observe behavior  For this experiment  I am using  NET 4 7 2   var names   new List lt string gt         Henry        Shirley        Ann        Peter        Nancy       Lets iterate over this with foreach first   foreach  var name in names        Console WriteLine name       We could expand this into   using  var enumerator   names GetEnumerator           With the enumerator in hand  looking under the covers we get   public List lt T gt  Enumerator GetEnumerator       return new List lt T gt  Enumerator this         internal Enumerator List lt T gt  list      this list   list    this index   0    this version   list  version    this current   default  T      public bool MoveNext       List lt T gt  list   this list    if  this version    list  version     uint  this index  gt    uint  list  size      return this MoveNextRare      this current   list  items this index       this index    return true     object IEnumerator Current           if  this index    0    this index    this list  size   1        ThrowHelper ThrowInvalidOperationException ExceptionResource InvalidOperation EnumOpCantHappen       return  object  this Current          Two things become immediate evident    We are returned a stateful object with intimate knowledge of the underlying collection  The copy of the collection is a shallow copy    This is of course in no way thread safe  As was pointed out above  changing the collection while iterating is just bad mojo   But what about the problem of the collection becoming invalid during iteration by means outside of us mucking with the collection during iteration  Best practices suggests versioning the collection during operations and iteration  and checking versions to detect when the underlying collection changes   Here s where things get really murky  According to the Microsoft documentation      If changes are made to the collection  such as adding  modifying  or   deleting elements  the behavior of the enumerator is undefined    Well  what does that mean  By way of example  just because List lt T gt  implements exception handling does not mean that all collections that implement IList lt T gt  will do the same  That seems to be a clear violation of the Liskov Substitution Principle      Objects of a superclass shall be replaceable with objects of its   subclasses without breaking the application    Another problem is that the enumerator must implement IDisposable -- that means another source of potential memory leaks  not only if the caller gets it wrong  but if the author does not implement the Dispose pattern correctly   Lastly  we have a lifetime issue    what happens if the iterator is valid  but the underlying collection is gone  We now a snapshot of what was    when you separate the lifetime of a collection and its iterators  you are asking for trouble   Lets now examine ForEach x   gt         names ForEach name   gt           This expands to   public void ForEach Action lt T gt  action      if  action    null      ThrowHelper ThrowArgumentNullException ExceptionArgument match     int version   this  version    for  int index   0  index  lt  this  size  amp  amp   version    this  version     BinaryCompatibility TargetsAtLeast Desktop V4 5     index      action this  items index      if  version    this  version     BinaryCompatibility TargetsAtLeast Desktop V4 5      return    ThrowHelper ThrowInvalidOperationException ExceptionResource InvalidOperation EnumFailedVersion       Of important note is the following   for  int index   0  index  lt  this  size  amp  amp          index          action this  items index     This code does not allocate any enumerators  nothing to Dispose   and does not pause while iterating   Note that this also performs a shallow copy of the underlying collection  but the collection is now a snapshot in time  If the author does not correctly implement a check for the collection changing or going  stale   the snapshot is still valid   This doesn t in any way protect you from the problem of the lifetime issues    if the underlying collection disappears  you now have a shallow copy that points to what was    but at least you don t have a Dispose problem to deal with on orphaned iterators     Yes  I said iterators    sometimes its advantageous to have state  Suppose you want to maintain something akin to a database cursor    maybe multiple foreach style Iterator lt T gt  s is the way to go  I personally dislike this style of design as there are too many lifetime issues  and you rely on the good graces of the authors of the collections you are relying on  unless you literally write everything yourself from scratch    There is always a third option     for  var i   0  i  lt  names Count  i          Console WriteLine names i        It ain t sexy  but its got teeth  apologies to Tom Cruise and the movie The Firm   Its your choice  but now you know and it can be an informed one

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We had some code here  in VS2005 and C 2 0  where the previous engineers went out of their way to use list ForEach  delegate item    foo     instead of foreach item in list   foo     for all the code that they wrote  e g  a block of code for reading rows from a dataReader   I still don t know exactly why they did this   The drawbacks of list ForEach   are    It is more verbose in C  2 0  However  in C  3 onwards  you can use the    gt   syntax to make some nicely terse expressions  It is less familiar  People who have to maintain this code will wonder why you did it that way  It took me awhile to decide that there wasn t any reason  except maybe to make the writer seem clever  the quality of the rest of the code undermined that   It was also less readable  with the      at the end of the delegate code block  See also Bill Wagner s book  Effective C   50 Specific Ways to Improve Your C   where he talks about why foreach is preferred to other loops like for or while loops - the main point is that you are letting the compiler decide the best way to construct the loop  If a future version of the compiler manages to use a faster technique  then you will get this for free by using foreach and rebuilding  rather than changing your code  a foreach item in list  construct allows you to use break or continue if you need to exit the iteration or the loop   But you cannot alter the list inside a foreach loop    I m surprised to see that list ForEach is slightly faster  But that s probably not a valid reason to use it throughout   that would be premature optimisation  If your application uses a database or web service that  not loop control  is almost always going to be be where the time goes  And have you benchmarked it against a for loop too  The list ForEach could be faster due to using that internally and a for loop without the wrapper would be even faster    I disagree that the list ForEach delegate  version is  more functional  in any significant way  It does pass a function to a function  but there s no big difference in the outcome or program organisation    I don t think that foreach item in list   says exactly how you want it done  - a for int 1   0  i  lt  count  i    loop does that  a foreach loop leaves the choice of control up to the compiler   My feeling is  on a new project  to use foreach item in list  for most loops in order to adhere to the common usage and for readability  and use list Foreach   only for short blocks  when you can do something more elegantly or compactly with the C  3    gt   operator  In cases like that  there may already be a LINQ extension method that is more specific than ForEach    See if Where    Select    Any    All    Max   or one of the many other LINQ methods doesn t already do what you want from the loop

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For fun  I popped List into reflector and this is the resulting C    public void ForEach Action lt T gt  action        if  action    null                ThrowHelper ThrowArgumentNullException ExceptionArgument match             for  int i   0  i  lt  this  size  i                  action this  items i              Similarly  the MoveNext in Enumerator which is what is used by foreach is this   public bool MoveNext         if  this version    this list  version                ThrowHelper ThrowInvalidOperationException ExceptionResource InvalidOperation EnumFailedVersion             if  this index  lt  this list  size                this current   this list  items this index           this index            return true            this index   this list  size   1      this current   default T       return false      The List ForEach is much more trimmed down than MoveNext - far less processing - will more likely JIT into something efficient    In addition  foreach   will allocate a new Enumerator no matter what   The GC is your friend  but if you re doing the same foreach repeatedly  this will make more throwaway objects  as opposed to reusing the same delegate - BUT - this is really a fringe case   In typical usage you will see little or no difference

User · Answer

One thing to be wary of is how to exit from the Generic  ForEach method - see this discussion  Although the link seems to say that this way is the fastest  Not sure why - you d think they would be equivalent once compiled

User · Answer

I know two obscure-ish things that make them different  Go me   Firstly  there s the classic bug of making a delegate for each item in the list  If you use the foreach keyword  all your delegates can end up referring to the last item of the list          A list of actions to execute later     List lt Action gt  actions   new List lt Action gt             Numbers 0 to 9     List lt int gt  numbers   Enumerable Range 0  10  ToList            Store an action that prints each number  WRONG       foreach  int number in numbers          actions Add      gt  Console WriteLine number            Run the actions  we actually print 10 copies of  9      foreach  Action action in actions          action            So try again     actions Clear            Store an action that prints each number  RIGHT       numbers ForEach number   gt          actions Add      gt  Console WriteLine number             Run the actions     foreach  Action action in actions          action      The List ForEach method doesn t have this problem  The current item of the iteration is passed by value as an argument to the outer lambda  and then the inner lambda correctly captures that argument in its own closure  Problem solved    Sadly I believe ForEach is a member of List  rather than an extension method  though it s easy to define it yourself so you have this facility on any enumerable type    Secondly  the ForEach method approach has a limitation  If you are implementing IEnumerable by using yield return  you can t do a yield return inside the lambda  So looping through the items in a collection in order to yield return things is not possible by this method  You ll have to use the foreach keyword and work around the closure problem by manually making a copy of the current loop value inside the loop   More here

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For fun  I popped List into reflector and this is the resulting C    public void ForEach Action lt T gt  action        if  action    null                ThrowHelper ThrowArgumentNullException ExceptionArgument match             for  int i   0  i  lt  this  size  i                  action this  items i              Similarly  the MoveNext in Enumerator which is what is used by foreach is this   public bool MoveNext         if  this version    this list  version                ThrowHelper ThrowInvalidOperationException ExceptionResource InvalidOperation EnumFailedVersion             if  this index  lt  this list  size                this current   this list  items this index           this index            return true            this index   this list  size   1      this current   default T       return false      The List ForEach is much more trimmed down than MoveNext - far less processing - will more likely JIT into something efficient    In addition  foreach   will allocate a new Enumerator no matter what   The GC is your friend  but if you re doing the same foreach repeatedly  this will make more throwaway objects  as opposed to reusing the same delegate - BUT - this is really a fringe case   In typical usage you will see little or no difference

User · Answer

The entire ForEach scope  delegate function  is treated as a single line of code  calling the function   and you cannot set breakpoints or step into the code  If an unhandled exception occurs the entire block is marked

User · Answer

There is one important  and useful  distinction between the two   Because  ForEach uses a for loop to iterate the collection  this is valid  edit  prior to  net 4 5 - the implementation changed and they both throw    someList ForEach x   gt    if x RemoveMe  someList Remove x          whereas foreach uses an enumerator  so this is not valid   foreach var item in someList    if item RemoveMe  someList Remove item       tl dr   Do NOT copypaste this code into your application    These examples aren t best practice  they are just to demonstrate the differences between ForEach   and foreach   Removing items from a list within a for loop can have side effects   The most common one is described in the comments to this question   Generally  if you are looking to remove multiple items from a list  you would want to separate the determination of which items to remove from the actual removal   It doesn t keep your code compact  but it guarantees that you do not miss any items

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We had some code here  in VS2005 and C 2 0  where the previous engineers went out of their way to use list ForEach  delegate item    foo     instead of foreach item in list   foo     for all the code that they wrote  e g  a block of code for reading rows from a dataReader   I still don t know exactly why they did this   The drawbacks of list ForEach   are    It is more verbose in C  2 0  However  in C  3 onwards  you can use the    gt   syntax to make some nicely terse expressions  It is less familiar  People who have to maintain this code will wonder why you did it that way  It took me awhile to decide that there wasn t any reason  except maybe to make the writer seem clever  the quality of the rest of the code undermined that   It was also less readable  with the      at the end of the delegate code block  See also Bill Wagner s book  Effective C   50 Specific Ways to Improve Your C   where he talks about why foreach is preferred to other loops like for or while loops - the main point is that you are letting the compiler decide the best way to construct the loop  If a future version of the compiler manages to use a faster technique  then you will get this for free by using foreach and rebuilding  rather than changing your code  a foreach item in list  construct allows you to use break or continue if you need to exit the iteration or the loop   But you cannot alter the list inside a foreach loop    I m surprised to see that list ForEach is slightly faster  But that s probably not a valid reason to use it throughout   that would be premature optimisation  If your application uses a database or web service that  not loop control  is almost always going to be be where the time goes  And have you benchmarked it against a for loop too  The list ForEach could be faster due to using that internally and a for loop without the wrapper would be even faster    I disagree that the list ForEach delegate  version is  more functional  in any significant way  It does pass a function to a function  but there s no big difference in the outcome or program organisation    I don t think that foreach item in list   says exactly how you want it done  - a for int 1   0  i  lt  count  i    loop does that  a foreach loop leaves the choice of control up to the compiler   My feeling is  on a new project  to use foreach item in list  for most loops in order to adhere to the common usage and for readability  and use list Foreach   only for short blocks  when you can do something more elegantly or compactly with the C  3    gt   operator  In cases like that  there may already be a LINQ extension method that is more specific than ForEach    See if Where    Select    Any    All    Max   or one of the many other LINQ methods doesn t already do what you want from the loop

User · Answer

I guess the someList ForEach   call could be easily parallelized whereas the normal foreach is not that easy to run parallel  You could easily run several different delegates on different cores  which is not that easy to do with a normal foreach  Just my 2 cents

User · Answer

The second way you showed uses an extension method to execute the delegate method for each of the elements in the list   This way  you have another delegate   method  call   Additionally  there is the possibility to iterate the list with a for loop

User · Answer

For fun  I popped List into reflector and this is the resulting C    public void ForEach Action lt T gt  action        if  action    null                ThrowHelper ThrowArgumentNullException ExceptionArgument match             for  int i   0  i  lt  this  size  i                  action this  items i              Similarly  the MoveNext in Enumerator which is what is used by foreach is this   public bool MoveNext         if  this version    this list  version                ThrowHelper ThrowInvalidOperationException ExceptionResource InvalidOperation EnumFailedVersion             if  this index  lt  this list  size                this current   this list  items this index           this index            return true            this index   this list  size   1      this current   default T       return false      The List ForEach is much more trimmed down than MoveNext - far less processing - will more likely JIT into something efficient    In addition  foreach   will allocate a new Enumerator no matter what   The GC is your friend  but if you re doing the same foreach repeatedly  this will make more throwaway objects  as opposed to reusing the same delegate - BUT - this is really a fringe case   In typical usage you will see little or no difference

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The ForEach function is member of the generic class List   I have created the following extension to reproduce the internal code   public static class MyExtension lt T gt                public static void MyForEach this IEnumerable lt T gt  collection  Action lt T gt  action                        foreach  T item in collection                  action Invoke item                     So a the end we are using a normal foreach  or a loop for if you want    On the other hand  using a delegate function is just another way to define a function  this code   delegate string s         lt process the string gt      is equivalent to   private static void myFunction string s   lt other variables    gt         lt process the string gt      or using labda expressions    s    gt   lt process the string gt

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I guess the someList ForEach   call could be easily parallelized whereas the normal foreach is not that easy to run parallel  You could easily run several different delegates on different cores  which is not that easy to do with a normal foreach  Just my 2 cents

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The second way you showed uses an extension method to execute the delegate method for each of the elements in the list   This way  you have another delegate   method  call   Additionally  there is the possibility to iterate the list with a for loop

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List ForEach   is considered to be more functional   List ForEach   says what you want done   foreach item in list  also says exactly how you want it done  This leaves List ForEach free to change the implementation of the how part in the future  For example  a hypothetical future version of  Net might always run List ForEach in parallel  under the assumption that at this point everyone has a number of cpu cores that are generally sitting idle     On the other hand  foreach  item in list  gives you a little more control over the loop  For example  you know that the items will be iterated in some kind of sequential order  and you could easily break in the middle if an item meets some condition     Some more recent remarks on this issue are available here      https   stackoverflow com a 529197 3043

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One thing to be wary of is how to exit from the Generic  ForEach method - see this discussion  Although the link seems to say that this way is the fastest  Not sure why - you d think they would be equivalent once compiled

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There is one important  and useful  distinction between the two   Because  ForEach uses a for loop to iterate the collection  this is valid  edit  prior to  net 4 5 - the implementation changed and they both throw    someList ForEach x   gt    if x RemoveMe  someList Remove x          whereas foreach uses an enumerator  so this is not valid   foreach var item in someList    if item RemoveMe  someList Remove item       tl dr   Do NOT copypaste this code into your application    These examples aren t best practice  they are just to demonstrate the differences between ForEach   and foreach   Removing items from a list within a for loop can have side effects   The most common one is described in the comments to this question   Generally  if you are looking to remove multiple items from a list  you would want to separate the determination of which items to remove from the actual removal   It doesn t keep your code compact  but it guarantees that you do not miss any items

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As they say  the devil is in the details     The biggest difference between the two methods of collection enumeration is that foreach carries state  whereas ForEach x   gt       does not    But lets dig a little deeper  because there are some things you should be aware of that can influence your decision  and there are some caveats you should be aware of when coding for either case   Lets use List lt T gt  in our little experiment to observe behavior  For this experiment  I am using  NET 4 7 2   var names   new List lt string gt         Henry        Shirley        Ann        Peter        Nancy       Lets iterate over this with foreach first   foreach  var name in names        Console WriteLine name       We could expand this into   using  var enumerator   names GetEnumerator           With the enumerator in hand  looking under the covers we get   public List lt T gt  Enumerator GetEnumerator       return new List lt T gt  Enumerator this         internal Enumerator List lt T gt  list      this list   list    this index   0    this version   list  version    this current   default  T      public bool MoveNext       List lt T gt  list   this list    if  this version    list  version     uint  this index  gt    uint  list  size      return this MoveNextRare      this current   list  items this index       this index    return true     object IEnumerator Current           if  this index    0    this index    this list  size   1        ThrowHelper ThrowInvalidOperationException ExceptionResource InvalidOperation EnumOpCantHappen       return  object  this Current          Two things become immediate evident    We are returned a stateful object with intimate knowledge of the underlying collection  The copy of the collection is a shallow copy    This is of course in no way thread safe  As was pointed out above  changing the collection while iterating is just bad mojo   But what about the problem of the collection becoming invalid during iteration by means outside of us mucking with the collection during iteration  Best practices suggests versioning the collection during operations and iteration  and checking versions to detect when the underlying collection changes   Here s where things get really murky  According to the Microsoft documentation      If changes are made to the collection  such as adding  modifying  or   deleting elements  the behavior of the enumerator is undefined    Well  what does that mean  By way of example  just because List lt T gt  implements exception handling does not mean that all collections that implement IList lt T gt  will do the same  That seems to be a clear violation of the Liskov Substitution Principle      Objects of a superclass shall be replaceable with objects of its   subclasses without breaking the application    Another problem is that the enumerator must implement IDisposable -- that means another source of potential memory leaks  not only if the caller gets it wrong  but if the author does not implement the Dispose pattern correctly   Lastly  we have a lifetime issue    what happens if the iterator is valid  but the underlying collection is gone  We now a snapshot of what was    when you separate the lifetime of a collection and its iterators  you are asking for trouble   Lets now examine ForEach x   gt         names ForEach name   gt           This expands to   public void ForEach Action lt T gt  action      if  action    null      ThrowHelper ThrowArgumentNullException ExceptionArgument match     int version   this  version    for  int index   0  index  lt  this  size  amp  amp   version    this  version     BinaryCompatibility TargetsAtLeast Desktop V4 5     index      action this  items index      if  version    this  version     BinaryCompatibility TargetsAtLeast Desktop V4 5      return    ThrowHelper ThrowInvalidOperationException ExceptionResource InvalidOperation EnumFailedVersion       Of important note is the following   for  int index   0  index  lt  this  size  amp  amp          index          action this  items index     This code does not allocate any enumerators  nothing to Dispose   and does not pause while iterating   Note that this also performs a shallow copy of the underlying collection  but the collection is now a snapshot in time  If the author does not correctly implement a check for the collection changing or going  stale   the snapshot is still valid   This doesn t in any way protect you from the problem of the lifetime issues    if the underlying collection disappears  you now have a shallow copy that points to what was    but at least you don t have a Dispose problem to deal with on orphaned iterators     Yes  I said iterators    sometimes its advantageous to have state  Suppose you want to maintain something akin to a database cursor    maybe multiple foreach style Iterator lt T gt  s is the way to go  I personally dislike this style of design as there are too many lifetime issues  and you rely on the good graces of the authors of the collections you are relying on  unless you literally write everything yourself from scratch    There is always a third option     for  var i   0  i  lt  names Count  i          Console WriteLine names i        It ain t sexy  but its got teeth  apologies to Tom Cruise and the movie The Firm   Its your choice  but now you know and it can be an informed one

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Behind the scenes  the anonymous delegate gets turned into an actual method so you could have some overhead with the second choice if the compiler didn t choose to inline the function  Additionally  any local variables referenced by the body of the anonymous delegate example would change in nature because of compiler tricks to hide the fact that it gets compiled to a new method  More info here on how C  does this magic   http   blogs msdn com oldnewthing archive 2006 08 04 688527 aspx

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There is one important  and useful  distinction between the two   Because  ForEach uses a for loop to iterate the collection  this is valid  edit  prior to  net 4 5 - the implementation changed and they both throw    someList ForEach x   gt    if x RemoveMe  someList Remove x          whereas foreach uses an enumerator  so this is not valid   foreach var item in someList    if item RemoveMe  someList Remove item       tl dr   Do NOT copypaste this code into your application    These examples aren t best practice  they are just to demonstrate the differences between ForEach   and foreach   Removing items from a list within a for loop can have side effects   The most common one is described in the comments to this question   Generally  if you are looking to remove multiple items from a list  you would want to separate the determination of which items to remove from the actual removal   It doesn t keep your code compact  but it guarantees that you do not miss any items

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One thing to be wary of is how to exit from the Generic  ForEach method - see this discussion  Although the link seems to say that this way is the fastest  Not sure why - you d think they would be equivalent once compiled

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The second way you showed uses an extension method to execute the delegate method for each of the elements in the list   This way  you have another delegate   method  call   Additionally  there is the possibility to iterate the list with a for loop

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We had some code here  in VS2005 and C 2 0  where the previous engineers went out of their way to use list ForEach  delegate item    foo     instead of foreach item in list   foo     for all the code that they wrote  e g  a block of code for reading rows from a dataReader   I still don t know exactly why they did this   The drawbacks of list ForEach   are    It is more verbose in C  2 0  However  in C  3 onwards  you can use the    gt   syntax to make some nicely terse expressions  It is less familiar  People who have to maintain this code will wonder why you did it that way  It took me awhile to decide that there wasn t any reason  except maybe to make the writer seem clever  the quality of the rest of the code undermined that   It was also less readable  with the      at the end of the delegate code block  See also Bill Wagner s book  Effective C   50 Specific Ways to Improve Your C   where he talks about why foreach is preferred to other loops like for or while loops - the main point is that you are letting the compiler decide the best way to construct the loop  If a future version of the compiler manages to use a faster technique  then you will get this for free by using foreach and rebuilding  rather than changing your code  a foreach item in list  construct allows you to use break or continue if you need to exit the iteration or the loop   But you cannot alter the list inside a foreach loop    I m surprised to see that list ForEach is slightly faster  But that s probably not a valid reason to use it throughout   that would be premature optimisation  If your application uses a database or web service that  not loop control  is almost always going to be be where the time goes  And have you benchmarked it against a for loop too  The list ForEach could be faster due to using that internally and a for loop without the wrapper would be even faster    I disagree that the list ForEach delegate  version is  more functional  in any significant way  It does pass a function to a function  but there s no big difference in the outcome or program organisation    I don t think that foreach item in list   says exactly how you want it done  - a for int 1   0  i  lt  count  i    loop does that  a foreach loop leaves the choice of control up to the compiler   My feeling is  on a new project  to use foreach item in list  for most loops in order to adhere to the common usage and for readability  and use list Foreach   only for short blocks  when you can do something more elegantly or compactly with the C  3    gt   operator  In cases like that  there may already be a LINQ extension method that is more specific than ForEach    See if Where    Select    Any    All    Max   or one of the many other LINQ methods doesn t already do what you want from the loop

[c#] foreach vs someList.ForEach(){}

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