Why is there no ForEach extension method on IEnumerable

Question

Inspired by another question asking about the missing Zip function  Why is there no ForEach extension method on the IEnumerable interface  Or anywhere  The only class that gets a ForEach method is List lt  gt   Is there a reason why it s missing  maybe performance

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Most of the LINQ extension methods return results  ForEach does not fit into this pattern as it returns nothing

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Coincoin  The real power of the foreach extension method involves reusability of the Action lt  gt  without adding unnecessary methods to your code  Say that you have 10 lists and you want to perform the same logic on them  and a corresponding function doesn t fit into your class and is not reused  Instead of having ten for loops  or a generic function that is obviously a helper that doesn t belong  you can keep all of your logic in one place  the Action lt  gt   So  dozens of lines get replaced with  Action lt blah blah gt  f     foo     List1 ForEach p   gt  f p   List2 ForEach p   gt  f p     etc     The logic is in one place and you haven t polluted your class

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Coincoin  The real power of the foreach extension method involves reusability of the Action lt  gt  without adding unnecessary methods to your code  Say that you have 10 lists and you want to perform the same logic on them  and a corresponding function doesn t fit into your class and is not reused  Instead of having ten for loops  or a generic function that is obviously a helper that doesn t belong  you can keep all of your logic in one place  the Action lt  gt   So  dozens of lines get replaced with  Action lt blah blah gt  f     foo     List1 ForEach p   gt  f p   List2 ForEach p   gt  f p     etc     The logic is in one place and you haven t polluted your class

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If you have F   which will be in the next version of  NET   you can use  Seq iter doSomething myIEnumerable

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You can use select when you want to return something  If you don t  you can use ToList first  because you probably don t want to modify anything in the collection

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I wrote a blog post about it  http   blogs msdn com kirillosenkov archive 2009 01 31 foreach aspx  You can vote here if you d like to see this method in  NET 4 0  http   connect microsoft com VisualStudio feedback ViewFeedback aspx FeedbackID 279093

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No one has yet pointed out that ForEach lt T gt  results in compile time type checking where the foreach keyword is runtime checked   Having done some refactoring where both methods were used in the code  I favor  ForEach  as I had to hunt down test failures   runtime failures to find the foreach problems

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The discussion here gives the answer      Actually  the specific discussion I witnessed did in fact hinge over functional purity   In an expression  there are frequently assumptions made about not having side-effects   Having ForEach is specifically inviting side-effects rather than just putting up with them  -- Keith Farmer  Partner    Basically the decision was made to keep the extension methods functionally  pure   A ForEach would encourage side-effects when using the Enumerable extension methods  which was not the intent

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There is already a foreach statement included in the language that does the job most of the time  I d hate to see the following  list ForEach  item   gt        item DoSomething          Instead of  foreach Item item in list         item DoSomething       The latter is clearer and easier to read in most situations  although maybe a bit longer to type  However  I must admit I changed my stance on that issue  a ForEach   extension method would indeed be useful in some situations  Here are the major differences between the statement and the method   Type checking  foreach is done at runtime  ForEach   is at compile time  Big Plus   The syntax to call a delegate is indeed much simpler  objects ForEach DoSomething   ForEach   could be chained  although evilness usefulness of such a feature is open to discussion   Those are all great points made by many people here and I can see why people are missing the function  I wouldn t mind Microsoft adding a standard ForEach method in the next framework iteration

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No one has yet pointed out that ForEach lt T gt  results in compile time type checking where the foreach keyword is runtime checked   Having done some refactoring where both methods were used in the code  I favor  ForEach  as I had to hunt down test failures   runtime failures to find the foreach problems

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I would like to expand on Aku s answer   If you want to call a method for the sole purpose of it s side-effect without iterating the whole enumerable first you can use this   private static IEnumerable lt T gt  ForEach lt T gt  IEnumerable lt T gt  xs  Action lt T gt  f        foreach  var x in xs            f x   yield return x

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One workaround is to write  ToList   ForEach x   gt            pros  Easy to understand - reader only needs to know what ships with C   not any additional extension methods   Syntactic noise is very mild  only adds a little extranious code    Doesn t usually cost extra memory  since a native  ForEach   would have to realize the whole collection  anyway   cons  Order of operations isn t ideal   I d rather realize one element  then act on it  then repeat   This code realizes all elements first  then acts on them each in sequence   If realizing the list throws an exception  you never get to act on a single element   If the enumeration is infinite  like the natural numbers   you re out of luck

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In 3 5  all the extension methods added to IEnumerable are there for LINQ support  notice that they are defined in the System Linq Enumerable class   In this post  I explain why foreach doesn t belong in LINQ   Existing LINQ extension method similar to Parallel For

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So there has been a lot of comments about the fact that a ForEach extension method isn t appropriate because it doesn t return a value like the LINQ extension methods  While this is a factual statement  it isn t entirely true   The LINQ extension methods do all return a value so they can be chained together   collection Where i   gt  i Name    hello   Select i   gt  i FullName     However  just because LINQ is implemented using extension methods does not mean that extension methods must be used in the same way and return a value  Writing an extension method to expose common functionality that does not return a value is a perfectly valid use   The specific arguement about ForEach is that  based on the constraints on extension methods  namely that an extension method will never override an inherited method with the same signature   there may be a situation where the custom extension method is available on all classes that impelement IEnumerable lt T  except List lt T   This can cause confusion when the methods start to behave differently depending on whether or not the extension method or the inherit method is being called

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Note that the MoreLINQ NuGet provides the ForEach extension method you re looking for  as well as a Pipe method which executes the delegate and yields its result    See    https   www nuget org packages morelinq https   code google com p morelinq wiki OperatorsOverview

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Partially it s because the language designers disagree with it from a philosophical perspective    Not having  and testing     a feature is less work than having a feature  It s not really shorter  there s some passing function cases where it is  but that wouldn t be the primary use   It s purpose is to have side effects  which isn t what linq is about  Why have another way to do the same thing as a feature we ve already got   foreach keyword    https   blogs msdn microsoft com ericlippert 2009 05 18 foreach-vs-foreach

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You could write this extension method      Possibly call this  Do  IEnumerable lt T gt  Apply lt T gt   this IEnumerable lt T gt  source  Action lt T gt  action        foreach  var e in source                action e           yield return e            Pros  Allows chaining   MySequence      Apply           Apply           Apply         Cons  It won t actually do anything until you do something to force iteration   For that reason  it shouldn t be called  ForEach     You could write  ToList   at the end  or you could write this extension method  too      possibly call this  Realize  IEnumerable lt T gt  Done lt T gt   this IEnumerable lt T gt  source        foreach  var e in source                   do nothing                      return source      This may be too significant a departure from the shipping C  libraries  readers who are not familiar with your extension methods won t know what to make of your code

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You could use the  chainable  but lazily evaluated  Select  first doing your operation  and then returning identity  or something else if you prefer   IEnumerable lt string gt  people   new List lt string gt     alica    bob    john    pete    people Select p   gt    Console WriteLine p   return p        You will need to make sure it is still evaluated  either with Count    the cheapest operation to enumerate afaik  or another operation you needed anyway   I would love to see it brought in to the standard library though   static IEnumerable lt T gt  WithLazySideEffect this IEnumerable lt T gt  src  Action lt T gt  action      return src Select i   gt    action i   return i           The above code then becomes people WithLazySideEffect p   gt  Console WriteLine p   which is effectively equivalent to foreach  but lazy and chainable

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Is it me or is the List lt T gt  Foreach pretty much been made obsolete by Linq   Originally there was   foreach X x in Y     where Y simply had to be IEnumerable  Pre 2 0   and implement a GetEnumerator    If you look at the MSIL generated you can see that it is exactly the same as  IEnumerator lt int gt  enumerator   list GetEnumerator    while  enumerator MoveNext          int i   enumerator Current       Console WriteLine i        See http   alski net post 0a-for-foreach-forFirst-forLast0a-0a- aspx for the MSIL   Then in DotNet2 0 Generics came along and the List  Foreach has always felt to me to be an implementation of the Vistor pattern   see Design Patterns by Gamma  Helm  Johnson  Vlissides    Now of course in 3 5 we can instead use a Lambda to the same effect  for an example try  http   dotnet-developments blogs techtarget com 2008 09 02 iterators-lambda-and-linq-oh-my

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While I agree that it s better to use the built-in foreach construct in most cases  I find the use of this variation on the ForEach lt   extension to be a little nicer than having to manage the index in a regular foreach myself   public static int ForEach lt T gt  this IEnumerable lt T gt  list  Action lt int  T gt  action        if  action    null  throw new ArgumentNullException  action         var index   0       foreach  var elem in list          action index    elem        return index      Example  var people   new      Moe    Curly    Larry     people ForEach  i  p    gt  Console WriteLine  Person   0  is  1    i  p      Would give you   Person  0 is Moe Person  1 is Curly Person  2 is Larry

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You could write this extension method      Possibly call this  Do  IEnumerable lt T gt  Apply lt T gt   this IEnumerable lt T gt  source  Action lt T gt  action        foreach  var e in source                action e           yield return e            Pros  Allows chaining   MySequence      Apply           Apply           Apply         Cons  It won t actually do anything until you do something to force iteration   For that reason  it shouldn t be called  ForEach     You could write  ToList   at the end  or you could write this extension method  too      possibly call this  Realize  IEnumerable lt T gt  Done lt T gt   this IEnumerable lt T gt  source        foreach  var e in source                   do nothing                      return source      This may be too significant a departure from the shipping C  libraries  readers who are not familiar with your extension methods won t know what to make of your code

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I ve always wondered that myself  that is why that I always carry this with me   public static void ForEach lt T gt  this IEnumerable lt T gt  col  Action lt T gt  action        if  action    null                throw new ArgumentNullException  action              foreach  var item in col                action item             Nice little extension method

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I wrote a blog post about it  http   blogs msdn com kirillosenkov archive 2009 01 31 foreach aspx  You can vote here if you d like to see this method in  NET 4 0  http   connect microsoft com VisualStudio feedback ViewFeedback aspx FeedbackID 279093

User · Answer

One workaround is to write  ToList   ForEach x   gt            pros  Easy to understand - reader only needs to know what ships with C   not any additional extension methods   Syntactic noise is very mild  only adds a little extranious code    Doesn t usually cost extra memory  since a native  ForEach   would have to realize the whole collection  anyway   cons  Order of operations isn t ideal   I d rather realize one element  then act on it  then repeat   This code realizes all elements first  then acts on them each in sequence   If realizing the list throws an exception  you never get to act on a single element   If the enumeration is infinite  like the natural numbers   you re out of luck

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ForEach method was added before LINQ  If you add ForEach extension  it will never be called for List instances because of extension methods constraints   I think the reason it was not added is to not interference with existing one   However  if you really miss this little nice function  you can roll out your own version  public static void ForEach lt T gt       this IEnumerable lt T gt  source      Action lt T gt  action        foreach  T element in source           action element

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The discussion here gives the answer      Actually  the specific discussion I witnessed did in fact hinge over functional purity   In an expression  there are frequently assumptions made about not having side-effects   Having ForEach is specifically inviting side-effects rather than just putting up with them  -- Keith Farmer  Partner    Basically the decision was made to keep the extension methods functionally  pure   A ForEach would encourage side-effects when using the Enumerable extension methods  which was not the intent

[c#] Why is there no ForEach extension method on IEnumerable?

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