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C#: IEnumerable<T>.Select() inefficient in some cases?

I recently learned that the objects created by .NET's LINQ implementation is inefficient for specific enumeration types.
Take a look at this code:
public class DummyCollection : ICollection<int>
{
public IEnumerator<int> GetEnumerator()
{
throw new Exception();
}
public int Count
{
get
{
return 10;
}
}
//some more interface methods
}
basically, instances of DummyCollection have a size of 10, but throws an exception if it is actually enumerated.
now here:
var d = new DummyCollection();
Console.WriteLine(d.Count());
A 10 is printed without error, but this piece of code:
var l = d.Select(a=> a);
Console.WriteLine(l.Count());
throws an exception, despite it being trivial to say that l's size is 10 as well (since Select offers 1-to-1 mapping). What this basically means is, that when checking the length of an Ienumerable, the input might be a Select-wrapped Collection, thus extending the computation time from an O(1) to a staggering O(n) (could be even worse, if the selection function is particularly cumbersome).
I know that you sacrifice efficiency when you ask for LINQ's generics, but this seems like such a simple problem to fix. I checked online and couldn't find anyone addressing this. Is there a way to bypass this shortcoming? Is anyone looking into this? Is anyone fixing this? Is this just an edge case that isn't that much of a problem? Any insight is appreciated.

You can see how Count() extension method is implemented here. Basically is something like this:
public static int Count<TSource>(this IEnumerable<TSource> source)
{
if (source == null) throw Error.ArgumentNull("source");
ICollection<TSource> collectionoft = source as ICollection<TSource>;
if (collectionoft != null) return collectionoft.Count;
ICollection collection = source as ICollection;
if (collection != null) return collection.Count;
int count = 0;
using (IEnumerator<TSource> e = source.GetEnumerator()) {
checked {
while (e.MoveNext()) count++;
}
}
return count;
}
As you can see the method check first is the source is of type ICollection<TSource> or ICollection, if that is the case then there is no need to iterate counting the elements, just return Count property.
In your first case Count property is called returning 10 and GetEnumerator() method is never called.
When you use Select() method you're wrapping the collection into another type that isn't an ICollection (in above link you can also see Select() implementation), therefore the iteration is necessary.
In your second case, when you call Count() your GetEnumerator() method is called and the exception is thrown.

IEnumerable<T> doesn't have a concept of Count. This exists in implementations, which (apart from the odd shortcut here and there) have no role in LINQ to Objects. If you project an implementation of IEnumerable<T> (such as ICollection<T>), with Select, the only real guarantee you have is that the output will be IEnumerable<T>... which has no Count.
LINQ should be thought of as dealing with sequences of items, one at a time, only with a concept of current and next item (or the end of sequence). Knowing about the number of items is a (potentially) costly operation that requires iteration of all the items being counted, other than in a few, optimized cases.
Given that LINQ relies on iteration in preference to indexes and counts means that an IEnumerable that errors when you try to iterate it is going to need some super weird special-casing to fly. To me, it wouldn't be a very useful use-case.

Is there any method like ForEach for IList? [duplicate]

This question already has answers here:
Closed 10 years ago.
Possible Duplicate:
LINQ equivalent of foreach for IEnumerable<T>
List<T> has a method called ForEach which executes the passed action on each element of it.
var names = new List<String>{ "Bruce", "Alfred", "Tim", "Richard" };
names.ForEach(p => { Console.WriteLine(p); });
But what if names is not a List<T> but an IList<T>? IList<T> doesn't have a method like ForEach.
Is there some alternative?

Use a foreach loop:
foreach (var p in names) {
Console.WriteLine(p);
}
There is no reason to use delegates and extension methods all over the place if that doesn't actually improve readability; a foreach loop is not any less explicitly telling readers what's being done than a ForEach method.

If your IList<T> is an array (T[]), then you have Array.ForEach method on them similar to ForEach on List<T>. You can create an extension method for your custom IList<T> or IEnumerable<T> or whatever you prefer.
public static void ForEach<T>(this IList<T> list, Action<T> action)
{
foreach (T t in list)
action(t);
}
You just have to be wary of the fact that the objects in the original collection will be modified, but I guess the naming does imply that.
------------------------------------------------------------------------------------------------------------------------------------
I prefer to call:
people.Where(p => p.Tenure > 5)
.Select(p => p.Nationality)
.ForEach(n => AssignCitizenShip(n);
than
foreach (var n in people.Where(p => p.Tenure > 5).Select(p => p.Nationality))
{
AssignCitizenShip(n);
}
If so you can create the extension method on IEnumerable. Mind you the terminating call ForEach executes the Linq query. If you do not want it, you can defer it too by using yield statement and returning an IEnumerable<T> back:
public static IEnumerable<T> ForEach<T>(this IEnumerable<T> list, Action<T> action)
{
foreach (T t in list)
{
action(t);
yield return t;
}
}
That solves the side-effect issue, but I personally like a method named ForEach to finally execute the call.
-----------------------------------------------------------------------------------------------------------------------------------
To address the opposing views on preferences, here is a better link from Eric Lippert than this. To quote him:
"The first reason is that doing so violates the functional programming
principles that all the other sequence operators are based upon.
Clearly the sole purpose of a call to this method is to cause side
effects. The purpose of an expression is to compute a value, not to
cause a side effect. The purpose of a statement is to cause a side
effect. The call site of this thing would look an awful lot like an
expression (though, admittedly, since the method is void-returning,
the expression could only be used in a “statement expression”
context.) It does not sit well with me to make the one and only
sequence operator that is only useful for its side effects.
The second reason is that doing so adds zero new representational
power to the language".
Eric's not saying it's a bad thing to do - just the philosophical reasons behind the decision to not include the construct in Linq by default. If you believe a function on an IEnumerable shouldn't act on the contents, then don't do it. Personally I dont mind it since I'm well aware what it does. I treat it as any other method that causes side-effect on a collection class. I can enter into the function and debug it too if I want. Here is another one from Linq itself.
people.Where(p => p.Tenure > 5)
.Select(p => p.Nationality)
.AsParallel()
.ForAll(n => AssignCitizenShip(n);
As I would say, there is nothing bad about these. Its just personal preference. I wouldn't use this for nested foreachs or if it involves more than one line of code to execute inside the foreach loop since thats plain unreadable. But for simple example I posted, I like it. Looks clean and concise.
Edit: See a performance link btw: Why is List<T>.ForEach faster than standard foreach?

You could make an extension method and use most of the implementation of void List<T>.ForEach(Action<T> action). You can download the source code at the Shared Source Initiative site.
Basically you will end to something like this:
public static void ForEach<T>(this IList<T> list, Action<T> action)
{
if (list == null) throw new ArgumentNullException("null");
if (action == null) throw new ArgumentNullException("action");
for (int i = 0; i < list.Count; i++)
{
action(list[i]);
}
}
It is slightly better than the other implementations that use the foreach statement since it takes advantage of the fact that IList includes an indexer.
Although I aggree with the answer of O. R. Mapper, sometimes in big projects with many developers it is hard to convicne everybody that a foreach statement is clearer. Even worse, if your API is based on interfaces (IList) instead of concrete types (List) then developers that are used to the List<T>.ForEach method might start calling ToList on your IList references! I know because it happened in my previous project. I was using the collection interfaces everywhere in our public APIs following the Framework Design Guidelines. It took me a while to notice that many developers where not used to this and call to ToList started apprearing with an alarming rate. Finally I added this extension method to a common assembly that everybody was using and made sure that all unecessary call to ToList were removed from the codebase.

Add this code to static class and call it extensions:
public static void ForEach<T>(this IList<T> list, Action<T> action) {
foreach(var item in list) {
action.Invoke(item);
}
}

How to make [example] extension method more generic/functional/efficient?

I needed a double[] split into groups of x elements by stride y returning a List. Pretty basic...a loop and/or some linq and your all set. However, I have not been spending much time on extension methods and this looked like a good candidate for some practice. The naive version returns what I am looking for in my current application....
(A)
public static IList<T[]> Split<T>(this IEnumerable<T> source, int every, int take)
{
/*... throw E if X is insane ...*/
var result = source
.Where ((t, i) => i % every == 0)
.Select((t, i) => source.Skip(i * every).Take(take).ToArray())
.ToList();
return result;
}
...the return type is sort of generic...depending on your definition of generic.
I would think...
(B)
public static IEnumerable<IEnumerable<T>> Split<T>
(this IEnumerable<T> source,int every, int take){/*...*/}
...is a better solution...maybe.
Question(s):
Is (B) preferred ?...Why ?
How would you cast (B) as IList <T[]> ?
Any benefit in refactoring ? possibly
two methods that might be chained or the like.
Is the approach sound ?...or have I
missed something basic.
Comments, opinions and harsh language are always appreciated.
Usage Context: C# .Net 4.0

B is probably the better option. Really the major change is that the consumer of the code has the option to make it a list using ToList() on the end of your method, instead of being forced to deal with a List (an IList, actually, which cannot be iterated).
This has a LOT of advantages in method chaining and general use. It's easy to ToList() an enumerable, but hard to go the other way. So, you can call Select().Split().OrderBy() on a list and use the results in a foreach statement without having to have Linq iterate through the whole thing at once.
Refactoring to yield return single values MIGHT get you a performance bonus, but since you're basically just returning the iterator that the Select gave you (which will yield one item at a time itself) I don't think you'll get much benefit in yielding through it yourself.

I would prefer (B) as it looks more flexible. One way of casting the output of the (B) method to an IList<T[]> is as simple as chaining .Select(x => x.ToArray()).ToList() to it, e.g.,
var foo =
bar.Split(someEvery, someTake).Select(x => x.ToArray()).ToList();

In .Net 4, you can just change the return type to IEnumerable<IEnumerable<T>> and it will work.
Before .Net 4, you would have to cast the internal lists to IEnumerable first, by just calling .Cast<IEnumerable<T>>() on your result before returning.

Is this C# extension method impure and if so, bad code?

I'm learning a bit about function programming, and I'm wondering:
1) If my ForEach extension method is pure? The way I'm calling it seems violate the "don't mess with the object getting passed in", right?
public static void ForEach<T>(this IEnumerable<T> source, Action<T> action)
{
foreach ( var item in source )
action(item);
}
static void Main(string[] args)
{
List<Cat> cats = new List<Cat>()
{
new Cat{ Purring=true,Name="Marcus",Age=10},
new Cat{ Purring=false, Name="Fuzzbucket",Age=25 },
new Cat{ Purring=false, Name="Beanhead",Age=9 },
new Cat{Purring=true,Name="Doofus",Age=3}
};
cats.Where(x=>x.Purring==true).ForEach(x =>
{
Console.WriteLine("{0} is a purring cat... purr!", x.Name);
});
// *************************************************
// Does this code make the extension method impure?
// *************************************************
cats.Where(x => x.Purring == false).ForEach(x =>
{
x.Purring = true; // purr,baby
});
// all the cats now purr
cats.Where(x=>x.Purring==true).ForEach(x =>
{
Console.WriteLine("{0} is a purring cat... purr!", x.Name);
});
}
public class Cat {
public bool Purring;
public string Name;
public int Age;
}
2) If it is impure, is it bad code? I personally think it makes cleaner looking code than the old foreach ( var item in items) { blah; }, but I worry that since it might be impure, it could make a mess.
3) Would it be bad code if it returned IEnumerable<T> instead of void? I'd say as long as it is impure, yes it would be very bad code as it would encourage chaining something that would modify the chain. For example, is this bad code?
// possibly bad extension
public static IEnumerable<T> ForEach<T>(this IEnumerable<T> source, Action<T> action)
{
foreach ( var item in source )
action(item);
return source;
}

Impurity doesn't necesarily mean bad code. Many people find it easy and useful to use side effects to solve a problem. The key is first knowing how to do it in a pure way, so you'll know when impurity is appropriate :).
.NET doesn't have the concept of purity in the type system, so a "pure" method that takes in arbitrary delegates can always be impure, depending on how it's called. For instance, "Where", aka "filter", would usually be considered a pure function, since it doesn't modify its arguments or modify global state.
But, there's nothing stopping you from putting such code inside the argument to Where. For example:
things.Where(x => { Console.WriteLine("um?");
return true; })
.Count();
So that's definately an impure usage of Where. Enumerables can do whatever they want as they iterate.
Is your code bad? No. Using a foreach loop is just as "impure" -- you're still modifying the source objects. I write code like that all the time. Chain together some selects, filters, etc., then execute a ForEach on it to invoke some work. You're right, it's cleaner and easier.
Example: ObservableCollection. It has no AddRange method for some reason. So, if I want to add a bunch of things to it, what do I do?
foreach(var x in things.Where(y => y.Foo > 0)) { collection.Add(x)); }
or
things.Where(x => x.Foo > 0).ForEach(collection.Add);
I prefer the second one. At a minimum, I don't see how it can be construed as being worse than the first way.
When is it bad code? When it does side effecting code in a place that's not expected. This is the case for my first example using Where. And even then, there are times when the scope is very limited and the usage is clear.
Chaining ForEach
I've written code that does things like that. To avoid confusion, I would give it another name. The main confusion is "is this immediately evaluated or lazy?". ForEach implies that it'll go execute a loop right away. But something returning an IEnumerable implies that the items will be processed as needed. So I'd suggest giving it another name ("Process", "ModifySeq", "OnEach"... something like that), and making it lazy:
public static IEnumerable<T> OnEach(this IEnumerable<T> src, Action<T> f) {
foreach(var x in src) {
f(x);
yield return x;
}
}

It is not pure, as it can call impure methods. I think by typical definitions, purity is a transitive closure - a function is pure only if all the functions it calls (directly or indirectly) are also pure, or if the effects of those functions are encapsulated (e.g. they only mutate a non-escaping local variable).

Yes, it's not pure, but that's kind of a moot point as it's not even a function.
As the method doesn't return anything, the only option for it to do anything at all is to either affect the objects that you are sending in, or affecting something unrelated (like writing to the console window).
Edit:
To answer your third question; yes, that is bad code, as it seems to be doing something that it doesn't. The method returns a collection so it seems to be pure, but as it just returns the collection that was sent in, it's actually not any more pure than the first version. To make any sense the method should take a Func<T,T> delegate to use as conversion, and return a collection of the converted items:
public static IEnumerable<T> ForEach<T>(this IEnumerable<T> source, Func<T,T> converter) {
foreach (T item in source) {
yield return converter(item);
}
}
It's of course still up to the converter function if the extension call is pure. If it doesn't make a copy of the input item but just changes it and returns it, the call is still not pure.

Indeed, because your lambda expression contains an assignment, the function is now by definition impure. Whether the assignment is related to one of the arguments or another object defined outside the current function is irrelevant... A function must have no side-effects whatsoever in order to be called pure. See Wikipedia for a more precise (though quite straightforward) definition, which details the two conditions a function must satisfy to be deemed pure (having no side-effects is one of them). I believe lambda expressions are typically meant to be used as pure functions (at least I would imagine they were originally studied as such from the mathematical perspective), though clearly C# isn't stringent about this, where purely functional languages are. So it's probably not bad pratice, though it's definitely worth being away that such a function is impure.

Why is there no ForEach extension method on IEnumerable?

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<>. Is there a reason why it's missing, maybe performance?

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 =>
{
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.

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<T>(
this IEnumerable<T> source,
Action<T> action)
{
foreach (T element in source)
action(element);
}

You could write this extension method:
// Possibly call this "Do"
IEnumerable<T> Apply<T> (this IEnumerable<T> source, Action<T> 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<T> Done<T> (this IEnumerable<T> 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.

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.

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<> extension to be a little nicer than having to manage the index in a regular foreach myself:
public static int ForEach<T>(this IEnumerable<T> list, Action<int, T> 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) => Console.WriteLine("Person #{0} is {1}", i, p));
Would give you:
Person #0 is Moe
Person #1 is Curly
Person #2 is Larry

One workaround is to write .ToList().ForEach(x => ...).
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.

I've always wondered that myself, that is why that I always carry this with me:
public static void ForEach<T>(this IEnumerable<T> col, Action<T> action)
{
if (action == null)
{
throw new ArgumentNullException("action");
}
foreach (var item in col)
{
action(item);
}
}
Nice little extension method.

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 => i.Name = "hello").Select(i => 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<T> except List<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.

You could use the (chainable, but lazily evaluated) Select, first doing your operation, and then returning identity (or something else if you prefer)
IEnumerable<string> people = new List<string>(){"alica", "bob", "john", "pete"};
people.Select(p => { 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<T> WithLazySideEffect(this IEnumerable<T> src, Action<T> action) {
return src.Select(i => { action(i); return i; } );
}
The above code then becomes people.WithLazySideEffect(p => Console.WriteLine(p)) which is effectively equivalent to foreach, but lazy and chainable.

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

#Coincoin
The real power of the foreach extension method involves reusability of the Action<> 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<>. So, dozens of lines get replaced with
Action<blah,blah> f = { foo };
List1.ForEach(p => f(p))
List2.ForEach(p => f(p))
etc...
The logic is in one place and you haven't polluted your class.

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

If you have F# (which will be in the next version of .NET), you can use
Seq.iter doSomething myIEnumerable

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/

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.

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

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?

Is it me or is the List<T>.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<int> 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/

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<T> ForEach<T>(IEnumerable<T> xs, Action<T> f) {
foreach (var x in xs) {
f(x); yield return x;
}
}

My version an extension method which would allow you to use ForEach on IEnumerable of T
public static class EnumerableExtension
{
public static void ForEach<T>(this IEnumerable<T> source, Action<T> action)
{
source.All(x =>
{
action.Invoke(x);
return true;
});
}
}

No one has yet pointed out that ForEach<T> 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.