A linq query Where clause can apply a func to an item in the original set and return a bool to include or not include the item based on the item's characteristics. Great stuff:
var q = myColl.Where(o => o.EffectiveDate = LastThursday);
But what if I want to find a set of items where each item is related to the last item in some way? Like:
var q = myColl.Where(o => o.EffectiveDate = thePreviousItem.ExpirationDate);
How do you make a Where (or other linq function) "jump out" of the current item?
Here's what I tried, trying to be clever. I made every item an array just so I can use the Aggregate function:
public IQueryable<T> CurrentVersions
{
get => AllVersions
.Select(vo => new T[] { vo })
.Aggregate((voa1, voa2) => voa1[0].BusinessExpirationDate.Value == voa2[0].BusinessEffectiveDate.Value ? voa1.Concat(voa2).ToArray() : voa1)
.SelectMany(vo => vo);
}
but that doesn't compile on the SelectMany:
The type arguments for method Enumerable.SelectMany<TSource,
TResult>(IEnumerable<TSource>, Func<TSource, IEnumerable<TResult>>)
cannot be inferred from the usage. Try specifying the type arguments
explicitly.
EDIT (SOLUTION)
As it turns out, I was on the right track, but was just confused about what SelectMany does. I didn't need it. I also needed to change IQueryable to IEnumerable because I'm using EF and you can't query after you let go of the DbContext. So, here is the actual solution.
public IEnumerable<T> CurrentVersions
{
get => AllVersions
.Select(vo => new T[] { vo })
.Aggregate((voa1, voa2) => voa1[0].BusinessExpirationDate.Value == voa2[0].BusinessEffectiveDate.Value ? voa1.Concat(voa2).ToArray() : voa1);
}
Linq queries are most effective when each item is processed in isolation. It doesn't work well when trying to relate items within the same collection, without having to process the same collection multiple times and standard linq operators.
The MoreLINQ library helps provide additional operators to fill in some of those gaps. I'm not sure what operators it provides that could be used in this instance, but I know it has a Pairwise() method that combines the current and previous items in the iteration.
In general, for situations like this, if you needed to roll out your own, it would be far easier to write it using a generator to generate your sequence. Either as a general purpose extension method:
public static IEnumerable<TSource> WhereWithPrevious<TSource>(
this IEnumerable<TSource> source,
Func<TSource, TSource, bool> predicate)
{
using (var iter = source.GetEnumerator())
{
if (!iter.MoveNext())
yield break;
var previous = iter.Current;
while (iter.MoveNext())
{
var current = iter.Current;
if (predicate(current, previous))
yield return current;
}
}
}
or one specifically for the problem you're trying to solve.
public static IEnumerable<MyType> GetVersions(IEnumerable<MyType> source)
{
using (var iter = source.GetEnumerator())
{
if (!iter.MoveNext())
yield break;
var previous = iter.Current;
while (iter.MoveNext())
{
var current = iter.Current;
if (current.EffectiveDate == previous.ExpirationDate)
yield return current;
}
}
}
An alternative approach which while standard practice in other languages but terribly inefficient here would be to zip the collection with itself offset by one.
var query = Collection.Skip(1).Zip(Collection, (c, p) => (current:c,previous:p))
.Where(x => x.current.EffectiveDate == x.previous.ExpirationDate)
...;
And with all of that said, using any of these options will most likely make your query incompatible with query providers. It's not something you would want expressed as a single query anyway.
I am using Entity Framework 4.0 and trying to use the "Contains" function of one the object sets in my context object. to do so i coded a Comparer class:
public class RatingInfoComparer : IEqualityComparer<RatingInfo>
{
public bool Equals(RatingInfo x, RatingInfo y)
{
var a = new {x.PlugInID,x.RatingInfoUserIP};
var b = new {y.PlugInID,y.RatingInfoUserIP};
if(a.PlugInID == b.PlugInID && a.RatingInfoUserIP.Equals(b.RatingInfoUserIP))
return true;
else
return false;
}
public int GetHashCode(RatingInfo obj)
{
var a = new { obj.PlugInID, obj.RatingInfoUserIP };
if (Object.ReferenceEquals(obj, null))
return 0;
return a.GetHashCode();
}
}
when i try to use the comparer with this code:
public void SaveRatingInfo2(int plugInId, string userInfo)
{
RatingInfo ri = new RatingInfo()
{
PlugInID = plugInId,
RatingInfoUser = userInfo,
RatingInfoUserIP = "192.168.1.100"
};
//This is where i get the execption
if (!context.RatingInfoes.Contains<RatingInfo>(ri, new RatingInfoComparer()))
{
//my Entity Framework context object
context.RatingInfoes.AddObject(ri);
context.SaveChanges();
}
}
i get an execption:
"LINQ to Entities does not recognize the method 'Boolean Contains[RatingInfo](System.Linq.IQueryable1[OlafCMSLibrary.Models.RatingInfo], OlafCMSLibrary.Models.RatingInfo,
System.Collections.Generic.IEqualityComparer1[OlafCMSLibrary.Models.RatingInfo])' method, and his method cannot be translated into a store expression."
Since i am not proficient with linQ and Entity Framework i might be making a mistake with my use of the "var" either in the "GetHashCode" function or in general.
If my mistake is clear to you do tell me :) it does not stop my project! but it is essential for me to understand why a simple comparer doesnt work.
Thanks
Aaron
LINQ to Entities works by converting an expression tree into queries against an object model through the IQueryable interface. This means than you can only put things into the expression tree which LINQ to Entities understands.
It doesn't understand the Contains method you are using, so it throws the exception you see. Here is a list of methods which it understands.
Under the Set Methods section header, it lists Contains using an item as supported, but it lists Contains with an IEqualityComparer as not supported. This is presumably because it would have to be able to work out how to convert your IEqualityComparer into a query against the object model, which would be difficult. You might be able to do what you want using multiple Where clauses, see which ones are supported further up the document.
I'd like to do the equivalent of the following in LINQ, but I can't figure out how:
IEnumerable<Item> items = GetItems();
items.ForEach(i => i.DoStuff());
What is the real syntax?
There is no ForEach extension for IEnumerable; only for List<T>. So you could do
items.ToList().ForEach(i => i.DoStuff());
Alternatively, write your own ForEach extension method:
public static void ForEach<T>(this IEnumerable<T> enumeration, Action<T> action)
{
foreach(T item in enumeration)
{
action(item);
}
}
Fredrik has provided the fix, but it may be worth considering why this isn't in the framework to start with. I believe the idea is that the LINQ query operators should be side-effect-free, fitting in with a reasonably functional way of looking at the world. Clearly ForEach is exactly the opposite - a purely side-effect-based construct.
That's not to say this is a bad thing to do - just thinking about the philosophical reasons behind the decision.
Update 7/17/2012: Apparently as of C# 5.0, the behavior of foreach described below has been changed and "the use of a foreach iteration variable in a nested lambda expression no longer produces unexpected results." This answer does not apply to C# ≥ 5.0.
#John Skeet and everyone who prefers the foreach keyword.
The problem with "foreach" in C# prior to 5.0, is that it is inconsistent with how the equivalent "for comprehension" works in other languages, and with how I would expect it to work (personal opinion stated here only because others have mentioned their opinion regarding readability). See all of the questions concerning "Access to modified closure"
as well as "Closing over the loop variable considered harmful". This is only "harmful" because of the way "foreach" is implemented in C#.
Take the following examples using the functionally equivalent extension method to that in #Fredrik Kalseth's answer.
public static class Enumerables
{
public static void ForEach<T>(this IEnumerable<T> #this, Action<T> action)
{
foreach (T item in #this)
{
action(item);
}
}
}
Apologies for the overly contrived example. I'm only using Observable because it's not entirely far fetched to do something like this. Obviously there are better ways to create this observable, I am only attempting to demonstrate a point. Typically the code subscribed to the observable is executed asynchronously and potentially in another thread. If using "foreach", this could produce very strange and potentially non-deterministic results.
The following test using "ForEach" extension method passes:
[Test]
public void ForEachExtensionWin()
{
//Yes, I know there is an Observable.Range.
var values = Enumerable.Range(0, 10);
var observable = Observable.Create<Func<int>>(source =>
{
values.ForEach(value =>
source.OnNext(() => value));
source.OnCompleted();
return () => { };
});
//Simulate subscribing and evaluating Funcs
var evaluatedObservable = observable.ToEnumerable().Select(func => func()).ToList();
//Win
Assert.That(evaluatedObservable,
Is.EquivalentTo(values.ToList()));
}
The following fails with the error:
Expected: equivalent to < 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 >
But was: < 9, 9, 9, 9, 9, 9, 9, 9, 9, 9 >
[Test]
public void ForEachKeywordFail()
{
//Yes, I know there is an Observable.Range.
var values = Enumerable.Range(0, 10);
var observable = Observable.Create<Func<int>>(source =>
{
foreach (var value in values)
{
//If you have resharper, notice the warning
source.OnNext(() => value);
}
source.OnCompleted();
return () => { };
});
//Simulate subscribing and evaluating Funcs
var evaluatedObservable = observable.ToEnumerable().Select(func => func()).ToList();
//Fail
Assert.That(evaluatedObservable,
Is.EquivalentTo(values.ToList()));
}
You could use the FirstOrDefault() extension, which is available for IEnumerable<T>. By returning false from the predicate, it will be run for each element but will not care that it doesn't actually find a match. This will avoid the ToList() overhead.
IEnumerable<Item> items = GetItems();
items.FirstOrDefault(i => { i.DoStuff(); return false; });
Keep your Side Effects out of my IEnumerable
I'd like to do the equivalent of the following in LINQ, but I can't figure out how:
As others have pointed out here and abroad LINQ and IEnumerable methods are expected to be side-effect free.
Do you really want to "do something" to each item in the IEnumerable? Then foreach is the best choice. People aren't surprised when side-effects happen here.
foreach (var i in items) i.DoStuff();
I bet you don't want a side-effect
However in my experience side-effects are usually not required. More often than not there is a simple LINQ query waiting to be discovered accompanied by a StackOverflow.com answer by either Jon Skeet, Eric Lippert, or Marc Gravell explaining how to do what you want!
Some examples
If you are actually just aggregating (accumulating) some value then you should consider the Aggregate extension method.
items.Aggregate(initial, (acc, x) => ComputeAccumulatedValue(acc, x));
Perhaps you want to create a new IEnumerable from the existing values.
items.Select(x => Transform(x));
Or maybe you want to create a look-up table:
items.ToLookup(x, x => GetTheKey(x))
The list (pun not entirely intended) of possibilities goes on and on.
I took Fredrik's method and modified the return type.
This way, the method supports deferred execution like other LINQ methods.
EDIT: If this wasn't clear, any usage of this method must end with ToList() or any other way to force the method to work on the complete enumerable. Otherwise, the action would not be performed!
public static IEnumerable<T> ForEach<T>(this IEnumerable<T> enumeration, Action<T> action)
{
foreach (T item in enumeration)
{
action(item);
yield return item;
}
}
And here's the test to help see it:
[Test]
public void TestDefferedExecutionOfIEnumerableForEach()
{
IEnumerable<char> enumerable = new[] {'a', 'b', 'c'};
var sb = new StringBuilder();
enumerable
.ForEach(c => sb.Append("1"))
.ForEach(c => sb.Append("2"))
.ToList();
Assert.That(sb.ToString(), Is.EqualTo("121212"));
}
If you remove the ToList() in the end, you will see the test failing since the StringBuilder contains an empty string. This is because no method forced the ForEach to enumerate.
So many answers, yet ALL fail to pinpoint one very significant problem with a custom generic ForEach extension: Performance! And more specifically, memory usage and GC.
Consider the sample below. Targeting .NET Framework 4.7.2 or .NET Core 3.1.401, configuration is Release and platform is Any CPU.
public static class Enumerables
{
public static void ForEach<T>(this IEnumerable<T> #this, Action<T> action)
{
foreach (T item in #this)
{
action(item);
}
}
}
class Program
{
private static void NoOp(int value) {}
static void Main(string[] args)
{
var list = Enumerable.Range(0, 10).ToList();
for (int i = 0; i < 1000000; i++)
{
// WithLinq(list);
// WithoutLinqNoGood(list);
WithoutLinq(list);
}
}
private static void WithoutLinq(List<int> list)
{
foreach (var item in list)
{
NoOp(item);
}
}
private static void WithLinq(IEnumerable<int> list) => list.ForEach(NoOp);
private static void WithoutLinqNoGood(IEnumerable<int> enumerable)
{
foreach (var item in enumerable)
{
NoOp(item);
}
}
}
At a first glance, all three variants should perform equally well. However, when the ForEach extension method is called many, many times, you will end up with garbage that implies a costly GC. In fact, having this ForEach extension method on a hot path has been proven to totally kill performance in our loop-intensive application.
Similarly, the weakly typed foreach loop will also produce garbage, but it will still be faster and less memory-intensive than the ForEach extension (which also suffers from a delegate allocation).
Strongly typed foreach: Memory usage
Weakly typed foreach: Memory usage
ForEach extension: Memory usage
Analysis
For a strongly typed foreach the compiler is able to use any optimized enumerator (e.g. value based) of a class, whereas a generic ForEach extension must fall back to a generic enumerator which will be allocated on each run. Furthermore, the actual delegate will also imply an additional allocation.
You would get similar bad results with the WithoutLinqNoGood method. There, the argument is of type IEnumerable<int> instead of List<int> implying the same type of enumerator allocation.
Below are the relevant differences in IL. A value based enumerator is certainly preferable!
IL_0001: callvirt instance class
[mscorlib]System.Collections.Generic.IEnumerator`1<!0>
class [mscorlib]System.Collections.Generic.IEnumerable`1<!!T>::GetEnumerator()
vs
IL_0001: callvirt instance valuetype
[mscorlib]System.Collections.Generic.List`1/Enumerator<!0>
class [mscorlib]System.Collections.Generic.List`1<int32>::GetEnumerator()
Conclusion
The OP asked how to call ForEach() on an IEnumerable<T>. The original answer clearly shows how it can be done. Sure you can do it, but then again; my answer clearly shows that you shouldn't.
Verified the same behavior when targeting .NET Core 3.1.401 (compiling with Visual Studio 16.7.2).
If you want to act as the enumeration rolls you should yield each item.
public static class EnumerableExtensions
{
public static IEnumerable<T> ForEach<T>(this IEnumerable<T> enumeration, Action<T> action)
{
foreach (var item in enumeration)
{
action(item);
yield return item;
}
}
}
There is an experimental release by Microsoft of Interactive Extensions to LINQ (also on NuGet, see RxTeams's profile for more links). The Channel 9 video explains it well.
Its docs are only provided in XML format. I have run this documentation in Sandcastle to allow it to be in a more readable format. Unzip the docs archive and look for index.html.
Among many other goodies, it provides the expected ForEach implementation. It allows you to write code like this:
int[] numbers = { 1, 2, 3, 4, 5, 6, 7, 8 };
numbers.ForEach(x => Console.WriteLine(x*x));
According to PLINQ (available since .Net 4.0), you can do an
IEnumerable<T>.AsParallel().ForAll()
to do a parallel foreach loop on an IEnumerable.
The purpose of ForEach is to cause side effects.
IEnumerable is for lazy enumeration of a set.
This conceptual difference is quite visible when you consider it.
SomeEnumerable.ForEach(item=>DataStore.Synchronize(item));
This wont execute until you do a "count" or a "ToList()" or something on it.
It clearly is not what is expressed.
You should use the IEnumerable extensions for setting up chains of iteration, definining content by their respective sources and conditions. Expression Trees are powerful and efficient, but you should learn to appreciate their nature. And not just for programming around them to save a few characters overriding lazy evaluation.
Many people mentioned it, but I had to write it down. Isn't this most clear/most readable?
IEnumerable<Item> items = GetItems();
foreach (var item in items) item.DoStuff();
Short and simple(st).
Now we have the option of...
ParallelOptions parallelOptions = new ParallelOptions();
parallelOptions.MaxDegreeOfParallelism = 4;
#if DEBUG
parallelOptions.MaxDegreeOfParallelism = 1;
#endif
Parallel.ForEach(bookIdList, parallelOptions, bookID => UpdateStockCount(bookID));
Of course, this opens up a whole new can of threadworms.
ps (Sorry about the fonts, it's what the system decided)
As numerous answers already point out, you can easily add such an extension method yourself. However, if you don't want to do that, although I'm not aware of anything like this in the BCL, there's still an option in the System namespace, if you already have a reference to Reactive Extension (and if you don't, you should have):
using System.Reactive.Linq;
items.ToObservable().Subscribe(i => i.DoStuff());
Although the method names are a bit different, the end result is exactly what you're looking for.
ForEach can also be Chained, just put back to the pileline after the action. remain fluent
Employees.ForEach(e=>e.Act_A)
.ForEach(e=>e.Act_B)
.ForEach(e=>e.Act_C);
Orders //just for demo
.ForEach(o=> o.EmailBuyer() )
.ForEach(o=> o.ProcessBilling() )
.ForEach(o=> o.ProcessShipping());
//conditional
Employees
.ForEach(e=> { if(e.Salary<1000) e.Raise(0.10);})
.ForEach(e=> { if(e.Age >70 ) e.Retire();});
An Eager version of implementation.
public static IEnumerable<T> ForEach<T>(this IEnumerable<T> enu, Action<T> action)
{
foreach (T item in enu) action(item);
return enu; // make action Chainable/Fluent
}
Edit: a Lazy version is using yield return, like this.
public static IEnumerable<T> ForEachLazy<T>(this IEnumerable<T> enu, Action<T> action)
{
foreach (var item in enu)
{
action(item);
yield return item;
}
}
The Lazy version NEEDs to be materialized, ToList() for example, otherwise, nothing happens. see below great comments from ToolmakerSteve.
IQueryable<Product> query = Products.Where(...);
query.ForEachLazy(t => t.Price = t.Price + 1.00)
.ToList(); //without this line, below SubmitChanges() does nothing.
SubmitChanges();
I keep both ForEach() and ForEachLazy() in my library.
Inspired by Jon Skeet, I have extended his solution with the following:
Extension Method:
public static void Execute<TSource, TKey>(this IEnumerable<TSource> source, Action<TKey> applyBehavior, Func<TSource, TKey> keySelector)
{
foreach (var item in source)
{
var target = keySelector(item);
applyBehavior(target);
}
}
Client:
var jobs = new List<Job>()
{
new Job { Id = "XAML Developer" },
new Job { Id = "Assassin" },
new Job { Id = "Narco Trafficker" }
};
jobs.Execute(ApplyFilter, j => j.Id);
.
.
.
public void ApplyFilter(string filterId)
{
Debug.WriteLine(filterId);
}
This "functional approach" abstraction leaks big time. Nothing on the language level prevents side effects. As long as you can make it call your lambda/delegate for every element in the container - you will get the "ForEach" behavior.
Here for example one way of merging srcDictionary into destDictionary (if key already exists - overwrites)
this is a hack, and should not be used in any production code.
var b = srcDictionary.Select(
x=>
{
destDictionary[x.Key] = x.Value;
return true;
}
).Count();
MoreLinq has IEnumerable<T>.ForEach and a ton of other useful extensions. It's probably not worth taking the dependency just for ForEach, but there's a lot of useful stuff in there.
https://www.nuget.org/packages/morelinq/
https://github.com/morelinq/MoreLINQ
I respectually disagree with the notion that link extension methods should be side-effect free (not only because they aren't, any delegate can perform side effects).
Consider the following:
public class Element {}
public Enum ProcessType
{
This = 0, That = 1, SomethingElse = 2
}
public class Class1
{
private Dictionary<ProcessType, Action<Element>> actions =
new Dictionary<ProcessType,Action<Element>>();
public Class1()
{
actions.Add( ProcessType.This, DoThis );
actions.Add( ProcessType.That, DoThat );
actions.Add( ProcessType.SomethingElse, DoSomethingElse );
}
// Element actions:
// This example defines 3 distict actions
// that can be applied to individual elements,
// But for the sake of the argument, make
// no assumption about how many distict
// actions there may, and that there could
// possibly be many more.
public void DoThis( Element element )
{
// Do something to element
}
public void DoThat( Element element )
{
// Do something to element
}
public void DoSomethingElse( Element element )
{
// Do something to element
}
public void Apply( ProcessType processType, IEnumerable<Element> elements )
{
Action<Element> action = null;
if( ! actions.TryGetValue( processType, out action ) )
throw new ArgumentException("processType");
foreach( element in elements )
action(element);
}
}
What the example shows is really just a kind of late-binding that allows one invoke one of many possible actions having side-effects on a sequence of elements, without having to write a big switch construct to decode the value that defines the action and translate it into its corresponding method.
To stay fluent one can use such a trick:
GetItems()
.Select(i => new Action(i.DoStuf)))
.Aggregate((a, b) => a + b)
.Invoke();
For VB.NET you should use:
listVariable.ForEach(Sub(i) i.Property = "Value")
Yet another ForEach Example
public static IList<AddressEntry> MapToDomain(IList<AddressModel> addresses)
{
var workingAddresses = new List<AddressEntry>();
addresses.Select(a => a).ToList().ForEach(a => workingAddresses.Add(AddressModelMapper.MapToDomain(a)));
return workingAddresses;
}