I am looking for a better pattern for working with a list of elements which each need processed and then depending on the outcome are removed from the list.
You can't use .Remove(element) inside a foreach (var element in X) (because it results in Collection was modified; enumeration operation may not execute. exception)... you also can't use for (int i = 0; i < elements.Count(); i++) and .RemoveAt(i) because it disrupts your current position in the collection relative to i.
Is there an elegant way to do this?
Iterate your list in reverse with a for loop:
for (int i = safePendingList.Count - 1; i >= 0; i--)
{
// some code
// safePendingList.RemoveAt(i);
}
Example:
var list = new List<int>(Enumerable.Range(1, 10));
for (int i = list.Count - 1; i >= 0; i--)
{
if (list[i] > 5)
list.RemoveAt(i);
}
list.ForEach(i => Console.WriteLine(i));
Alternately, you can use the RemoveAll method with a predicate to test against:
safePendingList.RemoveAll(item => item.Value == someValue);
Here's a simplified example to demonstrate:
var list = new List<int>(Enumerable.Range(1, 10));
Console.WriteLine("Before:");
list.ForEach(i => Console.WriteLine(i));
list.RemoveAll(i => i > 5);
Console.WriteLine("After:");
list.ForEach(i => Console.WriteLine(i));
foreach (var item in list.ToList()) {
list.Remove(item);
}
If you add ".ToList()" to your list (or the results of a LINQ query), you can remove "item" directly from "list" without the dreaded "Collection was modified; enumeration operation may not execute." error. The compiler makes a copy of "list", so that you can safely do the remove on the array.
While this pattern is not super efficient, it has a natural feel and is flexible enough for almost any situation. Such as when you want to save each "item" to a DB and remove it from the list only when the DB save succeeds.
A simple and straightforward solution:
Use a standard for-loop running backwards on your collection and RemoveAt(i) to remove elements.
Reverse iteration should be the first thing to come to mind when you want to remove elements from a Collection while iterating over it.
Luckily, there is a more elegant solution than writing a for loop which involves needless typing and can be error prone.
ICollection<int> test = new List<int>(new int[] {1, 2, 3, 4, 5, 6, 7, 8, 9, 10});
foreach (int myInt in test.Reverse<int>())
{
if (myInt % 2 == 0)
{
test.Remove(myInt);
}
}
Using the ToArray() on a generic list allows you to do a Remove(item) on your generic List:
List<String> strings = new List<string>() { "a", "b", "c", "d" };
foreach (string s in strings.ToArray())
{
if (s == "b")
strings.Remove(s);
}
Select the elements you do want rather than trying to remove the elements you don't want. This is so much easier (and generally more efficient too) than removing elements.
var newSequence = (from el in list
where el.Something || el.AnotherThing < 0
select el);
I wanted to post this as a comment in response to the comment left by Michael Dillon below, but it's too long and probably useful to have in my answer anyway:
Personally, I'd never remove items one-by-one, if you do need removal, then call RemoveAll which takes a predicate and only rearranges the internal array once, whereas Remove does an Array.Copy operation for every element you remove. RemoveAll is vastly more efficient.
And when you're backwards iterating over a list, you already have the index of the element you want to remove, so it would be far more efficient to call RemoveAt, because Remove first does a traversal of the list to find the index of the element you're trying to remove, but you already know that index.
So all in all, I don't see any reason to ever call Remove in a for-loop. And ideally, if it is at all possible, use the above code to stream elements from the list as needed so no second data structure has to be created at all.
Using .ToList() will make a copy of your list, as explained in this question:
ToList()-- Does it Create a New List?
By using ToList(), you can remove from your original list, because you're actually iterating over a copy.
foreach (var item in listTracked.ToList()) {
if (DetermineIfRequiresRemoval(item)) {
listTracked.Remove(item)
}
}
If the function that determines which items to delete has no side effects and doesn't mutate the item (it's a pure function), a simple and efficient (linear time) solution is:
list.RemoveAll(condition);
If there are side effects, I'd use something like:
var toRemove = new HashSet<T>();
foreach(var item in items)
{
...
if(condition)
toRemove.Add(item);
}
items.RemoveAll(toRemove.Contains);
This is still linear time, assuming the hash is good. But it has an increased memory use due to the hashset.
Finally if your list is only an IList<T> instead of a List<T> I suggest my answer to How can I do this special foreach iterator?. This will have linear runtime given typical implementations of IList<T>, compared with quadratic runtime of many other answers.
As any remove is taken on a condition you can use
list.RemoveAll(item => item.Value == someValue);
List<T> TheList = new List<T>();
TheList.FindAll(element => element.Satisfies(Condition)).ForEach(element => TheList.Remove(element));
You can't use foreach, but you could iterate forwards and manage your loop index variable when you remove an item, like so:
for (int i = 0; i < elements.Count; i++)
{
if (<condition>)
{
// Decrement the loop counter to iterate this index again, since later elements will get moved down during the remove operation.
elements.RemoveAt(i--);
}
}
Note that in general all of these techniques rely on the behaviour of the collection being iterated. The technique shown here will work with the standard List(T). (It is quite possible to write your own collection class and iterator that does allow item removal during a foreach loop.)
For loops are a bad construct for this.
Using while
var numbers = new List<int>(Enumerable.Range(1, 3));
while (numbers.Count > 0)
{
numbers.RemoveAt(0);
}
But, if you absolutely must use for
var numbers = new List<int>(Enumerable.Range(1, 3));
for (; numbers.Count > 0;)
{
numbers.RemoveAt(0);
}
Or, this:
public static class Extensions
{
public static IList<T> Remove<T>(
this IList<T> numbers,
Func<T, bool> predicate)
{
numbers.ForEachBackwards(predicate, (n, index) => numbers.RemoveAt(index));
return numbers;
}
public static void ForEachBackwards<T>(
this IList<T> numbers,
Func<T, bool> predicate,
Action<T, int> action)
{
for (var i = numbers.Count - 1; i >= 0; i--)
{
if (predicate(numbers[i]))
{
action(numbers[i], i);
}
}
}
}
Usage:
var numbers = new List<int>(Enumerable.Range(1, 10)).Remove((n) => n > 5);
However, LINQ already has RemoveAll() to do this
var numbers = new List<int>(Enumerable.Range(1, 10));
numbers.RemoveAll((n) => n > 5);
Lastly, you are probably better off using LINQ's Where() to filter and create a new list instead of mutating the existing list. Immutability is usually good.
var numbers = new List<int>(Enumerable.Range(1, 10))
.Where((n) => n <= 5)
.ToList();
Using Remove or RemoveAt on a list while iterating over that list has intentionally been made difficult, because it is almost always the wrong thing to do. You might be able to get it working with some clever trick, but it would be extremely slow. Every time you call Remove it has to scan through the entire list to find the element you want to remove. Every time you call RemoveAt it has to move subsequent elements 1 position to the left. As such, any solution using Remove or RemoveAt, would require quadratic time, O(n²).
Use RemoveAll if you can. Otherwise, the following pattern will filter the list in-place in linear time, O(n).
// Create a list to be filtered
IList<int> elements = new List<int>(new int[] {1, 2, 3, 4, 5, 6, 7, 8, 9, 10});
// Filter the list
int kept = 0;
for (int i = 0; i < elements.Count; i++) {
// Test whether this is an element that we want to keep.
if (elements[i] % 3 > 0) {
// Add it to the list of kept elements.
elements[kept] = elements[i];
kept++;
}
}
// Unfortunately IList has no Resize method. So instead we
// remove the last element of the list until: elements.Count == kept.
while (kept < elements.Count) elements.RemoveAt(elements.Count-1);
I would reassign the list from a LINQ query that filtered out the elements you didn't want to keep.
list = list.Where(item => ...).ToList();
Unless the list is very large there should be no significant performance problems in doing this.
The best way to remove items from a list while iterating over it is to use RemoveAll(). But the main concern written by people is that they have to do some complex things inside the loop and/or have complex compare cases.
The solution is to still use RemoveAll() but use this notation:
var list = new List<int>(Enumerable.Range(1, 10));
list.RemoveAll(item =>
{
// Do some complex operations here
// Or even some operations on the items
SomeFunction(item);
// In the end return true if the item is to be removed. False otherwise
return item > 5;
});
By assuming that predicate is a Boolean property of an element, that if it is true, then the element should be removed:
int i = 0;
while (i < list.Count())
{
if (list[i].predicate == true)
{
list.RemoveAt(i);
continue;
}
i++;
}
In C# one easy way is to mark the ones you wish to delete then create a new list to iterate over...
foreach(var item in list.ToList()){if(item.Delete) list.Remove(item);}
or even simpler use linq....
list.RemoveAll(p=>p.Delete);
but it is worth considering if other tasks or threads will have access to the same list at the same time you are busy removing, and maybe use a ConcurrentList instead.
I wish the "pattern" was something like this:
foreach( thing in thingpile )
{
if( /* condition#1 */ )
{
foreach.markfordeleting( thing );
}
elseif( /* condition#2 */ )
{
foreach.markforkeeping( thing );
}
}
foreachcompleted
{
// then the programmer's choices would be:
// delete everything that was marked for deleting
foreach.deletenow(thingpile);
// ...or... keep only things that were marked for keeping
foreach.keepnow(thingpile);
// ...or even... make a new list of the unmarked items
others = foreach.unmarked(thingpile);
}
This would align the code with the process that goes on in the programmer's brain.
foreach(var item in list.ToList())
{
if(item.Delete) list.Remove(item);
}
Simply create an entirely new list from the first one. I say "Easy" rather than "Right" as creating an entirely new list probably comes at a performance premium over the previous method (I haven't bothered with any benchmarking.) I generally prefer this pattern, it can also be useful in overcoming Linq-To-Entities limitations.
for(i = list.Count()-1;i>=0;i--)
{
item=list[i];
if (item.Delete) list.Remove(item);
}
This way cycles through the list backwards with a plain old For loop. Doing this forwards could be problematic if the size of the collection changes, but backwards should always be safe.
Just wanted to add my 2 cents to this in case this helps anyone, I had a similar problem but needed to remove multiple elements from an array list while it was being iterated over. the highest upvoted answer did it for me for the most part until I ran into errors and realized that the index was greater than the size of the array list in some instances because multiple elements were being removed but the index of the loop didn't keep track of that. I fixed this with a simple check:
ArrayList place_holder = new ArrayList();
place_holder.Add("1");
place_holder.Add("2");
place_holder.Add("3");
place_holder.Add("4");
for(int i = place_holder.Count-1; i>= 0; i--){
if(i>= place_holder.Count){
i = place_holder.Count-1;
}
// some method that removes multiple elements here
}
There is an option that hasn't been mentioned here.
If you don't mind adding a bit of code somewhere in your project, you can add and extension to List to return an instance of a class that does iterate through the list in reverse.
You would use it like this :
foreach (var elem in list.AsReverse())
{
//Do stuff with elem
//list.Remove(elem); //Delete it if you want
}
And here is what the extension looks like:
public static class ReverseListExtension
{
public static ReverseList<T> AsReverse<T>(this List<T> list) => new ReverseList<T>(list);
public class ReverseList<T> : IEnumerable
{
List<T> list;
public ReverseList(List<T> list){ this.list = list; }
public IEnumerator GetEnumerator()
{
for (int i = list.Count - 1; i >= 0; i--)
yield return list[i];
yield break;
}
}
}
This is basically list.Reverse() without the allocation.
Like some have mentioned you still get the drawback of deleting elements one by one, and if your list is massively long some of the options here are better. But I think there is a world where someone would want the simplicity of list.Reverse(), without the memory overhead.
Copy the list you are iterating. Then remove from the copy and interate the original. Going backwards is confusing and doesn't work well when looping in parallel.
var ids = new List<int> { 1, 2, 3, 4 };
var iterableIds = ids.ToList();
Parallel.ForEach(iterableIds, id =>
{
ids.Remove(id);
});
I would do like this
using System.IO;
using System;
using System.Collections.Generic;
class Author
{
public string Firstname;
public string Lastname;
public int no;
}
class Program
{
private static bool isEven(int i)
{
return ((i % 2) == 0);
}
static void Main()
{
var authorsList = new List<Author>()
{
new Author{ Firstname = "Bob", Lastname = "Smith", no = 2 },
new Author{ Firstname = "Fred", Lastname = "Jones", no = 3 },
new Author{ Firstname = "Brian", Lastname = "Brains", no = 4 },
new Author{ Firstname = "Billy", Lastname = "TheKid", no = 1 }
};
authorsList.RemoveAll(item => isEven(item.no));
foreach(var auth in authorsList)
{
Console.WriteLine(auth.Firstname + " " + auth.Lastname);
}
}
}
OUTPUT
Fred Jones
Billy TheKid
I found myself in a similar situation where I had to remove every nth element in a given List<T>.
for (int i = 0, j = 0, n = 3; i < list.Count; i++)
{
if ((j + 1) % n == 0) //Check current iteration is at the nth interval
{
list.RemoveAt(i);
j++; //This extra addition is necessary. Without it j will wrap
//down to zero, which will throw off our index.
}
j++; //This will always advance the j counter
}
The cost of removing an item from the list is proportional to the number of items following the one to be removed. In the case where the first half of the items qualify for removal, any approach which is based upon removing items individually will end up having to perform about N*N/4 item-copy operations, which can get very expensive if the list is large.
A faster approach is to scan through the list to find the first item to be removed (if any), and then from that point forward copy each item which should be retained to the spot where it belongs. Once this is done, if R items should be retained, the first R items in the list will be those R items, and all of the items requiring deletion will be at the end. If those items are deleted in reverse order, the system won't end up having to copy any of them, so if the list had N items of which R items, including all of the first F, were retained,
it will be necessary to copy R-F items, and shrink the list by one item N-R times. All linear time.
My approach is that I first create a list of indices, which should get deleted. Afterwards I loop over the indices and remove the items from the initial list. This looks like this:
var messageList = ...;
// Restrict your list to certain criteria
var customMessageList = messageList.FindAll(m => m.UserId == someId);
if (customMessageList != null && customMessageList.Count > 0)
{
// Create list with positions in origin list
List<int> positionList = new List<int>();
foreach (var message in customMessageList)
{
var position = messageList.FindIndex(m => m.MessageId == message.MessageId);
if (position != -1)
positionList.Add(position);
}
// To be able to remove the items in the origin list, we do it backwards
// so that the order of indices stays the same
positionList = positionList.OrderByDescending(p => p).ToList();
foreach (var position in positionList)
{
messageList.RemoveAt(position);
}
}
Trace the elements to be removed with a property, and remove them all after process.
using System.Linq;
List<MyProperty> _Group = new List<MyProperty>();
// ... add elements
bool cond = false;
foreach (MyProperty currObj in _Group)
{
// here it is supposed that you decide the "remove conditions"...
cond = true; // set true or false...
if (cond)
{
// SET - element can be deleted
currObj.REMOVE_ME = true;
}
}
// RESET
_Group.RemoveAll(r => r.REMOVE_ME);
myList.RemoveAt(i--);
simples;
For every single LINQ query I've written, I've always used a foreach loop to go through the result. Now, I have a program where I want to get the first 50 rows of the result, do some calculations with those rows, then get the next 50 rows of the result etc.
What is the good standards way to do this using LINQ and C#?
.Skip().Take() is going to be the best method (as far as clarity goes) in this case. For example:
var results = db.Table.Select(); // Your query here
int rowCount = results.Count(); // Count rows once and store
for(int i = 0; i <= rowCount; i += 50)
{
var paged = results.Skip(i).Take(50);
// do the calculations with the groups of 50 here
}
It is worth noting that even though this seems more clear (you're taking groups of 50 from the results), each time you call Skip() and Take() there's a chance that the collection has to be enumerated over again...which is actually less efficient than simply using foreach to enumerate over the results a single time.
You could use a group by ie:
data.Select((item, index) => new {item, index}).GroupBy(g => g.index / 50)
then you'd do your operation on each group.
similar to this: How to use Linq to group every N number of rows
Without knowing anything about the nature of the data and/or the calculations, the "good standards" way to do this would probably be a foreach loop!
It would help if you could provide some information about the nature of the data and the calculations that you want to perform. Depending on that, the best answer might be foreach, Skip/Take, GroupBy or maybe something else altogether.
Using .Skip(...).Take(...) will work for paging. But something to consider is that again an IEnumerable<...> this will recalculate the enumerable each time it is processed. Is this is something that is consumed instead of being resettable you could have issues. This method set can fix this problem.
public static class EnumerableEx
{
public static IEnumerable<IEnumerable<T>> AsPages<T>(
this IEnumerable<T> set, int pageLength)
{
using (var e = set.GetEnumerator())
while (true)
yield return GetPage(e, pageLength);
}
private static IEnumerable<T> GetPage<T>(
IEnumerator<T> set, int pageLength)
{
for (var position = 0;
position < pageLength && set.MoveNext();
position++)
yield return set.Current;
}
}
... Here is a usage example of why it is important.
class Program
{
private static int _last = 0;
private static IEnumerable<int> GetValues()
{
while (true)
yield return _last++;
}
static void Main(string[] args)
{
for (var i = 0; i < 3; i++)
foreach (var value in GetValues().Skip(5 * i).Take(5))
Console.Write(value + ",");
// 0,1,2,3,4,10,11,12,13,14,25,26,27,28,29,
Console.WriteLine();
_last = 0;
foreach (var page in GetValues().AsPages(5).Take(3))
foreach (var value in page)
Console.Write(value + ",");
// 0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,
}
}
Something to this effect
var page = (From c In db.Customers Order By c.ContactName, c.City Descending
Select c).Skip(50).Take(50)
A new feature in C# / .NET 4.0 is that you can change your enumerable in a foreach without getting the exception. See Paul Jackson's blog entry An Interesting Side-Effect of Concurrency: Removing Items from a Collection While Enumerating for information on this change.
What is the best way to do the following?
foreach(var item in Enumerable)
{
foreach(var item2 in item.Enumerable)
{
item.Add(new item2)
}
}
Usually I use an IList as a cache/buffer until the end of the foreach, but is there better way?
The collection used in foreach is immutable. This is very much by design.
As it says on MSDN:
The foreach statement is used to
iterate through the collection to get
the information that you want, but can
not be used to add or remove items
from the source collection to avoid
unpredictable side effects. If you
need to add or remove items from the
source collection, use a for loop.
The post in the link provided by Poko indicates that this is allowed in the new concurrent collections.
Make a copy of the enumeration, using an IEnumerable extension method in this case, and enumerate over it. This would add a copy of every element in every inner enumerable to that enumeration.
foreach(var item in Enumerable)
{
foreach(var item2 in item.Enumerable.ToList())
{
item.Add(item2)
}
}
To illustrate Nippysaurus's answer: If you are going to add the new items to the list and want to process the newly added items too during the same enumeration then you can just use for loop instead of foreach loop, problem solved :)
var list = new List<YourData>();
... populate the list ...
//foreach (var entryToProcess in list)
for (int i = 0; i < list.Count; i++)
{
var entryToProcess = list[i];
var resultOfProcessing = DoStuffToEntry(entryToProcess);
if (... condition ...)
list.Add(new YourData(...));
}
For runnable example:
void Main()
{
var list = new List<int>();
for (int i = 0; i < 10; i++)
list.Add(i);
//foreach (var entry in list)
for (int i = 0; i < list.Count; i++)
{
var entry = list[i];
if (entry % 2 == 0)
list.Add(entry + 1);
Console.Write(entry + ", ");
}
Console.Write(list);
}
Output of last example:
0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 1, 3, 5, 7, 9,
List (15 items)
0
1
2
3
4
5
6
7
8
9
1
3
5
7
9
As mentioned, but with a code sample:
foreach(var item in collection.ToArray())
collection.Add(new Item...);
You should really use for() instead of foreach() in this case.
You can't change the enumerable collection while it is being enumerated, so you will have to make your changes before or after enumerating.
The for loop is a nice alternative, but if your IEnumerable collection does not implement ICollection, it is not possible.
Either:
1) Copy collection first. Enumerate the copied collection and change the original collection during the enumeration. (#tvanfosson)
or
2) Keep a list of changes and commit them after the enumeration.
LINQ is very effective for juggling with collections.
Your types and structure are unclear to me, but I will try to fit your example to the best of my ability.
From your code it appears that, for each item, you are adding to that item everything from its own 'Enumerable' property. This is very simple:
foreach (var item in Enumerable)
{
item = item.AddRange(item.Enumerable));
}
As a more general example, let's say we want to iterate a collection and remove items where a certain condition is true. Avoiding foreach, using LINQ:
myCollection = myCollection.Where(item => item.ShouldBeKept);
Add an item based on each existing item? No problem:
myCollection = myCollection.Concat(myCollection.Select(item => new Item(item.SomeProp)));
Here's how you can do that (quick and dirty solution. If you really need this kind of behavior, you should either reconsider your design or override all IList<T> members and aggregate the source list):
using System;
using System.Collections.Generic;
namespace ConsoleApplication3
{
public class ModifiableList<T> : List<T>
{
private readonly IList<T> pendingAdditions = new List<T>();
private int activeEnumerators = 0;
public ModifiableList(IEnumerable<T> collection) : base(collection)
{
}
public ModifiableList()
{
}
public new void Add(T t)
{
if(activeEnumerators == 0)
base.Add(t);
else
pendingAdditions.Add(t);
}
public new IEnumerator<T> GetEnumerator()
{
++activeEnumerators;
foreach(T t in ((IList<T>)this))
yield return t;
--activeEnumerators;
AddRange(pendingAdditions);
pendingAdditions.Clear();
}
}
class Program
{
static void Main(string[] args)
{
ModifiableList<int> ints = new ModifiableList<int>(new int[] { 2, 4, 6, 8 });
foreach(int i in ints)
ints.Add(i * 2);
foreach(int i in ints)
Console.WriteLine(i * 2);
}
}
}
To add to Timo's answer LINQ can be used like this as well:
items = items.Select(i => {
...
//perform some logic adding / updating.
return i / return new Item();
...
//To remove an item simply have logic to return null.
//Then attach the Where to filter out nulls
return null;
...
}).Where(i => i != null);
The best approach from a performance perspective is probably to use a one or two arrays. Copy the list to an array, do operations on the array, and then build a new list from the array. Accessing an array element is faster than accessing a list item, and conversions between a List<T> and a T[] can use a fast "bulk copy" operation which avoids the overhead associated accessing individual items.
For example, suppose you have a List<string> and wish to have every string in the list which starts with T be followed by an item "Boo", while every string that starts with "U" is dropped entirely. An optimal approach would probably be something like:
int srcPtr,destPtr;
string[] arr;
srcPtr = theList.Count;
arr = new string[srcPtr*2];
theList.CopyTo(arr, theList.Count); // Copy into second half of the array
destPtr = 0;
for (; srcPtr < arr.Length; srcPtr++)
{
string st = arr[srcPtr];
char ch = (st ?? "!")[0]; // Get first character of string, or "!" if empty
if (ch != 'U')
arr[destPtr++] = st;
if (ch == 'T')
arr[destPtr++] = "Boo";
}
if (destPtr > arr.Length/2) // More than half of dest. array is used
{
theList = new List<String>(arr); // Adds extra elements
if (destPtr != arr.Length)
theList.RemoveRange(destPtr, arr.Length-destPtr); // Chop to proper length
}
else
{
Array.Resize(ref arr, destPtr);
theList = new List<String>(arr); // Adds extra elements
}
It would have been helpful if List<T> provided a method to construct a list from a portion of an array, but I'm unaware of any efficient method for doing so. Still, operations on arrays are pretty fast. Of note is the fact that adding and removing items from the list does not require "pushing" around other items; each item gets written directly to its appropriate spot in the array.
I have written one easy step, but because of this performance will be degraded
Here is my code snippet:-
for (int tempReg = 0; tempReg < reg.Matches(lines).Count; tempReg++)
{
foreach (Match match in reg.Matches(lines))
{
var aStringBuilder = new StringBuilder(lines);
aStringBuilder.Insert(startIndex, match.ToString().Replace(",", " ");
lines[k] = aStringBuilder.ToString();
tempReg = 0;
break;
}
}