How to remove elements from a generic list while iterating over it?

Question

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?

Answer 1

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));

Answer 2

 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.

Answer 3

A simple and straightforward solution:

Use a standard for-loop running backwards on your collection and RemoveAt(i) to remove elements.

Answer 4

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);
    }
}

Answer 5

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);
        }

Answer 6

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.

Answer 7

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)
        }

     }

Answer 8

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.

Answer 9

As any remove is taken on a condition you can use

list.RemoveAll(item => item.Value == someValue);

Answer 10

List<T> TheList = new List<T>();

TheList.FindAll(element => element.Satisfies(Condition)).ForEach(element => TheList.Remove(element));

Answer 11

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.)

Answer 12

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();

Answer 13

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);

Answer 14

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;
});

Answer 15

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.

Answer 16

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++;
        }

Answer 17

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.

Answer 18

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.

Answer 19

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.

Answer 20

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
}

Answer 21

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.

Answer 22

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);
});

Answer 23

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

Answer 24

I found myself in a similar situation where I had to remove every n^th 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
}

Answer 25

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.

Answer 26

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);
    }
}

Answer 27

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);

Answer 28

myList.RemoveAt(i--);

simples;