Intelligent way of removing items from a List while enumerating in C#

Question

I have the classic case of trying to remove an item from a collection while enumerating it in a loop:

List myIntCollection = new List()         


        

Answer 1

How about

int[] tmp = new int[myIntCollection.Count ()];
myIntCollection.CopyTo(tmp);
foreach(int i in tmp)
{
    myIntCollection.Remove(42); //The error is no longer here.
}

Answer 2

If you're interested in high performance, you can use two lists. The following minimises garbage collection, maximises memory locality and never actually removes an item from a list, which is very inefficient if it's not the last item.

private void RemoveItems()
{
    _newList.Clear();

    foreach (var item in _list)
    {
        item.Process();
        if (!item.NeedsRemoving())
            _newList.Add(item);
    }

    var swap = _list;
    _list = _newList;
    _newList = swap;
}

Answer 3

The best solution is usually to use the RemoveAll() method:

myList.RemoveAll(x => x.SomeProp == "SomeValue");

Or, if you need certain elements removed:

MyListType[] elems = new[] { elem1, elem2 };
myList.RemoveAll(x => elems.Contains(x));

This assume that your loop is solely intended for removal purposes, of course. If you do need to additional processing, then the best method is usually to use a for or while loop, since then you're not using an enumerator:

for (int i = myList.Count - 1; i >= 0; i--)
{
    // Do processing here, then...
    if (shouldRemoveCondition)
    {
        myList.RemoveAt(i);
    }
}

Going backwards ensures that you don't skip any elements.

Response to Edit:

If you're going to have seemingly arbitrary elements removed, the easiest method might be to just keep track of the elements you want to remove, and then remove them all at once after. Something like this:

List<int> toRemove = new List<int>();
foreach (var elem in myList)
{
    // Do some stuff

    // Check for removal
    if (needToRemoveAnElement)
    {
        toRemove.Add(elem);
    }
}

// Remove everything here
myList.RemoveAll(x => toRemove.Contains(x));

Answer 4

Just figured I'll share my solution to a similar problem where i needed to remove items from a list while processing them.

So basically "foreach" that will remove the item from the list after it has been iterated.

My test:

var list = new List<TempLoopDto>();
list.Add(new TempLoopDto("Test1"));
list.Add(new TempLoopDto("Test2"));
list.Add(new TempLoopDto("Test3"));
list.Add(new TempLoopDto("Test4"));

list.PopForEach((item) =>
{
    Console.WriteLine($"Process {item.Name}");
});

Assert.That(list.Count, Is.EqualTo(0));

I solved this with a extension method "PopForEach" that will perform a action and then remove the item from the list.

public static class ListExtensions
{
    public static void PopForEach<T>(this List<T> list, Action<T> action)
    {
        var index = 0;
        while (index < list.Count) {
            action(list[index]);
            list.RemoveAt(index);
        }
    }
}

Hope this can be helpful to any one.