Getting InvalidOperationException when modifying the collection while loopng through it

Question

I have a collection lets say list of integers and looping through it using foreach, but if I have to insert data into the list during the loop, how do I do that ? I'm getting

Unhandled Exception: System.InvalidOperationException: Collection was modified enumeration operation may not execute.

This is the sample code below. I thought AsReadonly() would return a collection that was prior to modified state. That is not the case here.

ints = new List<int>(30);
ints.AddRange(Enumerable.Range(1, 10));
int y = 11;

foreach(int x in ints.AsReadOnly())
{
    ints.Add(y++);
    Console.WriteLine(x);
}

out of curiosity? what does the snipped above do? I am asking because it is not even possible to suggest you an alternative approach without understanding what is your real use case. — Davide Piras, Sep 11 '11 at 15:54
possible duplicate of [How to modify or delete items from an enumerable collection while iterating through it in C#](http://stackoverflow.com/questions/308466/how-to-modify-or-delete-items-from-an-enumerable-collection-while-iterating-throu) — adrianbanks, Sep 11 '11 at 16:07

score 2 · Answer 1 · answered Sep 11 '11 at 16:24

You need to use a copy of the list that can't be modified. This works:

foreach(int x in ints.ToArray())
{
    ints.Add(y++);
    Console.WriteLine(x);
}

The copy is unpleasant of course. In this particular case, since you are only adding elements, you can avoid the copy by using a for() loop:

int count = ints.Count;
for (int ix = 0; ix < count; ++ix) {
    ints.Add(y++);
    Console.WriteLine(ints[ix]);
}

BrokenGlass · Answer 2 · 2011-09-11T16:22:56.420

1

As the documentation states AsReadonly() is only a wrapper on the original collection so you are still operating on the same collection - the same rules apply.

A collection that is read-only is simply a collection with a wrapper that prevents modifying the collection; therefore, if changes are made to the underlying collection, the read-only collection reflects those changes.

edited Sep 11 '11 at 16:22

answered Sep 11 '11 at 16:02

BrokenGlass

158,293
28
286
335

It's not a decorator - more likely a [facade](http://en.wikipedia.org/wiki/Facade_pattern). – Jonathan Dickinson Sep 11 '11 at 16:12
@Jonathan: yes, you are right - decorator doesn't apply since the interface is actually different (since it can't allow modification) – BrokenGlass Sep 11 '11 at 16:22

score 1 · Accepted Answer · answered Sep 11 '11 at 16:09

As others mentioned AsReadonly() only produces a wrapper around the original list.

Something like this would work. But I am not sure what you are trying to do so it might not be helpful.

var ints = new List<int>();
ints.AddRange(Enumerable.Range(1, 10));
int y = 11;

var moreInts = new List<int>();

foreach(int x in ints.AsReadOnly())
{
    moreInts.Add(y++);
    Console.WriteLine(x);
}

ints.AddRange(moreInts);

score 0 · Answer 4 · answered Sep 11 '11 at 15:52

0

You cant modify a collection while inside foreach loop. Try to use for-loop instead.

answered Sep 11 '11 at 15:52

You can't modify the collection you are iterating on. But you can modify another collection, which makes the `ToArray()` approach Hans mentioned in a comment possible. – svick Sep 11 '11 at 16:26

score 0 · Answer 5 · answered Sep 11 '11 at 16:02

0

The AsReadOnlyMethod() returns a wrapper list that prohibits modification, it is still the same list.

answered Sep 11 '11 at 16:02

Mark Menchavez

1,651
12
15

Getting InvalidOperationException when modifying the collection while loopng through it

5 Answers5