What is the easiest way to foreach through a List<T> removing unwanted objects?

Question

In my application, _collection is a List from which I need to remove all User objects which do not match the criteria.

However, the following code gets an invalid operation error in its second iteration since the _collection itself has been changed:

foreach (User user in _collection)
{
    if (!user.IsApproved())
    {
        _collection.Remove(user);
    }
}

I could create another List collection and copy them back and forth but then I have the issue of non-cloned reference types, etc.

Is there a way to do the above more elegantly than copying _collection to another another List variable?

Answer 1

_collection.RemoveAll(user => !user.IsApproved());

If you're still on 2.0:

_collection.RemoveAll(delegate(User u) { return !u.IsApproved(); });

By the way, if you don't want to touch the original list, you can get another list of approved users with:

_collection.FindAll(user => user.IsApproved());

Answer 2

You can always start at the top index and iterate downward towards 0:

for (int i = _collection.Count - 1; i >= 0; i--)
{
    User user = _collection[i];
    if (!user.IsApproved())
    {
        _collection.RemoveAt(i);
    }
}

Mehrdad's answer looks pretty darn elegant, though.

Answer 3

Whenever there is a chance that a collection will be modified in a loop, opt for a for loop instead. The solution given by Mehrdad is lovely and definitely worth a try!

Here's code I find helpful when dealing with modifiable collections:

for(int index=0;index < _collection.Count; index++)
{
    if (!_collection[index].IsApproved)
    {
        _collection.RemoveAt(index);
        index--;
    }
}