Why does List.ForEach allow its list to be modified?

Question

If I use:

var strings = new List { \"sample\" };
foreach (string s in strings)
{
  Console.WriteLine(s);
  strings.Add(s + \"!\");
}

Answer 1

It's because the ForEach method doesn't use the enumerator, it loops through the items with a for loop:

public void ForEach(Action<T> action)
{
    if (action == null)
    {
        ThrowHelper.ThrowArgumentNullException(ExceptionArgument.match);
    }
    for (int i = 0; i < this._size; i++)
    {
        action(this._items[i]);
    }
}

(code obtained with JustDecompile)

Since the enumerator is not used, it never checks if the list has changed, and the end condition of the for loop is never reached because _size is increased at every iteration.

Answer 2

List<T>.ForEach is implemented through for inside, so it does not use enumerator and it allows to modify the collection.

Answer 3

Because the ForEach attached to the List class internally uses a for loop that is directly attached to its internal members -- which you can see by downloading the source code for the .NET framework.

http://referencesource.microsoft.com/netframework.aspx

Where as a foreach loop is first and foremost a compiler optimization but also must operate against the collection as an observer -- so if the collection is modified it throws an exception.

Answer 4

We know about this issue, it was an oversight when it was originally written. Unfortunately, we can't change it because it would now prevent this previously working code from running:

        var list = new List<string>();
        list.Add("Foo");
        list.Add("Bar");

        list.ForEach((item) => 
        { 
            if(item=="Foo") 
                list.Remove(item); 
        });

The usefulness of this method itself is questionable as Eric Lippert pointed out, so we didn't include it for .NET for Metro style apps (ie Windows 8 apps).

David Kean (BCL Team)