Question

If I use:

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

the Add in the foreach throws an InvalidOperationException (Collection was modified; enumeration operation may not execute), which I consider logical, since we are pulling the rug from under our feet.

However, if I use:

var strings = new List<string> { "sample" };
strings.ForEach(s =>
  {
    Console.WriteLine(s);
    strings.Add(s + "!");
  });

it promptly shoots itself in the foot by looping until it throws an OutOfMemoryException.

This comes as a suprise to me, as I always thought that List.ForEach was either just a wrapper for foreach or for for.
Does anyone have an explanation for the how and the why of this behavior?

(Inpired by ForEach loop for a Generic List repeated endlessly)

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.