What is the best way to modify a list in a 'foreach' loop?

Question

A new feature in C# / .NET 4.0 is that you can change your enumerable in a foreach without getting the exception. See Paul Jackson\'s blog entry An Interest

Answer 1

Make a copy of the enumeration, using an IEnumerable extension method in this case, and enumerate over it. This would add a copy of every element in every inner enumerable to that enumeration.

foreach(var item in Enumerable)
{
    foreach(var item2 in item.Enumerable.ToList())
    {
        item.Add(item2)
    }
}