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

To illustrate Nippysaurus's answer: If you are going to add the new items to the list and want to process the newly added items too during the same enumeration then you can just use for loop instead of foreach loop, problem solved :)

var list = new List();
... populate the list ...

//foreach (var entryToProcess in list)
for (int i = 0; i < list.Count; i++)
{
    var entryToProcess = list[i];

    var resultOfProcessing = DoStuffToEntry(entryToProcess);

    if (... condition ...)
        list.Add(new YourData(...));
}

For runnable example:

void Main()
{
    var list = new List();
    for (int i = 0; i < 10; i++)
        list.Add(i);

    //foreach (var entry in list)
    for (int i = 0; i < list.Count; i++)
    {
        var entry = list[i];
        if (entry % 2 == 0)
            list.Add(entry + 1);

        Console.Write(entry + ", ");
    }

    Console.Write(list);
}

Output of last example:

0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 1, 3, 5, 7, 9,

List (15 items)
0
1
2
3
4
5
6
7
8
9
1
3
5
7
9