Question

I need to iterate over a list (or whatever enumeration), but I’d like to add values into the list in the course of the iteration.

This is an example.

public static void RunSnippet()
{
    List<int> hello = new List<int>();

    hello.Add(1); hello.Add(2); hello.Add(3);

    foreach (var x in hello)
    {
        Console.WriteLine(x);
        if (x == 1) {
            hello.Add(100);
        }
    }
}

I expect to get “1,2,3,100”, but instead I got this error.

How can I iterate over a list that is changing in the process?

ADDED

What I want to accomplish is that I iterate over elements to process something. The thing is that some of the elements needs to be decomposed into sub elements on and on.

public static void RunSnippet()
{
    List<Element> hello = new List<Element>();

    hello.Add(Element); hello.Add(Element); hello.Add(Element);

    foreach (var x in hello)
    {
        List<Element> decomposed;
        decomposed = Decompose(x);
        if (decomposed != null) {
            foreach (var y in decomposed)
            {
                hello.Add(y);
            }
        }
    }
}

Answer 1

You can’t, basically. Not with a foreach loop, anyway. You can use a straight for loop:

for (int i = 0; i < hello.Count; i++)
{
    int x = hello[i];
    Console.WriteLine(x);
    if (x == 1) {
        hello.Add(100);
    }
}

I would personally try to avoid doing it in the first place though – it can get very hard to reason about whether you’ll ever complete, or if you’ll skip items (if you’re removing instead of adding, or adding before your current position).