Question

This seems like it should be answered but potential dupes I found were asking different things…

I noticed that this seems to work fine (sourceDirInclusion is a simple Dictionary<X,Y>)

    foreach (string dir in sourceDirInclusion.Keys)
    {
        if (sourceDirInclusion[dir] == null)
            sourceDirInclusion.Remove(dir);
    }

Does that mean removing items from a collection in foreach is safe, or that I got lucky?

What about if I was adding more elements to the dictionary rather than removing?

The problem I’m trying to solve is that sourceDirInclusion is initially populated, but then each value can contribute new items to the dictionary in a second pass. e.g what I want to do is like:

foreach (string dir in sourceDirInclusion.Keys)
{
  X x = sourceDirInclusion[dir];
  sourceDirInclusion.Add(X.dir,X.val);
}

Answer 1

Short answer: This is not safe.

Long answer: From the IEnumerator<T> documentation:

An enumerator remains valid as long as the collection remains unchanged. If changes are made to the collection, such as adding, modifying, or deleting elements, the enumerator is irrecoverably invalidated and its behavior is undefined.

Note that the docs say the behavior is undefined, which means that it might work and it might not. One should never rely on undefined behavior.

In this case, it depends on the behavior of the Keys enumerable, regarding whether or not it creates a copy of the list of keys when you begin enumerating. In this specific case, we know from the docs that the return value from Dictionary<,>.Keys is a collection that refers back to the dictionary:

The returned Dictionary<TKey, TValue>.KeyCollection is not a static copy; instead, the Dictionary<TKey, TValue>.KeyCollection refers back to the keys in the original Dictionary<TKey, TValue>. Therefore, changes to the Dictionary<TKey, TValue> continue to be reflected in the Dictionary<TKey, TValue>.KeyCollection.

So it should be considered unsafe to modify the dictionary while enumerating the dictionary’s keys.

You can correct this with one change. Alter this line:

foreach (string dir in sourceDirInclusion.Keys)

To this:

foreach (string dir in sourceDirInclusion.Keys.ToList())

The ToList() extension method will create an explicit copy of the list of keys, making it safe to modify the dictionary; the “underlying collection” will be the copy and not the original.