IEnumerable thread safety?

Question

I have a main thread that populates a List. Further I create a chain of objects that will execute on different threads, requiring access to the List. The

Answer 1

IEnumerable can't be modified. So what can be non thread safe with it? (If you don't modify the actual List).

For non thread safety you need writing and reading operations.

"Iterator in itself" is instantiated for each foreach.

Edit: I simplified my answer a bit, but @Eric Lippert added valuable comment. IEnumerable doesn't define modifying methods, but it doesn't mean that access operators are thread safe (GetEnumerator, MoveNext and etc.) Simplest example: GetEnumerator implemented as this:

• Every time returns same instance of IEnumerator
• Resets it's position

More sophisticated example is caching.

This is interesting point, but fortunately I don't know any standard class that has not thread-safe implementation of IEnumerable.