Thread-Safety with Enumerable collections and copies

Question

Imagine I have an interface IHeapObject:

interface IHeapObject
{
    string Id {get;}
    SomeEnum Type {get;}
}

and it's implementation HeapObject:

class HeapObject : IHeapObject
{
    public string Id {get; private set;}

    public SomeEnum Type {get; private set;}

    public object objectLocker {get; } = new object();
}

Imagine I have a class that has a public IEnumerable<IHeapObject> Readonly-Property property. This property points to a private readonly IList<HeapObject> field. Like this:

class MyClass
{
    public IEnumerable<IHeapObject> Foo => _bar;

    private readonly IList<HeapObject> _bar;
}

Suppose the way I handle this collection inside of this class is 100% thread-safe. Furthermore, any and all state changes in the list and HeapObject itself happen only within this class. Is the fact that I'm exposing the collection a thread-safety concern?

My assumptions is that I can't guarantee that the IHeapObject instance will be "up-to-date" when read from outside MyClass. Meaning, as long as I don't rely on the properties' values for critical processes outside of it, I should be fine. Is this assumption correct?

Secondly, is there a risk of throwing an exception while iterating over the IEnumerable collection from outside of the class?

Answer 1

The basic thread safety rules state that there can be multiple readers or a single writer, but not both at the same time.

You allow anyone to be a reader. You cannot control who is. Therefore, it is actually never safe to write to the structures.