Question

I have a Visual Studio 2008 C# .NET 3.5 project where I want to have a thread-safe pool of Foo objects.

public class FooPool
{
    private object pool_lock_ = new object();
    private Dictionary<int, Foo> foo_pool_ = new Dictionary<int, Foo>();

    // ...

    public void Add(Foo f)
    {
        lock (pool_lock_)
        {
            foo_pool_.Add(SomeFooDescriminator, f);
        }
    }

    public Foo this[string key]
    {
        get { return foo_pool_[key]; }
        set { lock (pool_lock_) { foo_pool_[key] = value; } }
    }

    public IEnumerable<Foo> Foos
    {
        get
        {
            lock (pool_lock_)
            {
                // is this thread-safe?
                return foo_pool_.Select(x => x.Value);
            }
        }
    }
}

Is the public IEnumerable<Foo> Foos { get; } function thread-safe? Or, do I need to clone the result and return a new list?

Answer 1

No, it isn’t.

If another thread adds to the dictionary while your caller enumerates that, you’ll get an error.

Instead, you can do:

lock (pool_lock_) {
    return foo_pool.Values.ToList();
}