Is returning an IEnumerable<> thread-safe?

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?

Thanks

---