Question

If a Queue is syncronized:

var _item = Queue.Synchronized(new Queue());

can I call methods like Enqueue and Dequeue on it without using lock statements?

My current code is:

lock (_item.SyncRoot)
{
    _item.Enqueue(obj);
}

Can I thread-safely use:

_item.Enqueue(obj);
var item = _item.Dequeue();

Answer 1

The call to Enqueue and the call to Dequeue are thread safe.
However, your sample code is not:
Between the call to Enqueue and the call to Dequeue there could have been a thread switch. This means, that item might be another instance than obj or the call to Dequeue throws an exception, because it now is empty.

To make your sample code thread safe, you still need to lock explicitly:

lock(_item.SyncRoot)
{
    _item.Enqueue(obj);
    var item = _item.Dequeue);
}

Only now it is guaranteed, that item reference-equals obj in all circumstances.