Threadsafe factory pattern

Question

I'am coding some simple graph manager and i decied to use factory pattern to create edges for my graph. I'am just beginning to learn design-patterns and threads.

I created factory with return some objects(edges), create method, code:

public Edge<T> CreateEdge(Vertex<T> firstVertex, Vertex<T> secondVertex)
{
    var edge = new Edge<T>(firstVertex, secondVertex, _nextIndex);

    _nextIndex++;

    return edge;
}

And here is my problem. What if that factory will be shared by multiple threads is possible that the return several edges with same index.

What is the best solution of this problem?
Should i use lock or some mutex?
Or it is the fault of my bad design classes, use factory?

Edit #1: Change IncreaseIndex() -> _nextIndex++; mistake when wrote it.

Answer 1

You can use:

long _index;

var nextIndex = Interlocked.Increment(ref _index);

That will ensure your index remains unique across threads. Using a simple field makes the most sense here, since it's a property of your edge factory class.