how to handle events from variable number of sources?
<p>I have such class (sorry about posible mistakes, i'm writing it right here.) Class is simplified for this example, it must be more complex of course.</p>

<pre><code>class SP500Index {

    SP500Index(List&lt;OrderBook&gt; stocks) {
        foreach (var stock in stocks) {
            stock.StockUpdated += stockUpdated; // how to handle?
        }
    }

}
</code></pre>

<p>So I have a lot of sources and I need to handle StockUpdated event from them. In handler I need to know <code>index</code> of stock in <code>stocks</code> list which raised the event. How to do that?</p>

<p><strong>upd</strong> for perfomance reasons I don't want "sender look-up" instead I want index. Lookup is not trivial operation and likely involves <code>Hashcode</code> calculation <code>Equals</code> method call etc. Imagine how ofthen SP500 index changes...</p>
