Question

I am using a plugin system on an ASP.NET MVC application I am working on (loosely based on Rob Connery’s MVC storefront project implementation).

In addition to executing plugin methods, I also want to allow classes to provide various event hooks that a plugin can subscribe to.

I am publishing my events like so:

        #region Events

    public static event ItemAddedEventHandler ItemAdded;
    internal virtual void OnItemAdded(EventArgs e)
    {
        if (ItemAdded != null)
            ItemAdded(this, new EventArgs());
    }

    public static event LineRemovedEventHandler LineRemoved;
    internal virtual void OnLineRemoved(EventArgs e)
    {
        if (LineRemoved != null)
            LineRemoved(this, new EventArgs());
    }

    #endregion

    public delegate void ItemAddedEventHandler(object sender, EventArgs e);
    public delegate void LineRemovedEventHandler(object sender, EventArgs e);

And an example plugin:

        public EventHooks() {
        Cart.ItemAdded += new Cart.ItemAddedEventHandler(Cart_ItemAdded);
        Cart.LineRemoved += new Cart.LineRemovedEventHandler(Cart_LineRemoved);
    }

    void Cart_ItemAdded(object sender, EventArgs e) {
        _loggerService.Info("An item was added to the cart.");
    }

    void Cart_LineRemoved(object sender, EventArgs e) {
        _loggerService.Info("A line was removed from the cart.");
    }

Is this a good and thread safe way of publishing and subscribing to these events?

Any thoughts or suggestions appreciated.

Thanks
Ben

Answer 1

Assign your EventHandler’s to a local var before firing them.

ItemAddedEventHandler handler = ItemAdded;

if (handler != null) 
{
   handler(this, EventArgs.Empty) 
}

There is a race condition between checking the null and firing the event. You’d better follow the guidelines anyhow.