Developing For .NET

Event Handling Made Easy is my favorite post I’ve written. Mostly, because I’m lazy and I have a bad memory. I love events, but I find it difficult to remember all the bits and pieces in my head, so I wrote the post to give me some documentation.

NOTE: the following feature is NOT new: it has been available since .NET 2.0, but I learned about it while examining the new features in 2008.

I recently came across the EventHandler<T> generic delegate. In this case, T represents any EventArgs object, so when you create your custom EventArgs class just make sure that you inherit from EventArgs. Now, when you register the Event Handler with the object, you reference EventHandler<T> instead of your custom event name:

// registering EventHandler<T>
person.MaritalStatusChangedEvent +=new EventHandler<MaritalStatusChangedEventArgs<(person_MaritalStatusChangedEvent);

This approach buys us a couple of things. First, it means you no longer need to declare a public delegate to define the EventHandler signature. Instead, in your class, you simply declare a public EventHandler<CustomEventArgs> CustomEventName. I don’t know about you, but anytime I can avoid using “delegate” I get a smile on my face! Also, it means that event registration code is a little more standardized, and is syntactically a little nicer.

So here is the whole process, revisited and updated:

Step 1: Create our custom EventArgs class. Be sure it inherits from EventArgs.

class MaritalStatusChangedEventArgs : EventArgs
{
    public readonly string Message;
    public MaritalStatusChangedEventArgs(string message)
    {
        Message = message;
    }
}

Step 2: Define the EventHandler<T> in the firing class.

// Generic EventHandle<T> delegate definition
public event EventHandler<MaritalStatusChangedEventArgs> MaritalStatusChangedEvent;

Step 3: Fire the Event.

// No change here
if (MaritalStatusChangedEvent != null)
{
    MaritalStatusChangedEvent(this, new MaritalStatusChangedEventArgs("Marital Status is now " + _maritalStatus));
}

Step 4: Register the EventHandler<T> listener.

// Use the EventHandler<T> format when registering
person.MaritalStatusChangedEvent +=new EventHandler<MaritalStatusChangedEventArgs>(person_MaritalStatusChangedEvent);

When you register the Event listener, Visual Studio will help out by offering to stub out the listening method. Just hit tab when prompted and it will insert the method stub for you and place your cursor inside the method. Naturally, it throws our old friend NotImplementedException. This is a nice little addition that means less coding for you!