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Simplest way to handle creating events in a class

While playing around in VS 2015, i was creating a property and mistyped prior to hitting tab and it gave me what appears to be a very clean simple method for creating an event. I am unsure how to use this to accept additional parameters to pass it data (as I usually want to pass data when raising an event with information about what is in progress, or what was done)
The code it gave is as follows :
public event EventHandler<EventArgs> MyEvent;
protected virtual void OnMyEvent(EventArgs e)
{
EventHandler<EventArgs> handler = MyEvent;
if (handler != null)
handler(this, e);
}
At first glance it is similar to most examples however this method does not appear to require a delegate to operate. Compare with the following example from How to call an event manually in C#?
public delegate void MyEventHandler(object sender, MyEventArgs e)
public event MyEventHandler MyEvent;
public void RaisesMyEvent()
{
if(MyEvent != null) //required in C# to ensure a handler is attached
MyEvent(this, new MyEventArgs(/*any info you want handlers to have*/));
}
What I would like to know, in light of having more elegant (clean) code, is the method that microsoft provided (first method in this question) better (it seems cleaner), and if so, how could I adjust it slightly to accept a string in addition to standard event args and/or extend the eventargs to pass information ?

To pass custom arguments on an EventHandler, you would create your own class derived from EventArgs and change the declaration like so:
public event EventHandler<MyCustomEventArgs> MyEvent;
Alternatively, if you don't like having object sender in your handlers, you can do:
public event Action<MyCustomEventArgs> MyEvent;
or even
public event Action<string> MyEvent;
As far as the invocation, Microsoft's is better. The reason is that the null check you wrote isn't thread-safe. Storing off the handler before doing the null check ensures that you don't invoke a null method.
Alternatively, use C# 6 invocations and take advantage of the null conditional operator:
MyEvent?.Invoke(whateverArgs);

The code that Visual Studio auto-generated for you is actually better for one small condition:
public void RaisesMyEvent()
{
if(MyEvent != null) //required in C# to ensure a handler is attached
// Now if someone in another thread sets MyEvent to null the next line will still execute
MyEvent(this, new MyEventArgs(/*any info you want handlers to have*/));
// And throw a NullReferenceException
}
This code has a race condition. It's fairly rare to hit it, but it is possible. The Visual Studio boilerplate code avoids this race condition by making a local copy of the event handler before calling it.
As for how to pass more parameters to the event handler:
There are multiple ways, but the 'standard' way is to create an class that extends EventArgs. For instance, PaintEventArgs extends EventArgs and adds a Graphics and Rectangle properties for the OnPaint handler to use.
I'd follow that basic pattern by creating a MyEventArgs class based off EventArgs, and adding the properties your method will need to use.

Just replace EventArgs with your own class:
class MyEventArgs : EventArgs
{
public string Text { get; private set; }
public MyEventArgs(string text)
{
Text = text;
}
}
The EventHandler<T> generic delegate type will accept EventArgs or any subclass of EventArgs.
Then when you call OnMyEvent(), just pass an instance of your class with the data you want to deliver with the event.
I agree with the other notes regarding thread-safe invocation. This isn't always required, but it's simple enough to implement and the IDE-provided version is safe while yours is not.

Is it OK to always add an empty event handler? [duplicate]

This question already has answers here:
Closed 10 years ago.
Possible Duplicate:
Is there a downside to adding an anonymous empty delegate on event declaration?
The following pattern is quite common when using event handlers (in C#):
public event Action handler;
…
// some method:
if(handler != null) handler();
Are there any downsides of assigning an empty delegate to this event? This would save the if !=null condition everywhere, where the event is fired. Of course, this only applies, when the we cannot guarantee that the event is always assigned a proper delegate.
public event Action handler;
…
// in constructor:
handler += ()=>{};
…
// some method:
handler();
Sure, there's a slight performance hit, but it makes the code much cleaner. What's the best practice in such a case? Any technical disadvantages?

Interesting idea, I've never thought of doing that. The way i do my custom events is i make a OnCustomEventName taking parameters for the event and just do the check in there for null. and call OnCustomEventName from the code wherever i want the event triggered from.
Avoids any performance hits and keeps the operational code cleaner than a 2-line if check every time you want the event fired.
That being said this isn't answering the question about technical disadvantages but more of a best practice when firing events.
example code for threadsafe "On" function.
private void OnCustomEventName()
{
DelegateName localhandler = CustomEventName;
if (localhandler != null)
localhandler();
}

Instead of adding an empty delegate in the constructor, you can wrap the handler in a function which first checks if the handler is null then calls it. The downside of this is if you have a lot of events, you will have a lot of functions that wrap each event.
private void HandlerWrapper()
{
Action localHandler = handler;
if (localHandler != null) handler();
}

I haven't really found any big downsides to do this and generally I prefer it over checking for null. The only issue I cant think of is that it might lead to annoying steps in the code when debugging (i.e.
having to step over the empty delegate whenever stepping into an event).
I think that performance isn't an issue - if application performance degenerates significantly by invoking events, the event should probably not have been there in the first place.

Firing an event in C# with no attatched delegate methods?

I've just encountered a bug in the program I'm writing where an exception was thrown stating an "object reference must be set to an instance of an object". Upon investigation, I found that this exception was thrown when trying to fire an event BUT the event didn't have any delegate methods added to it.
I wanted to check that my understanding was correct that as a developer you shouldn't fire events without first checking that the event doesn't equal null? For example:
if (this.MyEventThatIWantToFire != null)
{
this.MyEventThatIWantToFire();
}
Thanks in advance for the advice/thoughts.

The pattern you've shown is broken in a multi-threaded environment. MyEventThatIWantToFire could become null after the test but before the invocation. Here's a safer approach:
EventHandler handler = MyEventThatIWantToFire;
if (handler != null)
{
handler(...);
}
Note that unless you use some sort of memory barrier, there's no guarantee you'll see the latest set of subscribers, even ignoring the obvious race condition.
But yes, unless you know that it will be non-null, you need to perform a check or use a helper method to do the check for you.
One way of making sure there's always a subscriber is to add a no-op subscriber yourself, e.g.
public event EventHandler MyEventThatIWantToFire = delegate {};
Of course, events don't have to be implemented with simple delegate fields. For example, you could have an event backed by a List<EventHandler> instead, using an empty list to start with. That would be quite unusual though.
An event itself is never null, because the event itself is just a pair of methods (add/remove). See my article about events and delegates for more details.

You shouldn't do it that way - if someone where to remove a listener between the if and the call to the event, you'd get an exception. It is good practise to use it with a local variable:
protected void RaiseEvent()
{
var handler = Event;
if(handler != null)
handler(this, EventArgs.Empty);
}
Of course, this has the disadvantage of maybe calling a listener that already was removed between the creation of the local variable and the call to the handler.

Yes, you should check it for null but the way you're doing it leads to a race condition. It's possible the event may end up being null anyway if someone unsubscribes the last delegate in between your "if" and your event raise. The accepted way to do this is like:
var temp = this.MyEventThatIWantToFire;
if (temp != null) {
this.temp(this, EventArgs.Empty);
}
or alternatively, ensure there's always at least one delegate in the invocation list by declaring your event like this:
public event EventHandler MyEventThatIWantToFire = delegate {};
Make sense?

Yes, your understanding is correct. It's up to you to check the delegate's value before calling it.
Most delegates - which events are - are "multi-cast" delegates. This means that a delegate can manage a list of one or more methods that it will call when activated.
When the delegate evaluates to a null value, there are no registered methods; therefore, there is nothing to call. If it evaluates to something other than a null value, there is at least one method registered. Calling the delegate is an instruction to call all methods it has registered. The methods are called in no guaranteed order.
Using the addition-assignment operator (+=) or addition operator (+) adds a method to the delegate's list of methods. Using the subtraction-assignment operator (-=) or subtraction operator (-) removes a method from the delegate's list of methods.
Also note that the called methods are executed from the caller's thread. This is important if you are using events to update your user interface controls. In this situation, use of your control's InvokeRequired property lets you handle cross-threading calls (using Invoke() or BeginInvoke()) gracefully.

Yes in C# if an event has no delegates, it will be null, so you must check

Yes, you must check whether event is not null. Mostly you encounter protected method that does the check and invokes event, or even constructs event args. Something like this:
public event EventHandler Foo;
protected void OnFoo() {
if (Foo == null)
return;
Foo(this, new EventArgs());
}
This approach also alows your derived classes to invoke (or prevent invoking) the event.

How can I retrieve all methods of an event?

I have an event Load
public delegate void OnLoad(int i);
public event OnLoad Load;
I subscribe to it with a method:
public void Go()
{
Load += (x) => { };
}
Is it possible to retrieve this method using reflection? How?

In this particular case you could, with reflection. However, in general, you can't. Events encapsulate the idea of subscribers subscribing and unsubscribing - and that's all. A subscriber isn't meant to find out what other subscribers there are.
A field-like event as you've just shown is simply backed by a field of the relevant delegate type, with autogenerated add/remove handlers which just use the field. However, there's nothing to say they have to be implemented like that. For example, an event can store its subscribers in an EventHandlerList, which is efficient if you have several events in a class and only a few of them are likely to be subscribed to.
Now I suppose you could try to find the body of the "add" handler, decompile it and work out how the event handlers are being stored, and fetch them that way... but please don't. You're creating a lot of work, just to break encapsulation. Just redesign your code so that you don't need to do this.
EDIT: I've been assuming that you're talking about getting the subscribers from outside the class declaring the event. If you're inside the class declaring the event, then it's easy, because you know how the event is being stored.
At that point, the problem goes from "fetching the subscribers of an event" to "fetching the individual delegates making up a multicast delegate" - and that's easy. As others have said, you can call Delegate.GetInvocationList to get an array of delegates... and then use the Delegate.Method property to get the method that that particular delegate targets.
Now, let's look again at your subscription code:
public void Go()
{
Load += (x) => { };
}
The method that's used to create the delegate here isn't Go... it's a method created by the C# compiler. It will have an "unspeakable name" (usually with angle brackets) so will look something like this:
[CompilerGenerated]
private static void <Go>b__0(int x)
{
}
Now, is that actually what you want to retrieve? Or were you really looking to find out which method performed the subscription, rather than which method was used as the subscribed handler?

If you call Load.GetInvocationList() you will be handed back an array of Delegate types. From the these types, you can access the MethodInfo.

You could use the GetInvocationList method which will give you all the subscribers.

Why does C# require you to write a null check every time you fire an event?

This seems odd to me -- VB.NET handles the null check implicitly via its RaiseEvent keyword. It seems to raise the amount of boilerplate around events considerably and I don't see what benefit it provides.
I'm sure the language designers had a good reason to do this.. but I'm curious if anyone knows why.

It's certainly a point of annoyance.
When you write code which accesses a field-like event within a class, you're actually accessing the field itself (modulo a few changes in C# 4; let's not go there for the moment).
So, options would be:
Special-case field-like event invocations so that they didn't actually refer to the field directly, but instead added a wrapper
Handle all delegate invocations differently, such that:
Action<string> x = null;
x();
wouldn't throw an exception.
Of course, for non-void delegates (and events) both options raise a problem:
Func<int> x = null;
int y = x();
Should that silently return 0? (The default value of an int.) Or is it actually masking a bug (more likely). It would be somewhat inconsistent to make it silently ignore the fact that you're trying to invoke a null delegate. It would be even odder in this case, which doesn't use C#'s syntactic sugar:
Func<int> x = null;
int y = x.Invoke();
Basically things become tricky and inconsistent with the rest of the language almost whatever you do. I don't like it either, but I'm not sure what a practical but consistent solution might be...

we usually work around this by declaring our events like this:
public event EventHandler<FooEventArgs> Foo = delegate { };
this has two advantages. The first is that we don't have check for null. The second is that we avoid the critical section issue that is omnipresent in typical event firing:
// old, busted code that happens to work most of the time since
// a lot of code never unsubscribes from events
protected virtual void OnFoo(FooEventArgs e)
{
// two threads, first checks for null and succeeds and enters
if (Foo != null) {
// second thread removes event handler now, leaving Foo = null
// first thread resumes and crashes.
Foo(this, e);
}
}
// proper thread-safe code
protected virtual void OnFoo(FooEventArgs e)
{
EventHandler<FooEventArgs> handler = Foo;
if (handler != null)
handler(this, e);
}
But with the automatic initialization of Foo to an empty delegate, there is never any checking necessary and the code is automatically thread-safe, and easier to read to boot:
protected virtual void OnFoo(FooEventArgs e)
{
Foo(this, e); // always good
}
With apologies to Pat Morita in the Karate Kid, "Best way to avoid null is not have one."
As to the why, C# doesn't coddle you as much as VB. Although the event keyword hides most of the implementation details of multicast delegates, it does give you finer control than VB.

You need to consider what code would be required if setting up the plumbing to raise the event in the first place would be expensive (like SystemEvents) or when preparing the event arguments would be expensive (like the Paint event).
The Visual Basic style of event handling doesn't let you postpone the cost of supporting such an event. You cannot override the add/remove accessors to delay putting the expensive plumbing in place. And you cannot discover that there might not be any event handlers subscribed so that burning the cycles to prepare the event arguments is a waste of time.
Not an issue in C#. Classic trade-off between convenience and control.

Extension methods provide a very cool way, to get around this. Consider the following code:
static public class Extensions
{
public static void Raise(this EventHandler handler, object sender)
{
Raise(handler, sender, EventArgs.Empty);
}
public static void Raise(this EventHandler handler, object sender, EventArgs args)
{
if (handler != null) handler(sender, args);
}
public static void Raise<T>(this EventHandler<T> handler, object sender, T args)
where T : EventArgs
{
if (handler != null) handler(sender, args);
}
}
Now you can simply do this:
class Test
{
public event EventHandler SomeEvent;
public void DoSomething()
{
SomeEvent.Raise(this);
}
}
However as others already mentioned, you should be aware of the possible race condition in multi-threaded scenarios.

Don't really know why is this done, but there's a variation of a Null Object pattern specifically for delegates:
private event EventHandler Foo = (sender, args) => {};
This way you can freely invoke Foo without ever checking for null.

Because RaiseEvent carries a some overhead.
There's always a tradeoff between control and ease of use.
VB.Net: ease of use,
C#: more control as VB
C++: even more control, less guidance, easier to shoot yourself in the foot

Edit: As the OP points out, this answer does not address the body of the question. However, some may find it useful because it does provide an answer for the title of the question (when taken by itself):
Why does C# require you to write a null check every time you fire an event?
It also provides context for the intent of the body of the question which some may find useful. So, for those reasons and this advice on Meta, I'll let this answer stand.
Original Text:
In its MSDN article How to: Publish Events that Conform to .NET Framework Guidelines (C# Programming Guide) ( Visual Studio 2013), Microsoft includes the following comment in its example:
// Make a temporary copy of the event to avoid possibility of
// a race condition if the last subscriber unsubscribes
// immediately after the null check and before the event is raised.
Here is a larger excerpt from Microsoft's example code that gives context to that comment.
// Wrap event invocations inside a protected virtual method
// to allow derived classes to override the event invocation behavior
protected virtual void OnRaiseCustomEvent(CustomEventArgs e)
{
// Make a temporary copy of the event to avoid possibility of
// a race condition if the last subscriber unsubscribes
// immediately after the null check and before the event is raised.
EventHandler<CustomEventArgs> handler = RaiseCustomEvent;
// Event will be null if there are no subscribers
if (handler != null)
{
// Format the string to send inside the CustomEventArgs parameter
e.Message += String.Format(" at {0}", DateTime.Now.ToString());
// Use the () operator to raise the event.
handler(this, e);
}
}

Note that as of C# 6, the language now provides a concise syntax to perform this null check conveniently. E.g.:
public event EventHandler SomeEvent;
private void M()
{
// raise the event:
SomeEvent?.Invoke(this, EventArgs.Empty);
}
See Null Conditional Operator

The reason really boils down to C# giving you more control. In C# you do not have to do the null check if you so choose. In the following code MyEvent can never be null so why bother doing the check?
public class EventTest
{
private event EventHandler MyEvent = delegate { Console.WriteLine("Hello World"); }
public void Test()
{
MyEvent(this, new EventArgs());
}
}