We Keep Coding

How to raise weak events?

I'm reading the MS docs on how to implement the weak event pattern. I've tried the first two (since I'm looking for the simplest way) but I can't find a way to raise the event.
The existing weak event managers like the PropertyChangedEventManager, and the generic WeakEventManager only have methods to add and remove handlers but nothing to raise the event. Also, I can't inherit those it says [the base class] does not contain a constructor that takes 0 arguments (the constructors are private).
So I'm finding the docs confusing - how are these constituted the ways to implement the weak event pattern if there's no way to raise the event?
If the suggested way in the docs doesn't work I'm looking for the simplest way to raise and handle weak events.

Should event firing in C# always be protected by a null check?

This situation seem very interesting to me.
In C# you need to check if there is any listener to the event in a class before firing it.
Assuming event structure of C# is a non-standart(Meaning Microsoft did it) implementation of Observer observable pattern for ease of use.
Why didn't they implement this inside that structure? Is there solid reasons or documentation for this choice.
Is it a necessity do null checking, or am I wrong in my assumption for event structures needing null checks under all circumstances.
This is a more of a curiosity question looking for an answer to this implementation choice by microsoft. Which I hope will lead to further understanding of delegate and event keyword inner workings.

Yes, you must do the null check.
Calling a delegate that is null results in a NullReferenceException.
You might be tempted to intialize all delegates with a non-null, empty event handler. However, that would be far worse than testing for null, in terms of CPU usage, memory usage, and lines of code.

There's a reply to this question in a blog by Eric Gunnerson.
Basically it seems to say that Microsoft thought about changing it, but it would break existing code.
In other words, it was a mistake in the original design for events, but it's too late to fix it.

You can add a default event listener, thus avoiding the null check
public Action<object, EventArgs> SomeEvent = (o, e) => { };
in that way, you can call SomeEvent without checking for null, since it contains a default (empty implementation) listener. Please note that it might hurt performance.

Not understanding the point with delegates/events and publisher/subscribers

I'm reading and reading, but I still can't get the point and how to use delegates/events and publisher/subscribers? I know that a delegate is a class that contains references to one or more methods and that it's used to send methods to another methods.
The thing I don't get is how I should determine the role of publischer/suubscriber. Let's take an example. A taxicentral and it's cabs. Is the taxicentral the publischer and the cabs the subscribers, whating to get driving orders from the taxicentral? But the cabs could also be publisher and report back there position and the address they are heading for to the taxicentral that, then is a subscriber of the cabs!?
I need inspiration and I'm looking for som beginners code to get a view of how this is working. Is there someone who have some minutes left to reply to this answer with some simple code? Thanks!

Another way to think of events, publishers/subscribers, etc...is to think of the cloud and the way it tends to function. In a cloud based system, any entity can register to listen for any type of event, OR can publish any event that it wants. Anyone listening, will get that data.
In your example: Taxi central could publish/push events like Shift Change, Traffic Accident as (Location), Taxi Requested at (Location), or other similar things that the entire group of cabs would be listening for. The individual cabs would be publishing things like Pickup At (Location), Drop Off at (Location), Accident at (Location); that Central would be logging for its own purposes. Other cabs could if they wanted to ALSO be listening for these same events, so that they would know where and how close other cabs are to their own location, or if there is an accident that another cab reported, or similar.
Cloud events however are a specific implementation of the event system. Its much more common to specifically subscribe to events. A cab object, when created, would immediately subscribe itself to the Central events, and the central system when dispatching the cab would make certain that it subscribed itself to that cab's events.
In this way, both objects serve the role of both publisher and subscriber. Its only really specific to a single event. In my example PickupAt(Location) would be published by a Cab, and Subscribed to by Central. Thus, for that event, Cab is the publisher, and Central is the Subscriber. In general, Who serves what role depends entirely on the system design and what events are being created. Its not something that can really be generalized, because the entire setup could be changed, or even reversed, if you setup the events differently. That I think is the most important part.
Delegates as a whole
In its simplest term, a Delegate is a Reference. It can reference a class, more commonly it can reference a method in a class, or the call to a method in a class. It can even contain an entire method call inside itself. Its really a very versatile object, in that it can do many things. In the context of events, the delegate actually references the call to the function that implements the event.
Simple Event Code
public class c1
{
public event Eventhandler DoStuff;
public c1()
{
}
public void OnDoStuff()
{//this actually makes the event happen
if (DoStuff != null)
DoStuff(this, null);
}
}
public static void Main()
{
c1 x = new c1();
x.DoStuff += new EventHandler(ThingFunction);
x.OnDoStuff();//this is how you would fire the event deliberately
}
public void ThingFunction(object sender, EventArgs x)
{
Console.WriteLine("Something Happened");
}
c1 contains the event DoStuff, Main subscribes to this event. When the code calls x.OnDoStuff() from ANYWHERE, which is very handy if say you pass x, or a reference to it, into other classes where actual code is processed, then the handler way back in main, no matter how many layers deep the x.OnDoStuff() call originates from, will execute.
Rather more specifically, once X is created, it can be passed somewhere else. As long as some function is assigned to x.DoStuff, whenever and wherever you call the method x.OnDoStuff() from, the function that is assigned to x.DoStuff will execute. That is how event subscriptions work. x.OnDoStuff() is the code call that publishes the event x.DoStuff, and any class...in fact any number of classes, you can have multiple subscribers after all, will be able to catch the published event.

You have to think in "Event Driven" terms. In your example there could be publisher roles for both the taxicentral and the cabs as well as subscriber roles for the taxicentral and cab.
For example, if the taxicentral has an update to push to all the cabs it can have an event that it raises and the cabs listen to. Cabs then could have an event they raise when they pick up and drop off someone, and the taxicentral would listen to all those events.
Does this help?

Delegates are not used to send methods to other methods. They are used to call methods, which may be unknown at compile time.
Let's look at a simpe example. Consider, you write a calculator, where the user can choose the arguments and the operation. The operation can be a delegate to a method that executes the calculation. Therefore, you don't have to worry about how to store the operations. Furthermore, the calculator does not need to know every operation, because the user will provide it.
A common example of delegates are callback functions. If you start a long term procedure in a second thread, you may want to be notified, if the thread has finished. You could do this by providing a delegate to the thread that is invoked, when the procedure has finished.
Events are kind of a special type of delegates. They are used, when an object wants to notify others of a status change or similar actions. The special thing about that is that the object itself does not need to know, which objects are interested in the changes. Instead, those objects subscribe to the event. The object that provides the event only has to invoke the event's delegate and all subscriber delegates are invoked.
As Justin C already explained, both the taxi central and the cab can act as subscribers and publishers. It depends on who is interested in what. And if everyone is allowed to subscribe to the events. If only the taxi central is allowed to get the information, a callback method would be more suitable.

When to use callbacks instead of events in c#?

When would you favour using a callback (i.e, passing in a Func or Action), as opposed to exposing and using an event?
UPDATE
What motivated this question was the following problem:
I have a ThingsHandler class, which
can be associated with a ThingEditor.
The ThingsHandler handles a list of
Things, knows their order, which one is 'current', when new
ones are added or deleted etc.
The ThingEditors can just modify a single
thing.
The ThingsHandler needs to alert
the ThingEditor when the user selects
a new Thing to edit, and the
ThingEditor needs to alert the
ThingsHandler when the user says
'done'.
What bothers me is having these two classes holding references to each other - though I guess that's inevitable - or binding to events in both directions. I wondered if using a callback in one direction was 'cleaner'.
I suspect there is a design pattern for this.

Though the other answers thus far seem reasonable, I would take a more philosophical tack.
A class is a mechanism that models a particular kind of thing in a particular domain. It is very easy when writing the internal details of a class to conflate the implementation details of the mechanism with the semantics being modeled. A brief example of what I mean:
class Giraffe : Mammal, IDisposable
{
public override void Eat(Food f) { ... }
public void Dispose() { ... }
}
Notice how we've conflated the real-world thing being modeled (a giraffe is a kind of mammal, a giraffe eats food) with the details of the implementation (an instance of Giraffe is an object which can be disposed of with the "using" statement). I guarantee that if you go to the zoo, you will never see a giraffe being disposed of with the using statement. We've mixed up the levels here, which is unfortunate.
I try to use events (and properties) as part of the semantic model and use callback methods (and fields) as part of the mechanism. I would make GaveBirth an event of Giraffe, since that is part of the model of real-world giraffe behaviour we're attempting to capture. If I had some mechanism, like, say I wanted to implement an inorder-walk tree traversal algorithm that walked the family tree of giraffes and called a method back on every one, then I'd say that this was clearly a mechanism and not part of the model, and make it a callback rather than try to shoehorn that into the event model.

I use callbacks in a few cases where I know it will only ever fire once, and the callback is specific to a single method call (rather than to an object instance) - for example, as the return part of an async method.
This is particularly true of static utility methods (since you don't have an instance, and static events are deadly when used carelessly, and to be avoided), but of course the other option is to create a class instance with an event instead.

Generally, I use a callback if it is required, whereas an event is used when it should be optional.
Don't expose an event if you're expecting there to always be something listening.
Consider the following:
public class MyClass_Event
{
public event EventHandler MakeMeDoWork;
public void DoWork()
{
if (MakeMeDoWork == null)
throw new Exception("Set the event MakeMeDoWork before calling this method.");
MakeMeDoWork(this, EventArgs.Empty);
}
}
versus:
public class MyClass_Callback
{
public void DoWork(EventHandler callback)
{
if (callback == null)
throw new ArgumentException("Set the callback.", "callback"); // better design
callback(this, EventArgs.Empty);
}
}
The code is almost the same as the callback can be passed as null, but at least the exception thrown can be more relevant.

Callbacks are good when one object wishes to receive a single notification (e.g. an Async data read runs and then calls you with the result).
Events are good for recurring notifications that can be received by an arbitrary number of listeners.

One example is when the callback should return something. E.g. (stupid example):
public int Sum(Func<int> callbackA, Func<int> callbackB) {
return callbackA() + callbackB();
}
public void UseSum() {
return sum(() => 10, () => 20);
}

In terms of OO design and class coupling there isn't great deal of difference between a callback interface and an event.
However, I prefer events where they are things the class needs to "shout about" to whoever is interested in listening (usually multiple things) and callbacks where a specific class has requested an async operation.
Whatever you use, use them consistently across the codebase!

I would use Func or Action when I am going to call the function once or use a Lambda expression.
Events can be registered more than once which sometimes is ideal. With a callback, one has to implement a registration system for the callbacks if you want multiple.

Well, I think they are same things. There're many different tech terms to name the same concepts or things in the different languages.
So, what do you mean "Callback" or "event handler"?
According to MSDN: Callback function is code within a managed application that helps an unmanaged DLL function complete a task.
And, MADN also gives us a introduction of the difference between them.click here
Callbacks are extensibility points that allow a framework to call back into user code through a delegate. These delegates are usually passed to the framework through a parameter of a method.
Events are a special case of callbacks that supports convenient and consistent syntax for supplying the delegate (an event handler). In addition, Visual Studio’s statement completion and designers provide help in using event-based APIs
Also, in some books, such as this book, the author seemed say the same thing with MSDN.
Therefore, in my opinion, you can't say use callbacks instead of events in the C#.

C#: events or an observer interface? Pros/cons?

I've got the following (simplified):
interface IFindFilesObserver
{
void OnFoundFile(FileInfo fileInfo);
void OnFoundDirectory(DirectoryInfo directoryInfo);
}
class FindFiles
{
IFindFilesObserver _observer;
// ...
}
...and I'm conflicted. This is basically what I would have written in C++, but C# has events. Should I change the code to use events, or should I leave it alone?
What are the advantages or disadvantages of events over a traditional observer interface?

Consider an event to be a callback interface where the interface has only one method.
Only hook events you need
With events, you only need to implement handlers for events you're interested in handling. In the observer interface pattern, you'd have to implement all methods in the entire interface including implementing method bodies for notification types you don't actually care about handling. In your example, you always have to implement OnFoundDirectory and OnFoundFile, even if you only care about one of these events.
Less maintenance
Another good thing about events is you can add a new one to a particular class so that it will raise it, and you don't have to change every existing observer. Whereas if you want to add a new method to an interface, you have to go around every class that already implements that interface and implement the new method in all of them. With an event though, you only need to alter existing classes that actually want to do something in response to the new event you're adding.
The pattern is built into the language so everybody knows how to use it
Events are idiomatic, in that when you see an event, you know how to use it. With an observer interface, people often implement different ways of registering to receive notifications and hook up the observer.. with events though, once you've learnt how to register and use one (with the += operator), the rest are all the same.
Pros for interfaces
I haven't got many pros for interfaces. I guess they force someone to implement all methods in the interface. But, you can't really force somebody to implement all those methods correctly, so I don't think there's a lot of value on this.
Syntax
Some people don't like the way you have to declare a delegate type for each event. Also, standard event handlers in the .NET framework have these parameters: (object sender, EventArgs args). As sender doesn't specify a particular type, you have to down-cast if you want to use it. This often is fine in practice, it feels not quite right though because you're losing the protection of the static type system. But, if you implement your own events and don't follow the .NET framework convention on this, you can use the correct type so potential down-casting isn't required.

Hmm, events can be used to implement the Observer pattern. In fact, using events can be regarded as another implementation of the observer-pattern imho.

Events are harder to propagate through chain of objects, for example if you use FACADE pattern or delegate work to other class.
You need to be very careful with unsubscribing from events to allow object to be garbage collected.
Events are 2x time slower than simple function call, 3x slower if you do null check on every raise, and copy event delegate before null check and invocation to make it thread safe.
Also read MSDN about new (in 4.0) IObserver<T> interface.
Consider this example:
using System;
namespace Example
{
//Observer
public class SomeFacade
{
public void DoSomeWork(IObserver notificationObject)
{
Worker worker = new Worker(notificationObject);
worker.DoWork();
}
}
public class Worker
{
private readonly IObserver _notificationObject;
public Worker(IObserver notificationObject)
{
_notificationObject = notificationObject;
}
public void DoWork()
{
//...
_notificationObject.Progress(100);
_notificationObject.Done();
}
}
public interface IObserver
{
void Done();
void Progress(int amount);
}
//Events
public class SomeFacadeWithEvents
{
public event Action Done;
public event Action<int> Progress;
private void RaiseDone()
{
if (Done != null) Done();
}
private void RaiseProgress(int amount)
{
if (Progress != null) Progress(amount);
}
public void DoSomeWork()
{
WorkerWithEvents worker = new WorkerWithEvents();
worker.Done += RaiseDone;
worker.Progress += RaiseProgress;
worker.DoWork();
//Also we neede to unsubscribe...
worker.Done -= RaiseDone;
worker.Progress -= RaiseProgress;
}
}
public class WorkerWithEvents
{
public event Action Done;
public event Action<int> Progress;
public void DoWork()
{
//...
Progress(100);
Done();
}
}
}

Pros of an interface-solution:
If you add methods, existing observers needs to implement those methods. This means that you have less of a chance of forgetting to wire up existing observers to new functionality. You can of course implement them as empty methods which means you have the luxury of still doing nothing in response to certain "events". But you won't so easily forget.
If you use explicit implementation, you'll also get compiler errors the other way, if you remove or change existing interfaces, then observers implementing them will stop compiling.
Cons:
More thought has to go into planning, since a change in the observer interface might enforce changes all over your solution, which might require different planning. Since a simple event is optional, little or no other code has to change unless that other code should react to the event.

Some further benefits of events.
You get proper multicast behaviour for free.
If you change the subscribers of an event in response to that event the behaviour is well defined
They can be introspected (reflected) easily and consistently
Tool chain support for events (simply because they are the idiom in .net)
You get the option to use the asynchronous apis it provides
You can achieve all of these (except the tool chain) yourself but it's surprisingly hard. For example:
If you use a member variable like a List<> to store the list of observers.
If you use foreach to iterate over it then any attempt to add or remove a subscriber within one of the OnFoo() method callbacks will trigger an exception unless you write further code to deal with it cleanly.

The best way to decide is this: which one suits the situation better. That might sound like a silly or unhelpful answer, but I don't think you should regard one or the other as the "proper" solution.
We can throw a hundred tips at you. Events are best when the observer is expected to listen for arbitrary events. An interface is best when the observer is expected to listed to all of a given set of events. Events are best when dealing with GUI apps. Interfaces consume less memory (a single pointer for multiple events). Yadda yadda yadda. A bulleted list of pros and cons is something to think about, but not a definitive answer. What you really need to do is try both of them in actual applications and get a good feel for them. Then you can choose the one that suits the situation better. Learn form doing.
If you have to use a single defining question, then ask yourself which better describes your situation: A set of loosely related events any of which may be used or ignored, or a set of closely related events which will all generally need to be handled by one observer. But then, I'm just describing the event model and interface model, so I'm back at square one: which one suits the situation better?

Pros are that events are more 'dot-netty'. If you are designing non-visual components that can be dropped onto a form, you can hook them up using the designer.
Cons are that an event only signifies a single event - you need a separate event for each 'thing' that you want to notify the observer about. This doesn't really have much practical impact except that each observed object would need to hold a reference for every observer for every event, bloating memory in the case where there are lots of observed objects (one of the reasons they made a different way of managing the observer/observable relationship in WPF).
In your case I'd argue it doesn't make much difference. If the observer would typically be interested in all those events, use an observer interface rather than separate events.

I prefer an event base solution for the following reasons
It reduces the cost of entry. It's much easier to say "+= new EventHandler" than to implement a full fledged interface.
It reduces maintenance costs. If you add a new event into your class that's all that needs to be done. If you add a new event to an interface you must update every single consumer in your code base. Or define an entirely new interface which over time gets annoying to consumers "Do I implement IRandomEvent2 or IRandomEvent5?"
Events allow for handlers to be non-class based (ie a static method somewhere). There is no functional reason to force all event handlers to be an instance member
Grouping a bunch of events into an interface is making an assumption about how the events are used (and it's just that, an assumption)
Interfaces offer no real advantage over a raw event.

Java has language support for anonymous interfaces, so callback interfaces are the thing to use in Java.
C# has support for anonymous delegates - lambdas - and so events are the thing to use in C#.

A benefit of interfaces is that they are easier to apply decorators to. The standard example:
subject.RegisterObserver(new LoggingObserver(myRealObserver));
compared to:
subject.AnEvent += (sender, args) => { LogTheEvent(); realEventHandler(sender, args); };
(I'm a big fan of the decorator pattern).

If your objects will need to be serialized in some way that retains references such as with NetDataContractSerializer or perhaps protobuf events will not be able to cross the serialization boundary. Since observer pattern relies on nothing more than just object references, it can work with this type of serialization with no problem if that is what is desired.
Ex. You have a bunch of business objects that link to each other bidirectionally that you need to pass to a web service.