- Delegates and Events
- Your Answer
- How to: Subscribe to and Unsubscribe from Events - C# Programming Guide | Microsoft Docs
Now we know how a delegate type is declared and what it contains, let's look at how to create an instance of such a type, and what we can do with it. My article on closures talks about the features of C 2. By concentrating on the explicit manner of creating instances in C 1. When you understand the basics, it's clearly worth knowing the features these later versions provide - but if you try to use them without having a firm grasp on the basics, you may well get confused.
Delegates and Events
As mentioned earlier, the key points of data in any particular delegate instance are the method the delegate refers to, and a reference to call the method on the target. For static methods, no target is required. The CLR itself supports other slightly different forms of delegate, where either the first argument passed to a static method is held within the delegate, or the target of an instance method is provided as an argument when the method is called.
See the documentation for System. Delegate for more information on this if you're interested, but don't worry too much about it. So, now that we know the two pieces of data required to create an instance along with the type itself, of course , how do we tell the compiler what they are? We use what the C specification calls a delegate-creation-expression which is of the form new delegate-type expression. The expression must either be another delegate of the same type or a compatible delegate type in C 2.
Creating copies of a delegate is fairly rare, so we will concentrate on the more common form. A few examples are listed below:. The constructor we mentioned earlier has two parameters - an object and an IntPtr. The object is a reference to the target or null for static methods and the IntPtr is a pointer to the method itself. One point to note is that delegate instances can refer to methods and targets which wouldn't normally be visible at the point the call is actually made. For instance, a private method can be used to create a delegate instance, and then the delegate instance can be returned from a public member.
Alternatively, the target of an instance may be an object which the eventual caller knows nothing about. However, both the target and the method must be accessible to the creating code. In other words, if and only if you can call a particular method on a particular object, you can use that method and target for delegate creation. Access rights are effectively ignored at call time.
- Lesson 14: Introduction to Delegates and Events.
- Declaring Events.
- c# - Event subscription from nested classes - Software Engineering Stack Exchange?
Delegate instances are called just as if they were the methods themselves. For instance, to call the delegate referred to by variable d1 above, we could write:. The method referred to by the delegate instance is called on the target object if there is one , and the result is returned. Producing a complete program to demonstrate this without including a lot of seemingly irrelevant code is tricky.
However, here's a program which gives one example of a static method and one of an instance method. StaticMethod could be written as just StaticMethod in the same way that within an instance method you could write InstanceMethod instead of this. InstanceMethod - I've included the class name just to make it clear how you would reference methods from other classes. The C syntax is just a short-hand for calling the Invoke method provided by each delegate type. These are explained later. Delegates can be combined such that when you call the delegate, a whole list of methods are called - potentially with different targets.
When I said before that a delegate contained a target and a method, that was a slight simplification. That's what a delegate instance representing one method contains. For the sake of clarity, I'll refer to such delegate instances as simple delegates. The alternative is a delegate instance which is effectively a list of simple delegates, all of the same type i. I'll call these combined delegates.
Combined delegates can themselves be combined together, effectively creating one big list of simple delegates in the obvious fashion. It's important to understand that delegate instances are always immutable. This is just like strings: PadLeft for instance, it doesn't actually change the string you call it on - it just returns a new string with the appropriate padding.
Combining two delegate instances is usually done using the addition operator, as if the delegate instances were strings or numbers. Subtracting one from another is usually done with the subtraction operator.
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Note that when you subtract one combined delegate from another, the subtraction works in terms of lists. If the list to subtract is not found in the original list, the result is just the original list. Otherwise, the last occurrence of the list is removed.
This is best shown with some examples. Instead of actual code, the following uses lists of simple delegates d1 , d2 etc. For instance, [d1, d2, d3] is a combined delegate which, when executed, would call d1 then d2 then d3. An empty list is represented by null rather than an actual delegate instance. Delegate instances can also be combined with the static Delegate. Combine method, and one can be subtracted from another with the static Delegate.
The C compiler converts the addition and subtraction operators into calls to these methods. Because they are static methods, they work easily with null references. The addition and subtraction operators always work as part of assignment: Again, the original delegate instance remains unchanged; the value of d1 just changes to be a reference to the appropriate new combined delegate.
If a delegate type is declared to return a value i. It's unfortunate in some ways that C lets you use them in the same way in certain situations, but it's very important that you understand the difference. I find the easiest way to understand events is to think of them a bit like properties. While properties look like they're fields, they're definitely not - and you can write properties which don't use fields at all.
Similarly, while events look like delegate instances in terms of the way you express the add and remove operations, they're not. Events are pairs of methods, appropriately decorated in IL to tie them together and let languages know that the methods represent events. The methods correspond to add and remove operations, each of which take a delegate instance parameter of the same type the type of the event. What you do with those operations is pretty much up to you, but the typical use is to add or remove the delegate from a list of handlers for the event.
When the event is triggered whatever that trigger might be - a button click, a timeout, an unhandled exception the handlers are called in turn. Note that in C , the calling of the event handlers is not part of the event itself. Click or a type name e. Static events are relatively rare. Events themselves can be declared in two ways. The first is with explicit add and remove methods, declared in a very similar way to properties, but with the event keyword. Here's an example of an event for the System. Note how it doesn't actually do anything with the delegate instances which are passed to the add and remove methods - it just prints out which operation has been called.
Note that the remove operation is called even though we've told it to remove null. Although it would be very rare to ignore the value in this way, there are times when you don't want to back an event with a simple delegate variable. For instance, in situations where there are lots of events but only a few are likely to be subscribed to, you could have a map from some key describing the event to the delegate currently handling it.
This is what Windows Forms does - it means that you can have a huge number of events without wasting a lot of memory with variables which will usually just have null values. C provides a simple way of declaring both a delegate variable and an event at the same time. This is called a field-like event , and is declared very simply - it's the same as the "longhand" event declaration, but without the "body" part:.
How to: Subscribe to and Unsubscribe from Events - C# Programming Guide | Microsoft Docs
This creates a delegate variable and an event, both with the same type. The access to the event is determined by the event declaration so the example above creates a public event, for instance but the delegate variable is always private. That's for an instance member. For an event declared as static, the variable is also static and a lock is taken out on typeof XXX where XXX is the name of the class declaring the event. Note that this only holds for class events, not struct events - there are issues in terms of locking with struct events; in practice I don't remember ever seeing a struct with any events.
None of this is actually as useful as you might think - see the threading section for more details. So, what happens when you refer to MyEvent in code? In all other contexts, the compiler generates code which refers to the event. Now we know what they are, what's the point of having both delegates and events? The answer is encapsulation. How would another class subscribe to an event?
Option 1 is clearly horrible, for all the normal reasons we abhor public variables. Option 2 is slightly better, but allows subscribers to effectively override each other - it would be all too easy to write someInstance.
In addition, you still need to write the properties. Option 3 is basically what events give you, but with a guaranteed convention generated by the compiler and backed by extra flags in the IL and a "free" implementation if you're happy with the semantics that field-like events give you. Subscribing to and unsubscribing from events is encapsulated without allowing arbitrary access to the list of event handlers, and languages can make things simpler by providing syntax for both declaration and subscription.
During previous lessons, you learned how to implement reference types using language constructs such as classes and interfaces. These reference types allowed you to create instances of objects and use them in special ways to accomplish your software development goals. Classes allow you to create objects that contained members with attributes or behavior. Interfaces allow you to declare a set of attributes and behavior that all objects implementing them would publicly expose.
A delegate is a C language element that allows you to reference a method. The answer boils down to giving you maximum flexibility to implement any functionality you want at runtime. Think about how you use methods right now. You write an algorithm that does its thing by manipulating the values of variables and calling methods directly by name. What if you wanted an algorithm that was very flexible, reusable, and allowed you to implement different functionality as the need arises? Another alternative would be for all the types to implement an interface that declared a common method your algorithm would call, which is actually a nice solution.
You could solve this problem by passing a delegate to your algorithm and letting the contained method, which the delegate refers to, perform the comparison operation. Such an operation is performed in Listing The first thing the program in this C delegate example from Listing does is declare a delegate. Delegate declarations look somewhat like methods, except they have the delegate modifier, are terminated with a semi-colon ; , and have no implementation.
Below, is the delegate declaration from Listing This delegate declaration defines the signature of a delegate handler method that this delegate can refer to. The delegate handler method, for the Comparer delegate , can have any name but must have the first parameter of type object , the second parameter of type object , and return an int type. The CompareFirstNames method calls String. Compare to compare the FirstName properties of the two Name instances. What you should concentrate on is that CompareFirstNames is a handler method that a delegate can refer to, regardless of the code inside of that method.
To use a delegate , you must create an instance of it. The instance is created, similar to a class instance, with a single parameter identifying the appropriate delegate handler method, as shown below. The delegate , CMP , is then used as a parameter to the Sort method, which uses it just like a normal method.
Using this technique, any delegate handler method may be passed to the Sort method at run-time. You could define a method handler named CompareLastNames , instantiate a new Comparer delegate instance with it, and pass the new delegate to the Sort method. Traditional Console applications operate by waiting for a user to press a key or type a command and press the Enter key.
Then they perform some pre-defined operation and either quit or return to the original prompt that they started from. This works but is inflexible in that everything is hard-wired and follows a rigid path of execution. In stark contrast, modern GUI programs operate on an event-based model.
That is, some event in the system occurs and interested modules are notified so they can react appropriately. It is all built into the system with events.