I am confused in deciding whether I should use
a polymorphic (via overriding the virtual method A) method.
a delegate-type-parameterized method B.
an event C.
an instance of a class implementing an interface with a single method D. A Java's trick!
when writing callbacks.
using System;
namespace CallBack
{
interface IOptional
{
void D();
}
class Base
{
protected virtual void A() => Console.WriteLine("Base's extra jobs.");
public void Do(Action B = null, IOptional optional = null)
{
Console.WriteLine("Base's main jobs.");
// and call the optional jobs
A();
B?.Invoke();
C?.Invoke();
optional?.D();
}
public event Action C;
}
class Derived : Base
{
protected override void A()
{
base.A();
Console.WriteLine("Derived's extra jobs.");
}
}
class Optional : IOptional
{
public void D()
{
Console.WriteLine("D");
}
}
class Test
{
static void Main()
{
Derived d = new Derived();
d.C += () => Console.WriteLine("C");
d.Do(() => Console.WriteLine("B"), new Optional());
}
}
}
Question
Is there any commonly used guideline for agile programmers?
The use cases of the three seem quite distinct to me :)
The core idea is "who do you want to let know about the event".
If you want to allow everyone to be able to subscribe to it, use an event. This is also what most of .NET that I have experience with deal with callbacks.
By using the polymorphism approach, you only allow subclasses to know about the event. If some other object wants to do something when it happens, it can't, because it's not a subclass and can't override the method.
By using the delegate parameter approach, you limit the number of subscribers to 1. Any caller can subscribe to the event, but subscriber-event is now a one-to-one relationship.
In C++ we have the std::function type which can be used to wrap lambdas, functions and even custom classes. Here is an example of a custom class wrapped in std::function:
#include <functional>
struct A {
void operator()() {
}
};
std::function<void()> a = A();
I though the same was possible with delegates in C# but, I can not get it to work:
class Program
{
delegate void Foo();
class Bar
{
public void Invoke()
{
}
}
static void Main()
{
new Foo(new Bar()); // CS0149: Method name expected
}
}
I am convinced that this has to be possible somehow, because the delegate operator internally creates a new class that inherits from System.Delegate which suggests that I can create my own type to do the same somehow.
Simple answer is 'no' because C# doesn't have an operator() in the way C++ does.
However, a lambda can be used to store not just a function, but a function on a specific object.
class Program
{
delegate void Foo();
class Bar
{
public void Invoke()
{
}
}
static void Main()
{
var f = new Foo(new Bar().Invoke);
}
}
The difference is simply that in C# you have to specify the method name, rather than there being a default.
Also worth noting that similarly to C++, C# has generics to help this, so we can cut out the Foo declaration altogether:
var f = new Action(new Bar().Invoke);
I have a C++ function which performs a number of tasks "PerformJob()". I have a C# wrapper which calls PerformJob(). The job takes a while and I would like the C++ method to send "status updates" back up to the calling C# class. The C++ code is not exposed to the C# class. Adding the C# project as a reference would cause a circular dependency.
I've attempted to pass a delegate through as a parameter but I'm not familiar enough with C++ syntax to make this work (or if it is even possible?). Is there an appropriate way to pass a delegate into C++ as a parameter? Is there a better method to facilitate this communication? I'd like to avoid a dllimport, as I only need to receive updates from this one class.
CSharpClass.cs:
public delegate void CallbackDelegate(ref string status);
public CallbackDelegate jobStatusDelegate;
public void UpdateJobStatus(ref string status)
{
Job.JobStatus = status;
}
public void StartJob()
{
jobStatusDelegate = new CallbackDelegate(UpdateJobStatus);
CPlusClass jobHelper = new CPlusClass();
jobHelper.PerformJob(jobStatusDelegate);
}
CPlusClass.h:
public ref class CPlusClass
{
public:
void PerformJob(delegate del); // is there c++ delegate type?
};
CPlusClass.cpp:
void CPlusClass::PerformJob(delegate del)
{
// ....
}
C++ doesn't have delegates, but C++/CLI does, and based on ref class your code already is C++/CLI.
Delegates work much like any other reference type, they get stored as a tracking handle with the ^.
In order to not create a dependency on the C#, which would be in the wrong direction, I suggest you use a System::Action<System::String^>^ instead of defining your own delegate type.
public ref class CPlusClass
{
public:
void PerformJob(System::Action<System::String^>^ del)
{
del->Invoke(gcnew String("Hello World"));
}
};
I am a Java programmer trying to transition to C# and I'm hoping there's a way to do something in C# that I'm accustomed to in Java: overriding a method in the declaration of an abstract object like so:
//This is the way I do it in Java and want to do in C#
Keyword k = new Keyword("quit"){
public abstract void do(String context){
//TODO Do stuff
}
};
This is for some text game stuff I've been doing for a while in Java. I've looked into abstract and virtual and anonymous classes but none of them do exactly this. Abstract and virtual want me to create a whole new subclass, but this would be time consuming and unfeasible on a large scale. Anonymous classes don't (as far as I can tell) enable me to override methods, just fields and don't provide any stabilization for me to rely on.
If there is a way to do this or something similar please explain. Thanks for your time.
That doesn't work in C#. You'll have to create a new class that inherits from Keyword.
public class MyKeyword : Keyword
{
public MyKeyword(string s) : base(s)
{ }
public override void do(string context)
{
// TODO: Do stuff.
}
}
Anonymous Types in C# aren't classes that you can provide any public methods for. They only have properties, and are intended to be a quick, intra-method way of pasing complex data from one line to the next.
To be honest, I didn't know you could do what you show in Java. That is, if I'm understanding it as kind of an in-line class derivation.
Brian Rasmussen mentions using a delegate. That would look something like this:
public delegate void DoSomething(string context);
public class Keyword
{
public DoSomething Do;
private void CallsDo()
{
if (Do != null) Do("some string");
}
}
Then you can assign to it:
Keyword k = new Keyword();
k.Do = (ctx) => { /* Do something with ctx string */ };
Delegates are probably what you are after.
You can utilize a delegate for this approach: Note the example
public class Keyword
{
public delegate void Do();
}
//Area of Execution
{
//...
Keyword k = new Keyword();
k.Do = delegate()
{
Console.Writeln("Anonymous Inner function assigned to a callback function i.e a Delegate!");
};
}
These are much like function pointers in C/C++ but that may mean nothing to you depending on your background.
A delegate is, in the simplest terms, a type-safe object that encapsulates a method/function. What this means is that it maintains a reference to the method or methods and can invoke them later through the delegate object rather than explicitly on the method(s) themselves. You can assign an anonymous function to the right hand side much the same as you can to a method in Java as you described.
hope this helps. Read more here for delegates in-depth
Delegates
What are the differences between delegates and an events? Don't both hold references to functions that can be executed?
An Event declaration adds a layer of abstraction and protection on the delegate instance. This protection prevents clients of the delegate from resetting the delegate and its invocation list and only allows adding or removing targets from the invocation list.
To understand the differences you can look at this 2 examples
Example with Delegates (in this case, an Action - that is a kind of delegate that doesn't return a value)
public class Animal
{
public Action Run {get; set;}
public void RaiseEvent()
{
if (Run != null)
{
Run();
}
}
}
To use the delegate, you should do something like this:
Animal animal= new Animal();
animal.Run += () => Console.WriteLine("I'm running");
animal.Run += () => Console.WriteLine("I'm still running") ;
animal.RaiseEvent();
This code works well but you could have some weak spots.
For example, if I write this:
animal.Run += () => Console.WriteLine("I'm running");
animal.Run += () => Console.WriteLine("I'm still running");
animal.Run = () => Console.WriteLine("I'm sleeping") ;
with the last line of code, I have overridden the previous behaviors just with one missing + (I have used = instead of +=)
Another weak spot is that every class which uses your Animal class can invoke the delegate directly. For example, animal.Run() or animal.Run.Invoke() are valid outside the Animal class.
To avoid these weak spots you can use events in c#.
Your Animal class will change in this way:
public class ArgsSpecial : EventArgs
{
public ArgsSpecial (string val)
{
Operation=val;
}
public string Operation {get; set;}
}
public class Animal
{
// Empty delegate. In this way you are sure that value is always != null
// because no one outside of the class can change it.
public event EventHandler<ArgsSpecial> Run = delegate{}
public void RaiseEvent()
{
Run(this, new ArgsSpecial("Run faster"));
}
}
to call events
Animal animal= new Animal();
animal.Run += (sender, e) => Console.WriteLine("I'm running. My value is {0}", e.Operation);
animal.RaiseEvent();
Differences:
You aren't using a public property but a public field (using events, the compiler protects your fields from unwanted access)
Events can't be assigned directly. In this case, it won't give rise to the previous error that I have showed with overriding the behavior.
No one outside of your class can raise or invoke the event. For example, animal.Run() or animal.Run.Invoke() are invalid outside the Animal class and will produce compiler errors.
Events can be included in an interface declaration, whereas a field cannot
Notes:
EventHandler is declared as the following delegate:
public delegate void EventHandler (object sender, EventArgs e)
it takes a sender (of Object type) and event arguments. The sender is null if it comes from static methods.
This example, which uses EventHandler<ArgsSpecial>, can also be written using EventHandler instead.
Refer here for documentation about EventHandler
In addition to the syntactic and operational properties, there's also a semantical difference.
Delegates are, conceptually, function templates; that is, they express a contract a function must adhere to in order to be considered of the "type" of the delegate.
Events represent ... well, events. They are intended to alert someone when something happens and yes, they adhere to a delegate definition but they're not the same thing.
Even if they were exactly the same thing (syntactically and in the IL code) there will still remain the semantical difference. In general I prefer to have two different names for two different concepts, even if they are implemented in the same way (which doesn't mean I like to have the same code twice).
Here is another good link to refer to.
http://csharpindepth.com/Articles/Chapter2/Events.aspx
Briefly, the take away from the article - Events are encapsulation over delegates.
Quote from article:
Suppose events didn't exist as a concept in C#/.NET. How would another class subscribe to an event? Three options:
A public delegate variable
A delegate variable backed by a property
A delegate variable with AddXXXHandler and RemoveXXXHandler methods
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.MyEvent = eventHandler; which would replace any existing event handlers rather than adding a new one. 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.
What a great misunderstanding between events and delegates!!! A delegate specifies a TYPE (such as a class, or an interface does), whereas an event is just a kind of MEMBER (such as fields, properties, etc). And, just like any other kind of member an event also has a type. Yet, in the case of an event, the type of the event must be specified by a delegate. For instance, you CANNOT declare an event of a type defined by an interface.
Concluding, we can make the following Observation: the type of an event MUST be defined by a delegate. This is the main relation between an event and a delegate and is described in the section II.18 Defining events of ECMA-335 (CLI) Partitions I to VI:
In typical usage, the TypeSpec (if present) identifies a delegate whose signature matches the arguments passed to the event’s fire method.
However, this fact does NOT imply that an event uses a backing delegate field. In truth, an event may use a backing field of any different data structure type of your choice. If you implement an event explicitly in C#, you are free to choose the way you store the event handlers (note that event handlers are instances of the type of the event, which in turn is mandatorily a delegate type---from the previous Observation). But, you can store those event handlers (which are delegate instances) in a data structure such as a List or a Dictionary or any other else, or even in a backing delegate field. But don’t forget that it is NOT mandatory that you use a delegate field.
NOTE: If you have access to C# 5.0 Unleashed, read the "Limitations on Plain Use of Delegates" in Chapter 18 titled "Events" to understand better the differences between the two.
It always helps me to have a simple, concrete example. So here's one for the community. First I show how you can use delegates alone to do what Events do for us. Then I show how the same solution would work with an instance of EventHandler. And then I explain why we DON'T want to do what I explain in the first example. This post was inspired by an article by John Skeet.
Example 1: Using public delegate
Suppose I have a WinForms app with a single drop-down box. The drop-down is bound to an List<Person>. Where Person has properties of Id, Name, NickName, HairColor. On the main form is a custom user control that shows the properties of that person. When someone selects a person in the drop-down the labels in the user control update to show the properties of the person selected.
Here is how that works. We have three files that help us put this together:
Mediator.cs -- static class holds the delegates
Form1.cs -- main form
DetailView.cs -- user control shows all details
Here is the relevant code for each of the classes:
class Mediator
{
public delegate void PersonChangedDelegate(Person p); //delegate type definition
public static PersonChangedDelegate PersonChangedDel; //delegate instance. Detail view will "subscribe" to this.
public static void OnPersonChanged(Person p) //Form1 will call this when the drop-down changes.
{
if (PersonChangedDel != null)
{
PersonChangedDel(p);
}
}
}
Here is our user control:
public partial class DetailView : UserControl
{
public DetailView()
{
InitializeComponent();
Mediator.PersonChangedDel += DetailView_PersonChanged;
}
void DetailView_PersonChanged(Person p)
{
BindData(p);
}
public void BindData(Person p)
{
lblPersonHairColor.Text = p.HairColor;
lblPersonId.Text = p.IdPerson.ToString();
lblPersonName.Text = p.Name;
lblPersonNickName.Text = p.NickName;
}
}
Finally we have the following code in our Form1.cs. Here we are Calling OnPersonChanged, which calls any code subscribed to the delegate.
private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
{
Mediator.OnPersonChanged((Person)comboBox1.SelectedItem); //Call the mediator's OnPersonChanged method. This will in turn call all the methods assigned (i.e. subscribed to) to the delegate -- in this case `DetailView_PersonChanged`.
}
Ok. So that's how you would get this working without using events and just using delegates. We just put a public delegate into a class -- you can make it static or a singleton, or whatever. Great.
BUT, BUT, BUT, we do not want to do what I just described above. Because public fields are bad for many, many reason. So what are our options? As John Skeet describes, here are our options:
A public delegate variable (this is what we just did above. don't do this. i just told you above why it's bad)
Put the delegate into a property with a get/set (problem here is that subscribers could override each other -- so we could subscribe a bunch of methods to the delegate and then we could accidentally say PersonChangedDel = null, wiping out all of the other subscriptions. The other problem that remains here is that since the users have access to the delegate, they can invoke the targets in the invocation list -- we don't want external users having access to when to raise our events.
A delegate variable with AddXXXHandler and RemoveXXXHandler methods
This third option is essentially what an event gives us. When we declare an EventHandler, it gives us access to a delegate -- not publicly, not as a property, but as this thing we call an event that has just add/remove accessors.
Let's see what the same program looks like, but now using an Event instead of the public delegate (I've also changed our Mediator to a singleton):
Example 2: With EventHandler instead of a public delegate
Mediator:
class Mediator
{
private static readonly Mediator _Instance = new Mediator();
private Mediator() { }
public static Mediator GetInstance()
{
return _Instance;
}
public event EventHandler<PersonChangedEventArgs> PersonChanged; //this is just a property we expose to add items to the delegate.
public void OnPersonChanged(object sender, Person p)
{
var personChangedDelegate = PersonChanged as EventHandler<PersonChangedEventArgs>;
if (personChangedDelegate != null)
{
personChangedDelegate(sender, new PersonChangedEventArgs() { Person = p });
}
}
}
Notice that if you F12 on the EventHandler, it will show you the definition is just a generic-ified delegate with the extra "sender" object:
public delegate void EventHandler<TEventArgs>(object sender, TEventArgs e);
The User Control:
public partial class DetailView : UserControl
{
public DetailView()
{
InitializeComponent();
Mediator.GetInstance().PersonChanged += DetailView_PersonChanged;
}
void DetailView_PersonChanged(object sender, PersonChangedEventArgs e)
{
BindData(e.Person);
}
public void BindData(Person p)
{
lblPersonHairColor.Text = p.HairColor;
lblPersonId.Text = p.IdPerson.ToString();
lblPersonName.Text = p.Name;
lblPersonNickName.Text = p.NickName;
}
}
Finally, here's the Form1.cs code:
private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
{
Mediator.GetInstance().OnPersonChanged(this, (Person)comboBox1.SelectedItem);
}
Because the EventHandler wants and EventArgs as a parameter, I created this class with just a single property in it:
class PersonChangedEventArgs
{
public Person Person { get; set; }
}
Hopefully that shows you a bit about why we have events and how they are different -- but functionally the same -- as delegates.
You can also use events in interface declarations, not so for delegates.
Delegate is a type-safe function pointer. Event is an implementation of publisher-subscriber design pattern using delegate.
An event in .net is a designated combination of an Add method and a Remove method, both of which expect some particular type of delegate. Both C# and vb.net can auto-generate code for the add and remove methods which will define a delegate to hold the event subscriptions, and add/remove the passed in delegagte to/from that subscription delegate. VB.net will also auto-generate code (with the RaiseEvent statement) to invoke the subscription list if and only if it is non-empty; for some reason, C# doesn't generate the latter.
Note that while it is common to manage event subscriptions using a multicast delegate, that is not the only means of doing so. From a public perspective, a would-be event subscriber needs to know how to let an object know it wants to receive events, but it does not need to know what mechanism the publisher will use to raise the events. Note also that while whoever defined the event data structure in .net apparently thought there should be a public means of raising them, neither C# nor vb.net makes use of that feature.
To define about event in simple way:
Event is a REFERENCE to a delegate with two restrictions
Cannot be invoked directly
Cannot be assigned values directly (e.g eventObj = delegateMethod)
Above two are the weak points for delegates and it is addressed in event. Complete code sample to show the difference in fiddler is here https://dotnetfiddle.net/5iR3fB .
Toggle the comment between Event and Delegate and client code that invokes/assign values to delegate to understand the difference
Here is the inline code.
/*
This is working program in Visual Studio. It is not running in fiddler because of infinite loop in code.
This code demonstrates the difference between event and delegate
Event is an delegate reference with two restrictions for increased protection
1. Cannot be invoked directly
2. Cannot assign value to delegate reference directly
Toggle between Event vs Delegate in the code by commenting/un commenting the relevant lines
*/
public class RoomTemperatureController
{
private int _roomTemperature = 25;//Default/Starting room Temperature
private bool _isAirConditionTurnedOn = false;//Default AC is Off
private bool _isHeatTurnedOn = false;//Default Heat is Off
private bool _tempSimulator = false;
public delegate void OnRoomTemperatureChange(int roomTemperature); //OnRoomTemperatureChange is a type of Delegate (Check next line for proof)
// public OnRoomTemperatureChange WhenRoomTemperatureChange;// { get; set; }//Exposing the delegate to outside world, cannot directly expose the delegate (line above),
public event OnRoomTemperatureChange WhenRoomTemperatureChange;// { get; set; }//Exposing the delegate to outside world, cannot directly expose the delegate (line above),
public RoomTemperatureController()
{
WhenRoomTemperatureChange += InternalRoomTemperatuerHandler;
}
private void InternalRoomTemperatuerHandler(int roomTemp)
{
System.Console.WriteLine("Internal Room Temperature Handler - Mandatory to handle/ Should not be removed by external consumer of ths class: Note, if it is delegate this can be removed, if event cannot be removed");
}
//User cannot directly asign values to delegate (e.g. roomTempControllerObj.OnRoomTemperatureChange = delegateMethod (System will throw error)
public bool TurnRoomTeperatureSimulator
{
set
{
_tempSimulator = value;
if (value)
{
SimulateRoomTemperature(); //Turn on Simulator
}
}
get { return _tempSimulator; }
}
public void TurnAirCondition(bool val)
{
_isAirConditionTurnedOn = val;
_isHeatTurnedOn = !val;//Binary switch If Heat is ON - AC will turned off automatically (binary)
System.Console.WriteLine("Aircondition :" + _isAirConditionTurnedOn);
System.Console.WriteLine("Heat :" + _isHeatTurnedOn);
}
public void TurnHeat(bool val)
{
_isHeatTurnedOn = val;
_isAirConditionTurnedOn = !val;//Binary switch If Heat is ON - AC will turned off automatically (binary)
System.Console.WriteLine("Aircondition :" + _isAirConditionTurnedOn);
System.Console.WriteLine("Heat :" + _isHeatTurnedOn);
}
public async void SimulateRoomTemperature()
{
while (_tempSimulator)
{
if (_isAirConditionTurnedOn)
_roomTemperature--;//Decrease Room Temperature if AC is turned On
if (_isHeatTurnedOn)
_roomTemperature++;//Decrease Room Temperature if AC is turned On
System.Console.WriteLine("Temperature :" + _roomTemperature);
if (WhenRoomTemperatureChange != null)
WhenRoomTemperatureChange(_roomTemperature);
System.Threading.Thread.Sleep(500);//Every second Temperature changes based on AC/Heat Status
}
}
}
public class MySweetHome
{
RoomTemperatureController roomController = null;
public MySweetHome()
{
roomController = new RoomTemperatureController();
roomController.WhenRoomTemperatureChange += TurnHeatOrACBasedOnTemp;
//roomController.WhenRoomTemperatureChange = null; //Setting NULL to delegate reference is possible where as for Event it is not possible.
//roomController.WhenRoomTemperatureChange.DynamicInvoke();//Dynamic Invoke is possible for Delgate and not possible with Event
roomController.SimulateRoomTemperature();
System.Threading.Thread.Sleep(5000);
roomController.TurnAirCondition (true);
roomController.TurnRoomTeperatureSimulator = true;
}
public void TurnHeatOrACBasedOnTemp(int temp)
{
if (temp >= 30)
roomController.TurnAirCondition(true);
if (temp <= 15)
roomController.TurnHeat(true);
}
public static void Main(string []args)
{
MySweetHome home = new MySweetHome();
}
}
For people live in 2020, and want a clean answer...
Definitions:
delegate: defines a function pointer.
event: defines
(1) protected interfaces, and
(2) operations(+=, -=), and
(3) advantage: you don't need to use new keyword anymore.
Regarding the adjective protected:
// eventTest.SomeoneSay = null; // Compile Error.
// eventTest.SomeoneSay = new Say(SayHello); // Compile Error.
Also notice this section from Microsoft: https://learn.microsoft.com/en-us/dotnet/standard/events/#raising-multiple-events
Code Example:
with delegate:
public class DelegateTest
{
public delegate void Say(); // Define a pointer type "void <- ()" named "Say".
private Say say;
public DelegateTest() {
say = new Say(SayHello); // Setup the field, Say say, first.
say += new Say(SayGoodBye);
say.Invoke();
}
public void SayHello() { /* display "Hello World!" to your GUI. */ }
public void SayGoodBye() { /* display "Good bye!" to your GUI. */ }
}
with event:
public class EventTest
{
public delegate void Say();
public event Say SomeoneSay; // Use the type "Say" to define event, an
// auto-setup-everything-good field for you.
public EventTest() {
SomeoneSay += SayHello;
SomeoneSay += SayGoodBye;
SomeoneSay();
}
public void SayHello() { /* display "Hello World!" to your GUI. */ }
public void SayGoodBye() { /* display "Good bye!" to your GUI. */ }
}
Reference:
Event vs. Delegate - Explaining the important differences between the Event and Delegate patterns in C# and why they're useful.: https://dzone.com/articles/event-vs-delegate