When why to use delegates

Question

I m relatively new in C    amp  I m wondering when to use Delegates appropriately  they are widely used in events declaration  but when should I use them in my own code and why are they useful  why not to use something else   I m also wondering when I have to use delegates and I have no other alternative   Thank you for the help   EDIT  I think I ve found a necessary use of Delegates here

User · Accepted Answer

I agree with everything that is said already, just trying to put some other words on it.

A delegate can be seen as a placeholder for a/some method(s).

By defining a delegate, you are saying to the user of your class, "Please feel free to assign, any method that matches this signature, to the delegate and it will be called each time my delegate is called".

Typical use is of course events. All the OnEventX delegate to the methods the user defines.

Delegates are useful to offer to the user of your objects some ability to customize their behavior. Most of the time, you can use other ways to achieve the same purpose and I do not believe you can ever be forced to create delegates. It is just the easiest way in some situations to get the thing done.

User · Answer

I ve just go my head around these  and so I ll share an example as you already have descriptions but at the moment one advantage I see is to get around the Circular Reference style warnings where you can t have 2 projects referencing each other   Let s assume an application downloads an XML  and then saves the XML to a database    I have 2 projects here which build my solution  FTP and a SaveDatabase   So  our application starts by looking for any downloads and downloading the file s  then  it calls the SaveDatabase project   Now  our application needs to notify the FTP site when a file is saved to the database by uploading a file with Meta data  ignore why  it s a request from the owner of the FTP site   The issue is at what point and how  We need a new method called NotifyFtpComplete    but in which of our projects should it be saved too - FTP or SaveDatabase  Logically  the code should live in our FTP project  But  this would mean our NotifyFtpComplete will have to be triggered or  it will have to wait until the save is complete  and then query the database to ensure it is in there  What we need to do is tell our SaveDatabase project to call the NotifyFtpComplete   method direct but we can t  we d get a ciruclar reference and the NotifyFtpComplete   is a private method  What a shame  this would have worked  Well  it can    During our application s code  we would have passed parameters between methods  but what if one of those parameters was the NotifyFtpComplete method  Yup  we pass the method  with all of the code inside as well  This would mean we could execute the method at any point  from any project  Well  this is what the delegate is  This means  we can pass the NotifyFtpComplete   method as a parameter to our SaveDatabase   class  At the point it saves  it simply executes the delegate    See if this crude example helps  pseudo code   We will also assume that the application starts with the Begin   method of the FTP class   class FTP       public void Begin                 string filePath   DownloadFileFromFtpAndReturnPathName             SaveDatabase sd   new SaveDatabase            sd Begin filePath  NotifyFtpComplete                private void NotifyFtpComplete                   Code to send file to FTP site           class SaveDatabase       private void Begin string filePath  delegateType NotifyJobComplete                  SaveToTheDatabase filePath               InvokeTheDelegate -             here we can execute the NotifyJobComplete            method at our preferred moment in the application             despite the method being private and belonging            to a different class                      NotifyJobComplete Invoke              So  with that explained  we can do it for real now with this Console Application using C   using System   namespace ConsoleApplication1          I ve made this class private to demonstrate that        the SaveToDatabase cannot have any knowledge of this Program class             class Program               static void Main string   args                          Note  this NotifyDelegate type is defined in the SaveToDatabase project             NotifyDelegate nofityDelegate   new NotifyDelegate NotifyIfComplete                SaveToDatabase sd   new SaveToDatabase                            sd Start nofityDelegate               Console ReadKey                          this is the method which will be delegated -            the only thing it has in common with the NofityDelegate            is that it takes 0 parameters and that it returns void             However  it is these 2 which are essential             It is really important to notice that it writes            a variable which  due to no constructor             has not yet been called  so  notice is not initialized yet                    private static void NotifyIfComplete                 Console WriteLine  notice              private static string  notice    Notified               public class SaveToDatabase               public void Start NotifyDelegate nd                           I shouldn t write to the console from here                  just for demonstration purposes                             Console WriteLine  SaveToDatabase Complete                Console WriteLine                   nd Invoke                        public delegate void NotifyDelegate        I suggest you step through the code and see when  notice is called and when the method  delegate  is called as this  I hope  will make things very clear   However  lastly  we can make it more useful by changing the delegate type to include a parameter   using System Text   namespace ConsoleApplication1          I ve made this class private to demonstrate that the SaveToDatabase        cannot have any knowledge of this Program class              class Program               static void Main string   args                        SaveToDatabase sd   new SaveToDatabase                   Please note  that although NotifyIfComplete              takes a string parameter  we do not declare it             all we want to do is tell C  where the method is            so it can be referenced later             we will pass the parameter later                          var notifyDelegateWithMessage   new NotifyDelegateWithMessage NotifyIfComplete                sd Start notifyDelegateWithMessage                 Console ReadKey                       private static void NotifyIfComplete string message                        Console WriteLine message                         public class SaveToDatabase               public void Start NotifyDelegateWithMessage nd                                       To simulate a saving fail or success  I m just going            to check the current time  well  the seconds  and            store the value as variable                          string message   string Empty              if  DateTime Now Second  gt  30                  message    Saved               else                 message    Failed                  It is at this point we pass the parameter to our method              nd Invoke message                        public delegate void NotifyDelegateWithMessage string message

User · Answer

A delegate is a simple class that is used to point to methods with a specific signature  becoming essentially a type-safe function pointer  A delegate s purpose is to facilitate a call back to another method  or methods   after one has been completed  in a structured way   While it could be possible to create an extensive set of code to perform this functionality  you don   t need too  You can use a delegate   Creating a delegate is easy to do  Identify the class as a delegate with the  delegate  keyword  Then specify the signature of the type

User · Answer

Delegates Overview Delegates have the following properties   Delegates are similar to C   function pointers  but are type safe  Delegates allow methods to be passed as parameters  Delegates can be used to define callback methods  Delegates can be chained together  for example  multiple methods can be  called on a single event  Methods don t need to match the delegate signature exactly  For more information  see Covariance and Contra variance  C  version 2 0 introduces the concept of Anonymous Methods  which permit code blocks to be passed as parameters in place of a separately defined method

User · Answer

I consider delegates to be Anonymous Interfaces  In many cases you can use them whenever you need an interface with a single method  but you don t want the overhead of defining that interface

User · Answer

A delegate is a reference to a method  Whereas objects can easily be sent as parameters into methods  constructor or whatever  methods are a bit more tricky  But every once in a while you might feel the need to send a method as a parameter to another method  and that s when you ll need delegates    using System  using System Collections Generic  using System Linq  using System Text   namespace DelegateApp           lt summary gt        A class to define a person        lt  summary gt    public class Person       public string Name   get  set        public int Age   get  set           class Program         Our delegate     public delegate bool FilterDelegate Person p        static void Main string   args             Create 4 Person objects       Person p1   new Person     Name    John   Age   41          Person p2   new Person     Name    Jane   Age   69          Person p3   new Person     Name    Jake   Age   12          Person p4   new Person     Name    Jessie   Age   25             Create a list of Person objects and fill it       List lt Person gt  people   new List lt Person gt      p1  p2  p3  p4             Invoke DisplayPeople using appropriate delegate       DisplayPeople  Children    people  IsChild         DisplayPeople  Adults    people  IsAdult         DisplayPeople  Seniors    people  IsSenior          Console Read                    lt summary gt          A method to filter out the people you need          lt  summary gt           lt param name  people  gt A list of people lt  param gt           lt param name  filter  gt A filter lt  param gt           lt returns gt A filtered list lt  returns gt      static void DisplayPeople string title  List lt Person gt  people  FilterDelegate filter          Console WriteLine title          foreach  Person p in people            if  filter p               Console WriteLine   0    1  years old   p Name  p Age                            Console Write   n n                           FILTERS                        static bool IsChild Person p          return p Age  lt  18             static bool IsAdult Person p          return p Age  gt   18             static bool IsSenior Person p          return p Age  gt   65                Output   Children  Jake  12 years old   Adults  John  41 years old Jane  69 years old Jessie  25 years old   Seniors  Jane  69 years old

User · Answer

Delegates are extremely useful when wanting to declare a block of code that you want to pass around  For example when using a generic retry mechanism   Pseudo   function Retry Delegate func  int numberOfTimes      try              func Invoke              catch   if numberOfTimes blabla  func Invoke    etc  etc      Or when you want to do late evaluation of code blocks  like a function where you have some Transform action  and want to have a BeforeTransform and an AfterTransform action that you can evaluate within your Transform function  without having to know whether the BeginTransform is filled  or what it has to transform   And of course when creating event handlers  You don t want to evaluate the code now  but only when needed  so you register a delegate that can be invoked when the event occurs

User · Answer

Say you want to write a procedure to integrate some real-valued function f  x  over some interval  a  b   Say we want to use the 3-Point Gaussian method to do this  any will do  of course    Ideally we want some function that looks like       f  is the integrand we want to integrate over  a  b  with  n  subintervals  static double Gauss3 Integrand f  double a  double b  int n      double res   0        compute result             return res      So we can pass in any Integrand  f  and get its definite integral over the closed interval   Just what type should Integrand be    Without Delegates  Well  without delegates  we d need some sort of interface with a single method  say eval declared as follows      Interface describing real-valued functions of one variable  interface Integrand     double eval double x       Then we d need to create a whole bunch of classes implementing this interface  as follows      Some function class MyFunc1   Integrand     public double eval double x        return    some result                Some other function class MyFunc2   Integrand     public double eval double x        return    some result                etc   Then to use them in our Gauss3 method  we need to invoke it as follows   double res1   Gauss3 new MyFunc1    -1  1  16   double res2   Gauss3 new MyFunc2    0  Math PI  16     And Gauss3 needs to do the look like the following   static double Gauss3 Integrand f  double a  double b  int n         Use the integrand passed in    f eval x       So we need to do all that just to use our arbitrary functions in Guass3   With Delegates  public delegate double Integrand double x     Now we can define some static  or not  functions adhering to that prototype   class Program      public delegate double Integrand double x            Define implementations to above delegate        with similar input and output types    static double MyFunc1 double x                   static double MyFunc2 double x                          etc         public static double Gauss3 Integrand f                   Now just call the function naturally  no f eval   stuff        double a   f x                             Let s use it    static void Main             Just pass the function in naturally  well  its reference        double res   Gauss3 MyFunc1  a  b  n        double res   Gauss3 MyFunc2  a  b  n                No interfaces  no clunky  eval stuff  no object instantiation  just simple function-pointer like usage  for a simple task   Of course  delegates are more than just function pointers under the hood  but that s a separate issue  function chaining and events

[c#] When & why to use delegates?

Examples related to c#

Examples related to .net

Examples related to delegates