delegate (CLI)

Code to declare a delegate type, named SendMessageDelegate, which takes a Message as a parameter and returns void:

delegate void SendMessageDelegate(Message message);

Code to define a method that takes an instantiated delegate as its argument:

void SendMessage(SendMessageDelegate sendMessageDelegateReference)

{

// Call the delegate and any other chained delegates synchronously.

sendMessageDelegateReference(new Message("hello this is a sample message"));

}

The implemented method that runs when the delegate is called:

void HandleSendMessage(Message message)

{

// The implementation for the Sender and Message classes are not relevant to this example.

Sender.Send(message);

}

Code to call the SendMessage method, passing an instantiated delegate as an argument:

SendMessage(new SendMessageDelegate(HandleSendMessage));

delegate void Notifier(string sender); // Normal method signature with the keyword delegate

Notifier greetMe; // Delegate variable

void HowAreYou(string sender) {

Console.WriteLine("How are you, " + sender + '?');

}

greetMe = new Notifier(HowAreYou);

A delegate variable calls the associated method and is called as follows:

greetMe("Anton"); // Calls HowAreYou("Anton") and prints "How are you, Anton?"

Delegate variables are first-class objects of the form {{C sharp|new DelegateType(obj.Method)}} and can be assigned to any matching method, or to the value {{C sharp|null}}. They store a method and its receiver without any parameters:{{cite web| access-date = 2011-08-04| date = 2002-03-25| first = Hanspeter| last = Mössenböck| pages = 23–24| publisher = Institut für Systemsoftware, Johannes Kepler Universität Linz, Fachbereich Informatik| title = Advanced C#: Variable Number of Parameters| url = http://ssw.jku.at/Teaching/Lectures/CSharp/Tutorial/Part2.pdf}}

new DelegateType(funnyObj.HowAreYou);

The object {{C sharp|funnyObj}} can be {{C sharp|this}} and omitted. If the method is {{C sharp|static}}, it should not be the object (also called an instance in other languages), but the class itself. It should not be {{C sharp|abstract}}, but could be {{C sharp|new}}, {{C sharp|override}} or {{C sharp|virtual}}.

To call a method with a delegate successfully, the method signature has to match the {{C sharp|DelegateType}} with the same number of parameters of the same kind ({{C sharp|ref}}, {{C sharp|out}}, {{C sharp|value}}) with the same type (including return type).

A delegate variable can hold multiple values at the same time:

void HowAreYou(string sender)

{

Console.WriteLine($"How are you, {sender}?");

}

void HowAreYouToday(string sender)

{

Console.WriteLine($"How are you today, {sender}?");

}

Notifier greetMe;

greetMe = HowAreYou;

greetMe += HowAreYouToday;

greetMe("Leonardo"); // "How are you, Leonardo?"

// "How are you today, Leonardo?"

greetMe -= HowAreYou;

greetMe("Pereira"); // "How are you today, Pereira?"

If the multicast delegate is a function or has no {{C sharp|out}} parameter, the parameter of the last call is returned.{{cite web| access-date = 2011-08-04| date = 2002-03-25| first = Hanspeter| last = Mössenböck| url = http://ssw.jku.at/Teaching/Lectures/CSharp/Tutorial/| page = 25| publisher = Institut für Systemsoftware, Johannes Kepler Universität Linz, Fachbereich Informatik| title = Advanced C#: Variable Number of Parameters

}}

Although internal implementations may vary, delegate instances can be thought of as a tuple of an object and a method pointer and a reference (possibly null) to another delegate. Hence a reference to one delegate is possibly a reference to multiple delegates. When the first delegate has finished, if its chain reference is not null, the next will be invoked, and so on until the list is complete. This pattern allows an event to have overhead scaling easily from that of a single reference up to dispatch to a list of delegates, and is widely used in the CLI.

Performance of delegates used to be much slower than a virtual or interface method call (6 to 8 times slower in Microsoft's 2003 benchmarks),{{cite web| url=http://msdn2.microsoft.com/en-us/library/ms973852| title=Writing Faster Managed Code: Know What Things Cost| publisher=Microsoft| last=Gray|first=Jan| date=June 2003| access-date=2007-09-09}} but, since the .NET 2.0 CLR in 2005, it is about the same as interface calls.{{cite web| url=http://www.sturmnet.org/blog/2005/09/01/delegate-interface-speed| title=Delegate calls vastly sped up in .NET 2

|last=Sturm|first=Oliver| date=2005-09-01| access-date=2007-09-09}} This means there is a small added overhead compared to direct method invocations.

There are very stringent rules on the construction of delegate classes. These rules permit optimizing compilers a great deal of leeway when optimizing delegates while ensuring type safety.{{Citation needed|date=July 2009}}

• Continuation
• Delegation pattern
• Delegation (programming)
• Hooking

{{reflist}}

• [http://msdn2.microsoft.com/en-us/library/system.delegate.aspx MSDN documentation for Delegates]
• [https://web.archive.org/web/20120627043929/http://java.sun.com/docs/white/delegates.html Sun's White Paper on Delegates]
• [http://msdn.microsoft.com/en-us/vjsharp/bb188664.aspx Microsoft answer to Sun]

{{DEFAULTSORT:Delegate}}

Category:Common Language Infrastructure

Category:Subroutines