Why Delegates ?

[sde]

Ive spent the last day trying to wrap my head around the concept of delegates. I finally understand how to use them ( somewhat ) , but I dont understand entirely why to use them.

Anyone have a link to a great explanation or tutorial on this ?

 

[TechnoTone]

One of the most obvious uses is on a wizard. You can change the delegate for the "Next" button as you progress through the wizards stages, for example.

 

[Joe Mamma]

There are two area that traditional VB programmers are going to have a hard time wrapping their heads around the .NET framework:

Delegates and reflection.

Delegates are basically method references. They are an extremely strong feature of any OOP language. They take some inginuity to use them effectively. They give the ability to dynamically specify application behavior.

I refer you to this gentlemans post in the databases thread:

Database record problems...no clue!

this code could be reduced to 3 lines

[edit]I removed some inappropriate comments. -Derek[/edit]

private void CheckedChangedHandler(Object sender, 
                                         EventArgs e)
{
   System.Windows.Forms.RadioButton btn = sender as
                                     System.Windows.Forms.RadioButton;
   if (btn != null)
      strSearch = btn.text;
   }
}

and in the form load event,for each radio button do the following

  selectOption1.CheckedChanged += 
                               CheckedChangedHandler(EventArgs.Empty);
  selectOption2.CheckedChanged +=
                               CheckedChangedHandler(EventArgs.Empty);
  selectOption3.CheckedChanged +=
                               CheckedChangedHandler(EventArgs.Empty);
  selectOption4.CheckedChanged +=
                               CheckedChangedHandler(EventArgs.Empty);
  selectOption5.CheckedChanged +=
                               CheckedChangedHandler(EventArgs.Empty);
  selectOption6.CheckedChanged +=
                               CheckedChangedHandler(EventArgs.Empty);

You see, EventHandler is a delegate declared:

public delegate void EventHandler(Object sender, EventArgs e);

Events are a specific application of delegates, basically delegate properties:

    public MyClass
    {
       EventHandler onChange; 
       public Event Change 
       {
           get 
          { 
               return onChange;
          }   
           set 
          { 
               onChange = value;
          }
       }
    }

by assigning events at run-time, behavior can be dynamic.
By deriving custom EventArgs you can create some real neat behavior.
Boy too much to go into here, but they are important!!!!

Joe Mamma

 

[pas30339]

One why has to due with GUIs. The user performs an action on one form, and you want to update all controls that are interested in what the user did on an application wide basis. A value is changed in formA and a datagrid in formC recalculates its totals based on the new value. The delegate acts as the bridge allowing communication from the object raising the event to objects interested in knowing about the event.

 

[Denaes]

Not to get back on topic, but...

Im reading this pretty damn cool book by OReilly called Object-Oriented Programming with Visual Basic.Net.

Friggin genius book.

Talk about bare bones. 3 chapters in and youre still using notepad, the framework compiler and a console screen to compile modules, dlls and console based programs (with notes on changes youd make with a windows program)

I get to the section on delegates and a few pages are spent on them. True to what Joe Mama said, they were complex and promised to be very important. I didnt have any trouble with the concept (despite being a VB programmer), but I was in a bus reading it. I really need to go back over it with the computer.

The next section was on Events. They looked similar to Deleagates in the examples he was using them.

Then he had like a little notebox saying that An event is pretty much just like a Delegate for most purposes. In fact, an Event decompiles down to a delegate in MSIL, and youd use them to do pretty much the same exact thing.

He advised learning delegates, because they did have their uses, but said tha Events were much more common. One of the exercises in the book was to rewrite a twin pair of programs (a debugger console and debugger program) using events rather than the delegates he used.

Ive been reading that book for one day... just straight reading... and Ive learned so much.

 

[Joe Mamma]

If I am not mistaken, Event is just a keyword that modifies delegates so they appear in the property explorer and allow the compiler to perform consumption and chaining(is that the right term?)

delegates are simply a user defined type that is the format for a method -

At the risk of being censored , I am going to use delphi syntax here because it really drives the concept home:

in delphi you can do the following

type
  TMyMethod = procedure(sender: TObject);

var
  MyMethod: TMyMethod;
  
  procedure Method1(sender: TObject);
  begin
    // do domething with sender
  end;

  procedure Method2(sender: TObject);
  begin
    // do domething with sender
  end;

 begin
  if [some runtime specified boolean value such as a check box] then
    MyMethod = Method1
  else
    MyMethod = Method2;
  MyMethod(Application);
 end.

So, TMyMethod is a definition delegating any function that takes a single argument of type TObject;

I declare a variable of type TMyMethod, MyMethod .

I declare two methods that take a single argument of type TObject, making them eligable for delegation by a variable of type TMyMethod.

at run time, the program does some dynamic analysis and assigns the appropriate method to MyMethod and then executes the method using the application variable as the argument.

now. . . open mind time. . . use your imagination. . .
arrays can contain any type of data and can be loaded dynamically at runtime. . .
consider an array of delegates, the actual methods being delegated are specified at run time depending on the user specified option selections. . .
After iterating through the array and executing the user specfied methods, the user can select a set of different options. the program loads the new delegated methods based on the selections and the program executes with a completely different behavior.

Again, I highly suggest that anyone new to OOP download delphi. not because delphi is the end all be all of programming languages but because delphi comes with vitually ALL the source used to devlop the delphi components. It is a wonderful source for ideas and examples of real world OOP technique that transcends syntax.

Oh yeah, and its type library editor is phenomenal!

Joe Mamma. . . free your mind - vote Kerry!!!

 

[Nerseus]

A delegate creates a new variable type and is used to define what a function looks like (its params and return type) so that you can call a function dynamically, in a type-safe way.

Similar to defining a class, a delegate just provides a definition of what it should look like. This is similar to an interface in that youre not providing code, only what it should look like. Unlike an interface, its just for a function and you CAN instantiate a variable of that type.

The most common use for delegates is for handling events. If you use WinForms, then every event you use is defined in the system as a delegate. When you double click on a control in Winforms to create an event, it creates code something like the following (in C#):

// In InitializeComponent:
this.Load += new System.EventHandler(this.Form1_Load);

// Outside of InitializeComponent - just somewhere in the class:
private void Form1_Load(object sender, System.EventArgs e)
{
}

Somewhere in System MS has defined a delegate that looks like this:
delegate void EventHandler(object sender, System.EventArgs e);

Visual Studio takes care of adding the "this.Load += ..." code for you as well as defining a function that matches what the delegate wants.

Delegates can be single or multicast as well. What does that mean? Simply, if you add more than one "handler" for the delegate, it becomes multicast. That means that when you call the delegate, which looks like a function call, it will actually call more than one function. This is ideal for events since more than one piece of code may want to handle an event. Heres a sample in C#:

public class Form1 : System.Windows.Forms.Form
{
	// The delegate, just the definition of what a function will look like
	delegate void Test();

	// A function that matches the definition of the delegate above	
	private void TestA()
	{
		System.Diagnostics.Debug.WriteLine("TestA");
	}
	
	// A function that matches the definition of the delegate above	
	private void TestB()
	{
		System.Diagnostics.Debug.WriteLine("TestB");
	}
	
	// Constructor
	public Form1()
	{
		// Define an instance of the delegate 
		Test t = new Test(TestA);
		// Add a second handler to the delegate
		t+= new Test(TestB);
		// Call the delegate - will call both functions, TestA and TestB.
		t();
	}
...
}

-Nerseus