The usage of delegate

Question

i saw that delegate is used for custom events. as far example

delegate string FuncRef(string Val);

FuncRef fValue = GetFieldName;

fValue("hello");

what i do here just declare delegate and assign a function name to delegate and call like fValue("hello"); whenever it is required.

instead of calling the GetFieldName() through delegate i can call it directly. so i just want to know why should i use delegate to call function where as we can call function directly....what is the advantage of calling any function through delegate.

so please tell me in what kind of scenario delegate usage is required except event handling. please guide me with sample code and simulate a situation where i need to call function through delegate except event handling. please show me some real life scenario where we have to call function through delegate.

Answer 1

delegate string FuncRef(string Val);

FuncRef fValue;   // class member

if (condition1)
    fValue = GetFieldName1;
else if (condition2)
    fValue = GetFieldName2;
else
    fValue = GetFieldName3;

// Another function
fValue("hello");

Answer 2

Delegates are 1st class objects. IOW you can refer to them, reassign them, etc, just like any other object.

Answer 3

Microsoft's tutorial code on C# delegates presents another interesting use case. In their example, an object called Bookstore has a method ProcessPaperbackBooks which takes a book processing delegate and applies it to each item in the class.

The cool part is that then, if you need say, a method which collects all the ISBNs of all the books in the store, you don't need to change anything in the bookstore class, but only to define a function which acts on books and pass that into the ProcessPaperbackBooks method. Delegated goodness will occur, and your new function will be applied to every item in the bookstore. Snazzy, no?

http://msdn.microsoft.com/en-us/library/aa288459(v=vs.71).aspx

Answer 4

One place I use delegates is my input handler (C#, XNA)

I have a public void OnKeyboardPress() delegate, which I bind to various functions when certain keys are pressed.

Input is a good example; rather than polling for it (i.e. every update you go 'is it ready yet?' you wait for it to call you)..such as a GUI. WPF events are delegates.

Answer 5

Linq to objects makes extensive use of delegates:

myAnimals.Where(animal => animal.CanSwim)

the parameter supplied to the Where method ( animal => animal.CanSwim ) is a lambda expression that converts to a delegate that is applied to all elements in myAnimals.

Answer 6

Shortly: You can use delegates and events members in combination to be able for example to assign an event handler to a class which does not know it before, the button class knows when it has to trigger the click event and does not know yet, internally, you will bind mybutton_Click handler to it.

Answer 7

The reason to use delegates instead of calling the function directly is the same reason you do

var taxRate = 0.15;
var taxAmount = income * taxRate;

instead of

var taxAmount = income * 0.15;

In other words: using a variable to hold a reference to a callable entity (a delegate is exactly that) allows you to write code that can change its behavior depending on the parameters passed to it (the value of the delagate we 're passing in). This means more flexible code.

For examples of code that uses delegates you can look at LINQ (of course), but there's also the "delegates 101" example which is relevant in any language: filtering a list.

Answer 8

Basically any place where you want to be able to specify at runtime which function should be called.

The async BeginInvoke/EndInvoke pattern is a perfect example of this; the callback is specified via a delegate.