Where do I use delegates?

Question

What are some real world places that call for delegates? I'm curious what situations or patterns are present where this method is the best solution. No code required.

Answer 1

Binding Events to Event Handlers is usually your first introduction to delegates...You might not even know you were using them because the delegate is wrapped up in the EventHandler class.

Answer 2

A quick google search came up with this http://en.wikipedia.org/wiki/Delegation_pattern . Basically, anytime that you use an object that forwards it's calls to another object then you are delegating.

Answer 3

A delegate is a named type that defines a particular kind of method. Just as a class definition lays out all the members for the given kind of object it defines, the delegate lays out the method signature for the kind of method it defines.

Based on this statement, a delegate is a function pointer and it defines what that function looks like.

A great example for a real world application of a delegate is the Predicate. In the example from the link, you will notice that Array.Find takes the array to search and then a predicate to handle the criteria of what to find. In this case it passes a method ProductGT10 which matches the Predicate signature.

Answer 4

If you're interested in seeing how the Delegate pattern is used in real-world code, look no further than Cocoa on Mac OS X. Cocoa is Apple's preferred UI toolkit for programming under Mac OS X, and is coded in Objective C. It's designed so that each UI component is intended to be extended via delegation rather than subclassing or other means.

For more information, I recommend checking out what Apple has to say about delegates here.

Answer 5

One common use of delegates for generic Lists are via Action delegates (or its anonymous equivalent) to create a one-line foreach operation:

myList.Foreach( i => i.DoSomething());

I also find the Predicate delegate quite useful in searching or pruning a List:

myList.FindAll( i => i.Name == "Bob");    
myList.RemoveAll( i => i.Name == "Bob");

I know you said no code required, but I find it easier to express its usefulness via code. :)

Answer 6

It seems as if the answers given to this question relate to incompatible meanings of the word. The .NET idea of a delegate (as a function pointer) seems to be different from the Cocoa idea of a delegate (as a helper object). Am I right? And if so, could the original poster clarify their question?

Answer 7

@sadie

Looks like a good spot for someone to spell out the differences :)

Answer 8

I had a project which used win32 Python.

Due to various reasons, some modules used odbc.py to access the DB, and other modules - pyodbc.py.

There was a problem when a function needed to be used by both kinds of modules. It had an connection object passed to it as an argument, but then it had to know whether to use dbi.dbiDate or datetime to represent times.

This was because odbc.py expected, as values in SQL statements, dates as dbi.dbiDate whereas pyodbc.py expected datetime values.

One further complication was that the connection objects created by odbc.py and pyodbc.py did not allow one to set additional fields.

My solution was to wrap the connection objects returned by odbc.odbc(...) and pyodbc.pyodbc(...) by a delegate class, which contains the desired time representation function as the value of an extra field, and which delegates all other field requests to the original connection object.

Answer 9

Hello slipsec. I had the same question as you and went to this site for an answer.

Apparently, I didn't understood it better even though I skimmed through the examples on this thread.

I found a great use for delegates now that I read: http://www.c-sharpcorner.com/UploadFile/thiagu304/passdata05172006234318PM/passdata.aspx

This might seem more obvious for new users because Forms is much more complicated to pass values than ASP.NET websites with POST/GET (QueryString) ..

Basically you define a delegate which takes "TextBox text" as parameters.

// Form1

// Class Property Definition
public delegate void delPassData(TextBox text);


// Click Handler
private void btnSend_Click(object sender, System.EventArgs e)
{
     Form2 frm= new Form2();
     delPassData del=new delPassData(frm.funData);
     del(this.textBox1);
     frm.Show();
}

// SUMMARY: Define delegate, instantiate new Form2 class, assign funData() function to delegate, pass in your textBox to the delegate. Show the form.

// Form2

public void passData(TextBox txtForm1)
{

     label1.Text = txtForm1.Text;
}

// SUMMARY: Simply take TextBox txtForm1 as parameters (as defined in your delegate) and assign label text to textBox's text.

I hope this enlightens some use on delegates :) ..