S
Stephen
I am new to C# and can't get my head round what delegates
are and what they are for. can anyone enlighten me?
are and what they are for. can anyone enlighten me?
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M
Michael Bird
Delegates, at the simplest explanation, are function pointers. You use them
to do callbacks.
to do callbacks.
T
Tu-Thach
If you are from C/C++, they are similar to function
pointers. It is used for callbacks, etc.
Tu-Thach
pointers. It is used for callbacks, etc.
Tu-Thach
D
Daniel Pratt
Hi Stephen,
Delegates are glorified function pointers (if that means anything to
you). Delegates support two related concepts.
1. Plugging in functionality into classes that are already built.
2. Call-backs.
An example of #1 is the Thread class. How do you tell a thread what you
want it to do? In a world without delegates, the Thread class could have
designed to accept a reference to an interface that defines a single method.
The Thread object would call the single method of the interface when the
thread was started. Of course, to create a class which implements the single
method and then create an instance of that class to pass to the Thread
instance seems a bit much. With delegates you can just say "Hey Thread
object, call *this* function".
An example of #2 is asynchronous methods. You don't want to freeze your
app while you're waiting around for the web service method you just called
to get back to you. No problem, you can call the web service method
asynchronously (i.e. control returns to your app before the web service
method completes). But how do you know when the web service method is done?
Polling isn't very efficient (Are you done, yet?...Are you done, yet?...Are
you...). Call-backs are the answer. You just give the asynchronous method a
method (in your app, presumably) to call when it's done.
Delegates also support events, which are somewhat more managed versions
of delegates. For example, events prevent one object from inadvertently (or
"vertantly") wiping out the event sinks of another. Delegates are a relative
free-for-all.
I hope that was somewhat lucid.
Regards,
Dan
Stephen said:I am new to C# and can't get my head round what delegates
are and what they are for. can anyone enlighten me?
Delegates are glorified function pointers (if that means anything to
you). Delegates support two related concepts.
1. Plugging in functionality into classes that are already built.
2. Call-backs.
An example of #1 is the Thread class. How do you tell a thread what you
want it to do? In a world without delegates, the Thread class could have
designed to accept a reference to an interface that defines a single method.
The Thread object would call the single method of the interface when the
thread was started. Of course, to create a class which implements the single
method and then create an instance of that class to pass to the Thread
instance seems a bit much. With delegates you can just say "Hey Thread
object, call *this* function".
An example of #2 is asynchronous methods. You don't want to freeze your
app while you're waiting around for the web service method you just called
to get back to you. No problem, you can call the web service method
asynchronously (i.e. control returns to your app before the web service
method completes). But how do you know when the web service method is done?
Polling isn't very efficient (Are you done, yet?...Are you done, yet?...Are
you...). Call-backs are the answer. You just give the asynchronous method a
method (in your app, presumably) to call when it's done.
Delegates also support events, which are somewhat more managed versions
of delegates. For example, events prevent one object from inadvertently (or
"vertantly") wiping out the event sinks of another. Delegates are a relative
free-for-all.
I hope that was somewhat lucid.
Regards,
Dan
E
Empire City
I am new to C# and can't get my head round what delegates
I know exactly what you mean. Fortunatly I knew indirect addressing from my
assembly language days. I also futzed about with VC++ a bit before getting
into C#. A delegete is like a C++ function pointer. It's an indirect way of
calling a method. For example say you didn't know the name of the method you
wanted to call before your program started. Maybe based on your data you
should either call a MyAddCustomerMethod or MyDeleteCustomerMethod. You
could pull the method name from a database and subsitute it into a delegate.
You would then be calling your method based on your data. Here's the
delegate tutorial:
http://msdn.microsoft.com/library/d...y/en-us/csref/html/vcwlkdelegatestutorial.asp
Look also at the simple conceptual sample under the help for delegate
keyword.
Lots of C# and .Net functionality uses delegates. ThreadStart, BeginInvoke,
etc, take delegates. Events are delegates. Asynchronous programming and
remoting use them extensively. I finally got comfortable with them after
doing the Asynchronous programming tutorial. Once I saw that, not only can
you use a delegate to call a method, you can also pass the actual delegate
as a delegate to a method for use inside that method. Using the methods
paramaters is another level of complexity in understanding them as well as
the return type of the delegate.
are and what they are for. can anyone enlighten me?
I know exactly what you mean. Fortunatly I knew indirect addressing from my
assembly language days. I also futzed about with VC++ a bit before getting
into C#. A delegete is like a C++ function pointer. It's an indirect way of
calling a method. For example say you didn't know the name of the method you
wanted to call before your program started. Maybe based on your data you
should either call a MyAddCustomerMethod or MyDeleteCustomerMethod. You
could pull the method name from a database and subsitute it into a delegate.
You would then be calling your method based on your data. Here's the
delegate tutorial:
http://msdn.microsoft.com/library/d...y/en-us/csref/html/vcwlkdelegatestutorial.asp
Look also at the simple conceptual sample under the help for delegate
keyword.
Lots of C# and .Net functionality uses delegates. ThreadStart, BeginInvoke,
etc, take delegates. Events are delegates. Asynchronous programming and
remoting use them extensively. I finally got comfortable with them after
doing the Asynchronous programming tutorial. Once I saw that, not only can
you use a delegate to call a method, you can also pass the actual delegate
as a delegate to a method for use inside that method. Using the methods
paramaters is another level of complexity in understanding them as well as
the return type of the delegate.