I'm new to SO and programming and learning day by day with bits and pieces of tech (C#) jargons.
After Googling for a while, below is what I've researched about methods
A Method is a block of statements, which serves for code reusability
& it also supports overloading with different SIGNATURE....for ex:
drawShape(2pts), drawShape(3pts) etc...
An Anonymous method is one with block of statements, but no
name....(as its premature to ask, in wt situation we come across
anonymous method...any articles, samples ...)
Named method: Here's a link but at the end i didn't get what Named Method actually is...
Can anyone explain what a "Named" method is, and where do we use anonymous method?
A named method is a method you can call by its name (e.g. it is a function that has a name). For example, you have defined a function to add two numbers:
int f(int x, int y)
{
return x+y;
}
You would call this method by its name like so: f(1, 2);.
Anonymous method is a method that is passed as an argument to a function without the need for its name. These methods can be constructed at runtime or evaluated from a lambda expression at compile time.
These methods are often used in LINQ queries, for example:
int maxSmallerThan10 = array.Where(x => x < 10).Max();
The expression x => x < 10 is called a lambda expression and its result is an anonymous function that will be run by the method Where.
If you are a beginner, I would suggest you first read about more basic stuff. Check out the following links:
http://www.completecsharptutorial.com/
http://www.csharp-station.com/tutorial.aspx
http://www.homeandlearn.co.uk/csharp/csharp.html
Let's start from a simple method.
void MyMethod()
{
Console.WriteLine("Inside MyMethod"); //Write to output
}
The above method is a named-method which just writes Inside MyMethod to the output window.
Anonymous methods are some methods used in some special scenarios (when using delegates) where the method definition is usually smaller where you don't specify the name of the method.
For example, (delegate) => { Console.WriteLine("Inside Mymethod");}
Just start writing some simple programs and in the due course, when you use delegates or some advanced concepts, you will yourself learn. :)
Explanation by Analogy
Normally when we tell stories we refer to people by name:
"Freddie"
"Who's Freddie?"
"You know, Freddie, Freddie from Sales - the male guy with the red hair, who burned the building down...?"
In reality nobody cares who the person is, department he works etc. it's not like we'll refer to him every again. We want to be able to say: "Some guy burned down our building". All the other stuff (hair color, name etc.) is irrelevant and/or can be inferred.
What does this have to do with c#?
Typically in c# you would have to define a method if you want to use it: you must tell the compiler (typically):
what it is called,
and what goes into it (parameters + their types),
as well as what should come out (return type),
and whether it is something you can do in the privacy of your home or whether you can do it in public. (scope)
When you do that with methods, you are basically using named methods. But writing them out: that's a lot of effort. Especially if all of that can be inferred and you're never going to use it again.
That's basically where anonymous methods come in. It's like a disposable method - something quick and dirty - it reduces the amount you have to type in. That's basically the purpose of them.
Anonymous methods or anonymous functions, what seems to be the same, basically are delegates. As the link you point out: http://msdn.microsoft.com/en-us/library/bb882516.aspx describes, anonymous methods provide a simplified way to pass method to be executed by another method. Like a callback.
Another way to see it, is think about lambda expressions.
A named by the contrast is any common method.
From MSDN:
A delegate can be associated with a named method. When you instantiate a delegate by using a named method, the method is passed as a parameter. This is called using a named method. Delegates constructed with a named method can encapsulate either a static method or an instance method. Named methods are the only way to instantiate a delegate in earlier versions of C#. However, in a situation where creating a new method is unwanted overhead, C# enables you to instantiate a delegate and immediately specify a code block that the delegate will process when it is called. The block can contain either a lambda expression or an anonymous method.
and
In versions of C# before 2.0, the only way to declare a delegate was to use named methods. C# 2.0 introduced anonymous methods and in C# 3.0 and later, lambda expressions supersede anonymous methods as the preferred way to write inline code. However, the information about anonymous methods in this topic also applies to lambda expressions. There is one case in which an anonymous method provides functionality not found in lambda expressions. Anonymous methods enable you to omit the parameter list. This means that an anonymous method can be converted to delegates with a variety of signatures. This is not possible with lambda expressions. For more information specifically about lambda expressions, see Lambda Expressions (C# Programming Guide). Creating anonymous methods is essentially a way to pass a code block as a delegate parameter. By using anonymous methods, you reduce the coding overhead in instantiating delegates because you do not have to create a separate method.
So in answer to your question about when to use anonymous methods, then MSDN says: in a situation where creating a new method is unwanted overhead.
In my experience it's more down to a question of code reuse and readability.
Links:
http://msdn.microsoft.com/en-us/library/98dc08ac.aspx
http://msdn.microsoft.com/en-us/library/0yw3tz5k.aspx
Hope that helps
Related
From this answer I've learned that it is possible to strongly suggest inlining in C# as follows:
using System.Runtime.CompilerServices;
[MethodImpl(MethodImplOptions.AggressiveInlining)]
bool MyCondition() { return someObject != null && someObject.Count > 2; }
In a current project we use statemachines as defined by the Appccelerate StateMachine framework, which results in sequences like the following (which in our project are much longer):
fsm.In(States.A)
.On(Events.B)
.If(arguments => false).Goto(States.B1)
.If(() => someVariable && somethingElse == false).Goto(States.B3);
.If(MyCondition).Goto(States.B2)
In order to simplify these structures I would like to separate the lambda expressions (or Action delegates) into helper methods (i.e. the last statement). Reasons for doing so is that with proper method names it would increase readability of the code, and secondly when autogenerating documentation it will use the method name instead of the non-intuitive [anonymous] text.
The question however, is whether it is any point in using AggressiveInlining or will simple lambda expressions involving upto 4 variables with simple comparison operators be automatically inlined by JIT/compiler?
My gut feeling is to inline these methods, as I believe the different parts of the statemachine will get a lot of hits, and thusly to reduce method calling would be a benefit. But then again how smart is JIT/compiler to automagically do this?
The problem is that you understand lambdas in c# incorrectly. When compiler translate c# to MSIL lambdas become classes and so you have nothing to inline. You can take a look at great Marc Gravell post on SO. So, whether or not you define lambdas in external class, you'll need to get an object from heap (i simplify compiled code behaviour). And so, as i think, there'll be no difference in performance for your application.
Also keep in mind that the state machine keeps the delegate in its internal data structure. The guard action cannot be inlined there.
The only possible thing is to inline the methods that the helper method calls.
I never seem to understand why we need delegates?
I know they are immutable reference types that hold reference of a method but why can't we just call the method directly, instead of calling it via a delegate?
Thanks
Simple answer: the code needing to perform the action doesn't know the method to call when it's written. You can only call the method directly if you know at compile-time which method to call, right? So if you want to abstract out the idea of "perform action X at the appropriate time" you need some representation of the action, so that the method calling the action doesn't need to know the exact implementation ahead of time.
For example:
Enumerable.Select in LINQ can't know the projection you want to use unless you tell it
The author of Button didn't know what you want the action to be when the user clicks on it
If a new Thread only ever did one thing, it would be pretty boring...
It may help you to think of delegates as being like single-method interfaces, but with a lot of language syntax to make them easy to use, and funky support for asynchronous execution and multicasting.
Of course you can call method directly on the object but consider following scenarios:
You want to call series of method by using single delegate without writing lot of method calls.
You want to implement event based system elegantly.
You want to call two methods same in signature but reside in different classes.
You want to pass method as a parameter.
You don't want to write lot of polymorphic code like in LINQ , you can provide lot of implementation to the Select method.
Because you may not have the method written yet, or you have designed your class in such a way that a user of it can decide what method (that user wrote) the user wants your class to execute.
They also make certain designs cleaner (for example, instead of a switch statement where you call different methods, you call the delegate passed in) and easier to understand and allow for extending your code without changing it (think OCP).
Delegates are also the basis of the eventing system - writing and registering event handlers without delegates would be much harder than it is with them.
See the different Action and Func delegates in Linq - it would hardly be as useful without them.
Having said that, no one forces you to use delegates.
Delegates supports Events
Delegates give your program a way to execute methods without having to know precisely what those methods are at compile time
Anything that can be done with delegates can be done without them, but delegates provide a much cleaner way of doing them. If one didn't have delegates, one would have to define an interface or abstract base class for every possible function signature containing a function Invoke(appropriate parameters), and define a class for each function which was to be callable by pseudo-delegates. That class would inherit the appropriate interface for the function's signature, would contain a reference to the class containing the function it was supposed to represent, and a method implementing Invoke(appropriate parameters) which would call the appropriate function in the class to which it holds a reference. If class Foo has two methods Foo1 and Foo2, both taking a single parameter, both of which can be called by pseudo-delegates, there would be two extra classes created, one for each method.
Without compiler support for this technique, the source code would have to be pretty heinous. If the compiler could auto-generate the proper nested classes, though, things could be pretty clean. Dispatch speed for pseudo-delegates would probably generally be slower than with conventional delegates, but if pseudo-delegates were an interface rather than an abstract base class, a class which only needs to make a pseudo-delegate for one method of a given signature could implement the appropriate pseudo-delegate interface itself; the class instance could then be passed to any code expecting a pseudo-delegate of that signature, avoiding any need to create an extra object. Further, while the number of classes one would need when using pseudo-delegates would be greater than when using "real" delegates, each pseudo-delegate would only need to hold a single object instance.
Think of C/C++ function pointers, and how you treat javascript event-handling functions as "data" and pass them around. In Delphi language also there is procedural type.
Behind the scenes, C# delegate and lambda expressions, and all those things are essentially the same idea: code as data. And this constitutes the very basis for functional programming.
You asked for an example of why you would pass a function as a parameter, I have a perfect one and thought it might help you understand, it is pretty academic but shows a use. Say you have a ListResults() method and getFromCache() method. Rather then have lots of checks if the cache is null etc. you can just pass any method to getCache and then only invoke it if the cache is empty inside the getFromCache method:
_cacher.GetFromCache(delegate { return ListResults(); }, "ListResults");
public IEnumerable<T> GetFromCache(MethodForCache item, string key, int minutesToCache = 5)
{
var cache = _cacheProvider.GetCachedItem(key);
//you could even have a UseCache bool here for central control
if (cache == null)
{
//you could put timings, logging etc. here instead of all over your code
cache = item.Invoke();
_cacheProvider.AddCachedItem(cache, key, minutesToCache);
}
return cache;
}
You can think of them as a construct similar with pointers to functions in C/C++. But they are more than that in C#. Details.
Could you give me some reasons for limitations of the dynamic type in C#? I read about them in "Pro C# 2010 and the .NET 4 platform". Here is an excerpt (if quoting books is illegal here, tell me and I will remove the excerpt):
While a great many things can be
defined using the dynamic keyword,
there are some limitations regarding
its usage. While they are not show
stoppers, do know that a dynamic data
item cannot make use of lambda
expressions or C# anonymous methods
when calling a method. For example,
the following code will always result
in errors, even if the target method
does indeed take a delegate parameter
which takes a string value and returns
void.
dynamic a = GetDynamicObject();
// Error! Methods on dynamic data can’t use lambdas!
a.Method(arg => Console.WriteLine(arg));
To circumvent this restriction, you
will need to work with the underlying
delegate directly, using the
techniques described in Chapter 11
(anonymous methods and lambda
expressions, etc). Another limitation
is that a dynamic point of data cannot
understand any extension methods (see
Chapter 12). Unfortunately, this would
also include any of the extension
methods which come from the LINQ APIs.
Therefore, a variable declared with
the dynamic keyword has very limited
use within LINQ to Objects and other
LINQ technologies:
dynamic a = GetDynamicObject();
// Error! Dynamic data can’t find the Select() extension method!
var data = from d in a select d;
Thanks in advance.
Tomas's conjectures are pretty good. His reasoning on extension methods is spot on. Basically, to make extension methods work we need the call site to at runtime somehow know what using directives were in force at compile time. We simply did not have enough time or budget to develop a system whereby this information could be persisted into the call site.
For lambdas, the situation is actually more complex than the simple problem of determining whether the lambda is going to expression tree or delegate. Consider the following:
d.M(123)
where d is an expression of type dynamic. *What object should get passed at runtime as the argument to the call site "M"? Clearly we box 123 and pass that. Then the overload resolution algorithm in the runtime binder looks at the runtime type of d and the compile-time type of the int 123 and works with that.
Now what if it was
d.M(x=>x.Foo())
Now what object should we pass as the argument? We have no way to represent "lambda method of one variable that calls an unknown function called Foo on whatever the type of x turns out to be".
Suppose we wanted to implement this feature: what would we have to implement? First, we'd need a way to represent an unbound lambda. Expression trees are by design only for representing lambdas where all types and methods are known. We'd need to invent a new kind of "untyped" expression tree. And then we'd need to implement all of the rules for lambda binding in the runtime binder.
Consider that last point. Lambdas can contain statements. Implementing this feature requires that the runtime binder contain the entire semantic analyzer for every possible statement in C#.
That was orders of magnitude out of our budget. We'd still be working on C# 4 today if we'd wanted to implement that feature.
Unfortunately this means that LINQ doesn't work very well with dynamic, because LINQ of course uses untyped lambdas all over the place. Hopefully in some hypothetical future version of C# we will have a more fully-featured runtime binder and the ability to do homoiconic representations of unbound lambdas. But I wouldn't hold my breath waiting if I were you.
UPDATE: A comment asks for clarification on the point about the semantic analyzer.
Consider the following overloads:
class C {
public void M(Func<IDisposable, int> f) { ... }
public void M(Func<int, int> f) { ... }
...
}
and a call
d.M(x=> { using(x) { return 123; } });
Suppose d is of compile time type dynamic and runtime type C. What must the runtime binder do?
The runtime binder must determine at runtime whether the expression x=>{...} is convertible to each of the delegate types in each of the overloads of M.
In order to do that, the runtime binder must be able to determine that the second overload is not applicable. If it were applicable then you could have an int as the argument to a using statement, but the argument to a using statement must be disposable. That means that the runtime binder must know all the rules for the using statement and be able to correctly report whether any possible use of the using statement is legal or illegal.
Clearly that is not restricted to the using statement. The runtime binder must know all the rules for all of C# in order to determine whether a given statement lambda is convertible to a given delegate type.
We did not have time to write a runtime binder that was essentially an entire new C# compiler that generates DLR trees rather than IL. By not allowing lambdas we only have to write a runtime binder that knows how to bind method calls, arithmetic expressions and a few other simple kinds of call sites. Allowing lambdas makes the problem of runtime binding on the order of dozens or hundreds of times more expensive to implement, test and maintain.
Lambdas: I think that one reason for not supporting lambdas as parameters to dynamic objects is that the compiler wouldn't know whether to compile the lambda as a delegate or as an expression tree.
When you use a lambda, the compiler decides based on the type of the target parameter or variable. When it is Func<...> (or other delegate) it compiles the lambda as an executable delegate. When the target is Expression<...> it compiles lambda into an expression tree.
Now, when you have a dynamic type, you don't know whether the parameter is delegate or expression, so the compiler cannot decide what to do!
Extension methods: I think that the reason here is that finding extension methods at runtime would be quite difficult (and perhaps also inefficient). First of all, the runtime would need to know what namespaces were referenced using using. Then it would need to search all classes in all loaded assemblies, filter those that are accessible (by namespace) and then search those for extension methods...
Eric (and Tomas) says it well, but here is how I think of it.
This C# statement
a.Method(arg => Console.WriteLine(arg));
has no meaning without a lot of context. Lambda expressions themselves have no types, rather they are convertible to delegate (or Expression) types. So the only way to gather the meaning is to provide some context which forces the lambda to be converted to a specific delegate type. That context is typically (as in this example) overload resolution; given the type of a, and the available overloads Method on that type (including extension members), we can possibly place some context that gives the lambda meaning.
Without that context to produce the meaning, you end up having to bundle up all kinds of information about the lambda in the hopes of somehow binding the unknowns at runtime. (What IL could you possibly generate?)
In vast contrast, one you put a specific delegate type there,
a.Method(new Action<int>(arg => Console.WriteLine(arg)));
Kazam! Things just got easy. No matter what code is inside the lambda, we now know exactly what type it has, which means we can compile IL just as we would any method body (we now know, for example, which of the many overloads of Console.WriteLine we're calling). And that code has one specific type (Action<int>), which means it is easy for the runtime binder to see if a has a Method that takes that type of argument.
In C#, a naked lambda is almost meaningless. C# lambdas need static context to give them meaning and rule out ambiguities that arise from many possible coercisons and overloads. A typical program provides this context with ease, but the dynamic case lacks this important context.
This question already has answers here:
Closed 13 years ago.
Possible Duplicates:
Delegate Usage : Business Applications
Where do I use delegates?
Hi,
I'm new to the concept of a delegate in .NET - I haven't really used them yet and think they are probably there for a good reason - when should I be using a delegate?
Examples are very welcome.
Delegates provide a way for you to pass behavior as a parameter.
Common examples are Events and asynchronous programming where something other than your object is responsible for calling into your object. You provide that event with a delegate, and now it's able to run the behavior associated with that delegate.
This can also be a useful technique when implementing a general algorithm. There've been times where I'm writing multiple methods that are very similar. Perhaps they loop over the same set of data, but they perform slightly different tasks. I can pass in a delegate to a single function to perform that task, then call the delegate from inside the loop over the data. This way I don't have to implement the loop over the data multiple times, I only have to write new code that does new things-- the common behavior is all captured in a common way.
In response to the comment:
The difference between calling the delegate from within the loop and calling the method from within the loop is that the delegate is a parameter to the function that contains the loop. This means that that function could do anything, not just what's defined within a particular method. That flexibility can and has been leveraged to supply generic algorithms in libraries completely independent of the specifics of what the algorithms are working with. Linq is a fine example of the generality that is allowed via the flexibility of delegates.
Delegates are useful when you need to tell another piece of code how to do something. Events are the simplest example - separating the event (something happened) from how to handle it. Other examples are some common iterative operations, such as a custom find:
List<Foo> foos = new List<Foo>();
foos.Find(delegate(Foo foo)
{
if(foo.CustomProperty.Contains("special value"))
{
return false;
}
return true;
});
The above is a totally arbitrary example but makes the point that you can separate the what (iterating and running a "find" criteria) from the how (how to determine whether something is found).
Also, I would recommend using generic delegate types instead of creating your own. Examples below:
EventHandler< TEventArgs>
Func< TResult>
Func< T1, T2, TResult>
Func etc...
Action< T1>
Action< T1, T2>
Action etc...
Events use delegates behind the scenes.
If you use events you've used delegates but just with a nicer syntax.
Basically one use for them is for callbacks or event based programming.
Delegates are used for event driven programming. Best practices are often about decoupling code (or 'loose' coupling). Using delegates you can subscribe methods to events, instead of having X call Y if something happens, and then Y call Z under a certain condition and so forth.
Events and Callbacks in the .NET guidelines explains it well.
In summary, you should prefer events in simple APIs because there is strong IDE support and most developers are comfortable with events. However, if you need the user to provide code that will be executed, you should consider using delegates or virtual members. Callbacks are less performant, but if you use delegates like Action and Func you allow lambdas.
Example case: a single resource is being used by multiple objects. The resource needs an asynchronous callback to these objects, but the nature of the resource demands that only one request be active at a given time, and that request is tied to a particular object. Due to the architecture, I don't want to pass the objects into the resource, so instead, I send the resource a delegate from each object and store these delegates along with object identifiers. When a request is made, I can look up the corresponding object identifier and call its specific delegate.
I originally implemented this with an event, but since the event couldn't be 'aimed' at a particular object, I was having issues. I'm not sure this is best practice, but it seems to work well for me.
I was going to post a question, but figured it out ahead of time and decided to post the question and the answer - or at least my observations.
When using an anonymous delegate as the WaitCallback, where ThreadPool.QueueUserWorkItem is called in a foreach loop, it appears that the same one foreach-value is passed into each thread.
List< Thing > things = MyDb.GetTheThings();
foreach( Thing t in Things)
{
localLogger.DebugFormat( "About to queue thing [{0}].", t.Id );
ThreadPool.QueueUserWorkItem(
delegate()
{
try
{
WorkWithOneThing( t );
}
finally
{
Cleanup();
localLogger.DebugFormat("Thing [{0}] has been queued and run by the delegate.", t.Id );
}
});
}
For a collection of 16 Thing instances in Things I observed that each 'Thing' passed to WorkWithOneThing corresponded to the last item in the 'things' list.
I suspect this is because the delegate is accessing the 't' outer variable. Note that I also experimented with passing the Thing as a parameter to the anonymous delegate, but the behavior remained incorrect.
When I re-factored the code to use a named WaitCallback method and passed the Thing 't' to the method, voilà ... the i'th instance of Things was correctly passed into WorkWithOneThing.
A lesson in parallelism I guess. I also imagine that the Parallel.For family addresses this, but that library was not an option for us at this point.
Hope this saves someone else some time.
Howard Hoffman
This is correct, and describes how C# captures outside variables inside closures. It's not directly an issue about parallelism, but rather about anonymous methods and lambda expressions.
This question discusses this language feature and its implications in detail.
This is a common occurrence when using closures and is especially evident when constructing LINQ queries. The closure references the variable, not its contents, therefore, to make your example work, you can just specify a variable inside the loop that takes the value of t and then reference that in the closure. This will ensure each version of your anonymous delegate references a different variable.
Below is a link detailing why that happens. It's written for VB but C# has the same semantics.
http://blogs.msdn.com/jaredpar/archive/2007/07/26/closures-in-vb-part-5-looping.aspx