I'm pretty sure that is possible (at least in java it is) and I'm C# beginner.
So I have a function which includes a callback (notify some other method that some work is finished).
I don't want to call another function because I'm losing a parameter there (and can't pass parameters in callback functions). How can I do everything in the same function?
What I'm doing now:
public static Tween Play(Tween tweenToPlay)
{
return tweenToPlay.Play().OnComplete(RewindCallback);
}
private static void RewindCallback()
{
// Execute some code after Tween is completed
}
What I actually want:
public static Tween Play(Tween tweenToPlay)
{
return tweenToPlay.Play().OnComplete(/*Create a function that will execute here*/);
}
Do you mean a lambda expression, like this?
public static Tween Play(Tween tweenToPlay)
{
return tweenToPlay
.Play()
.OnComplete(() => {
// Do stuff
});
}
You just want an anonymous method?
public static Tween Play(Tween tweenToPlay)
{
return tweenToPlay.Play().OnComplete(() =>
{
//... your code
});
}
Anonymous/lambdas are shorter to write, but depending on the complexity - you might want to use a full fledged class as follows.
Create a class with a field for that variable and an appropriate callback function.
When you want to subscribe to the callback - create an instance of that class with that field set, and set the callback to the callback in that instance.
Example:
class Temp
{
public Tween Tween1;
public void RewindCallback()
{
// Execute some code after Tween is completed
}
}
And usage:
Temp temp;
public Tween Play(Tween tweenToPlay)
{
temp = new Temp { Tween1 = tweenToPlay };
return tweenToPlay.Play().OnComplete(temp.RewindCallback);
}
I have been using javascript and I made a lot of use of functions inside of functions. I tried this in C# but it seems they don't exist. If I have the following:
public abc() {
}
How can I code a method d() that can only be called
from inside the method the method abc() ?
I wouldn't worry so much about the restriction of access to a method on the method level but more class level, you can use private to restrict access of the method to that specific class.
Another alternative would be to use lambdas/anonymous methods, or if you're using C# 4.0, Action/Tasks to create them inside your method.
An example of an anonymous method using a delegate (C# 1/2/3/4) for your specific example (incl. I need an action that can take a string parameter and return a string?) would be something like this:
delegate string MyDelegate(string);
public void abc() {
// Your code..
MyDelegate d = delegate(string a) { return a + "whatever"; };
var str = d("hello");
}
.. using C# 3/4:
public void abc() {
// Your code..
Func<string, string> d = (a) => { return a + "whatever"; };
var str = d("hello");
}
.. using a more ideal solution through private method:
private string d(string a)
{
return a + "whatever";
}
public void abc()
{
// Your code..
var str = d("hello");
}
Based on your comment for another answer: I would just like to have this at the bottom of the method and then call it from some earlier code.
This won't be possible, you would need to define a variable for your method using either delegates or Actions and so it would need to be fully initialised by time you call it. You wouldn't then be able to define this at the bottom of your method. A much better option would be to simply create a new private method on your class and call that.
It is not the way to define classes, but you could do:
public abc() {
Action d = () => {
// define your method
};
d();
}
You cannot declare a method inside another method, but you can create anonymous functions inside methods:
public void abc()
{
Action d = () => { ... };
// ...
d();
}
... that can only be called from inside the method the method abc() ?
The method can only be called if you have a reference to it. If you don't store the reference elsewhere then you should be fine.
how can I pass and return a string to the action?
Use a Func instead of an Action:
Func<string, string> d = s => {
return s + "foo";
};
The reason I would like to do this is to make my code more readable.
It's good to try to make your code more readable but I think this change will make it less readable. I suggest you use ordinary methods, and not anonymous functions. You can make them private so that they cannot be called from outside your class.
Use action delegates. More effective than you did.
public abc() {
Action <int> GetInt = (i) =>
{
//Write code here
Console.Writeline("Your integer is: {0}", i);
};
GetInt(10);
}
Action is a delegate so you can give parameter as a method, not variable. Action delegate encapsulates a method that has no parameters and does not return a value. Check it from MSDN.
Yes, they are called delegates and anonymous methods.
Delegate signatures must be predefined outside of the method for the body to be assigned, so it's not exactly like a function. You would first declare a delegate:
class MyClass {
public delegate boolean Decider(string message);
/* ... */
}
And then in MyClass.MyMethod you can say Decider IsAllLowerCase = /* method name or anonymous method */; and then use it with var result = IsAllLowerCase(s);.
The good news is that .NET already has delegate definitions for most signatures you could possibly need. System.Action has assorted signatures for methods which do not return anything, and System.Func is for the ones that do.
As shown elsewhere,
Action<int, string> a = (n, s) => { for(var i=0; i<n; i++) Console.WriteLine(s);};
Allows you to call a( /* inputs */ ); as if it was a local variable. (stuff) => { code } is "lambda expression" or an anonymous method, you can also just pass a name of a method (if the signature matches):
Action<string> a = Console.WriteLine;
If you want to return something, use Func:
Func<bool, string> f = (b) => { return b.ToString(); };
Allows you to call var result = f(b); in the same way.
As a footnote, delegates are a fun part of C#/.NET but usually, the way to control access is to make another method inside your class, and declare it private. If your issue is name conflicts, then you might want to refactor. For example, you can group methods in another class declared inside your original class (nested classes are supported) or move them to another class entirely.
You can use action delegates
public abc() {
Action action = () =>
{
//Your code here
}
action();
}
Edit: To pass parameter
public abc() {
Action <string>action = (str) =>
{
//Your code here
};
}
action("hello");
Using Func to return a value
public void abc() {
Func<string, string> func = (str) => { return "You sent " + str; };
string str = func("hello");
}
You CAN create a nested class:
public class ContainingClass
{
public static class NestedClass
{
public static void Method2()
{
}
public static void Method3()
{
}
}
}
Then yu can call:
ContainingClass.NestedClass.Method2();
or
ContainingClass.NestedClass.Method3();
I wouldn't recommend this though. Usually it's a bad idea to have public nested types.
I have referenced one DLL (I have source code of this) say Dll_A in which there is a function
private void uploadPic(int a)
In my main project, I have a function say
private void passMe(int b)
{
}
I need to pass the above function (passMe) to uploadPic function in Dll_A, how can I do that? Is it possible?
I am able to use functions of the Dll_A from my main project, so instantiating isn't a problem, I just need a way to pass function.
===
Thanks, giving it a try. If some can edit code below
//code in main project picbackman.cs
public delegate void delObj(int v);
private void uploadSome(string path, string fName, string str)
{
delObj del1 = new delObj(updatePValue);
UploadFileResponse response = boxProvider1.UploadFiles(args1, folderString, ((Picbackman.BoxProvider.delObj)( del1)));
}
//code in different dll which is referenced in main project //Dll_A
public delegate void delObj(int v);
public UploadFileResponse UploadFiles(string[] filePathes,string folderId, delObj d)
{}
First of all your method will need to accept a delegate as one of it's parameters. That would be something like
private void uploadPic(Action<int> func,int a){
//at some point do func(someInt);
}
at another point declare anothe method or function
public class someClasse {
public vois passMe(int b){
...
}
}
First of all notice that the access modifier has changed. if they are not in the same class you will need to be able to access one from the other so they can't both be private and since they are in different assemblies internal won't work either.
when you need the delegate do like this
var obj = new someClass();
var myInt = 5; //or whatever the value is
uploadPic(obj.passMe,myInt);
notice that the method is used with out arguments. When using a method without arguments the compiler will try and convert it to a suitable delegate.
I'd recommend you not to use delegate but stick with Func/Action they are delegates but more generic
public delegate void DelObj1();
public delegate void DelObj2()
public void F(){};
var del1 = new DelObj1(F);
var del2 = new DelObj2(F);
you can't pass a del1 where a DelObj2 is needed even though you use the same method for each. You will not have that issue if you use Action/Function
Action del1 = F;
Action del2 = F;
The reason is that DelObj1 and DelObj2 are two distinct classes with the same base class Ie they are siblings in the type tree. Using Action the type is the same for both del1 and del2
You should have a look at Delegates.
From the documentation:
A delegate is a type that references a method. Once a delegate is
assigned a method, it behaves exactly like that method. The delegate
method can be used like any other method, with parameters and a return
value
and
Delegates allow methods to be passed as parameters
So in your case you should be able to do something like this:
// define the delegate
public delegate int PictureDelegate(int value)
// define your passMe function (in the class MyClass for example)
public int passMe(int value)
{
return value + 1;
}
// when you want to use it
MyClass myInstance = new MyClass();
PictureDelegate passFunc = new PictureDelegate(myInstance.passMe);
myDll.uploadPic(passFunc, 12);
I'm having a problem with C#, I'd like to get a pointer of a method in my code, but it seems impossible. I need the pointer of the method because I want to no-op it using WriteProcessMemory. How would I get the pointer?
Example code
main()
{
function1();
function2();
}
function1()
{
//get function2 pointer
//use WPM to nop it (I know how, this is not the problem)
}
function2()
{
Writeline("bla"); //this will never happen because I added a no-op.
}
I know this is very old, but an example of something like a function pointer in C# would be like this:
class Temp
{
public void DoSomething() {}
public void DoSomethingElse() {}
public void DoSomethingWithAString(string myString) {}
public bool GetANewCat(string name) { return true; }
}
...and then in your main or wherever:
var temp = new Temp();
Action myPointer = null, myPointer2 = null;
myPointer = temp.DoSomething;
myPointer2 = temp.DoSomethingElse;
Then to call the original function,
myPointer();
myPointer2();
If you have arguments to your methods, then it's as simple as adding generic arguments to your Action:
Action<string> doItWithAString = null;
doItWithAString = temp.DoSomethingWithAString;
doItWithAString("help me");
Or if you need to return a value:
Func<string, bool> getACat = null;
getACat = temp.GetANewCat;
var gotIt = getACat("help me");
EDIT: I misread your question and didn't see the bit about wanting to NOP a statement with doing raw memory manipulation. I'm afraid this isn't recommended because, as Raymond Chen says, the GC moves stuff around in memory (hence the 'pinned' keyword in C#). You probably can do it with reflection, but your question suggests you don't have a strong grasp of the CLR. Anyway, back to my original irrelevant answer (where I thought you just wanted information on how to use delegates):
C# isn't a scripting language ;)
Anyway, C# (and the CLR) has "function pointers" - except they're called "delegates" and are strongly typed, which means you need to define the function's signature in addition to the function you want to call.
In your case, you'd have something like this:
public static void Main(String[] args) {
Function1();
}
// This is the "type" of the function pointer, known as a "delegate" in .NET.
// An instance of this delegate can point to any function that has the same signature (in this case, any function/method that returns void and accepts a single String argument).
public delegate void FooBarDelegate(String x);
public static void Function1() {
// Create a delegate to Function2
FooBarDelegate functionPointer = new FooBarDelegate( Function2 );
// call it
functionPointer("bla");
}
public static void Function2(String x) {
Console.WriteLine(x);
}
public string myFunction(string name)
{
return "Hello " + name;
}
public string functionPointerExample(Func<string,string> myFunction)
{
return myFunction("Theron");
}
Func functionName.. use this to pass methods around. Makes no sense in this context but thats basically how you would use it
I'd wish it is useful
class Program
{
static void Main(string[] args)
{
TestPointer test = new TestPointer();
test.function1();
}
}
class TestPointer
{
private delegate void fPointer(); // point to every functions that it has void as return value and with no input parameter
public void function1()
{
fPointer point = new fPointer(function2);
point();
}
private void function2()
{
Console.WriteLine("Bla");
}
}
Actually there are real function pointers introduced in C# 9
Official Documentation
From the link:
You can define a function pointer using the delegate* syntax. The compiler will call the function using the calli instruction rather than instantiating a delegate object and calling Invoke
Example for the example in the post:
static unsafe void function1()
{
//get function2 pointer
delegate*<void> ptr = &function2;
// do something with ptr
}
Rewriting a method cannot be done directly from managed code, however the unmanaged .net profiling api can be used to do this. See this msdn article for example on how to use it.
This question already has answers here:
Where do I use delegates? [closed]
(8 answers)
Closed 9 years ago.
I'm relatively new in C#, & I'm wondering when to use Delegates appropriately.
they are widely used in events declaration, but when should I use them in my own code and why are they useful? why not to use something else?
I'm also wondering when I have to use delegates and I have no other alternative.
Thank you for the help!
EDIT: I think I've found a necessary use of Delegates here
A delegate is a reference to a method. Whereas objects can easily be sent as parameters into methods, constructor or whatever, methods are a bit more tricky. But every once in a while you might feel the need to send a method as a parameter to another method, and that's when you'll need delegates.
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
namespace DelegateApp {
/// <summary>
/// A class to define a person
/// </summary>
public class Person {
public string Name { get; set; }
public int Age { get; set; }
}
class Program {
//Our delegate
public delegate bool FilterDelegate(Person p);
static void Main(string[] args) {
//Create 4 Person objects
Person p1 = new Person() { Name = "John", Age = 41 };
Person p2 = new Person() { Name = "Jane", Age = 69 };
Person p3 = new Person() { Name = "Jake", Age = 12 };
Person p4 = new Person() { Name = "Jessie", Age = 25 };
//Create a list of Person objects and fill it
List<Person> people = new List<Person>() { p1, p2, p3, p4 };
//Invoke DisplayPeople using appropriate delegate
DisplayPeople("Children:", people, IsChild);
DisplayPeople("Adults:", people, IsAdult);
DisplayPeople("Seniors:", people, IsSenior);
Console.Read();
}
/// <summary>
/// A method to filter out the people you need
/// </summary>
/// <param name="people">A list of people</param>
/// <param name="filter">A filter</param>
/// <returns>A filtered list</returns>
static void DisplayPeople(string title, List<Person> people, FilterDelegate filter) {
Console.WriteLine(title);
foreach (Person p in people) {
if (filter(p)) {
Console.WriteLine("{0}, {1} years old", p.Name, p.Age);
}
}
Console.Write("\n\n");
}
//==========FILTERS===================
static bool IsChild(Person p) {
return p.Age < 18;
}
static bool IsAdult(Person p) {
return p.Age >= 18;
}
static bool IsSenior(Person p) {
return p.Age >= 65;
}
}
}
Output:
Children:
Jake, 12 years old
Adults:
John, 41 years old
Jane, 69 years old
Jessie, 25 years old
Seniors:
Jane, 69 years old
I agree with everything that is said already, just trying to put some other words on it.
A delegate can be seen as a placeholder for a/some method(s).
By defining a delegate, you are saying to the user of your class, "Please feel free to assign any method that matches this signature to the delegate and it will be called each time my delegate is called".
Typical use is of course events. All the OnEventX delegate to the methods the user defines.
Delegates are useful to offer to the user of your objects some ability to customize their behavior.
Most of the time, you can use other ways to achieve the same purpose and I do not believe you can ever be forced to create delegates. It is just the easiest way in some situations to get the thing done.
Say you want to write a procedure to integrate some real-valued function f (x) over some interval [a, b]. Say we want to use the 3-Point Gaussian method to do this (any will do, of course).
Ideally we want some function that looks like:
// 'f' is the integrand we want to integrate over [a, b] with 'n' subintervals.
static double Gauss3(Integrand f, double a, double b, int n) {
double res = 0;
// compute result
// ...
return res;
}
So we can pass in any Integrand, f, and get its definite integral over the closed interval.
Just what type should Integrand be?
Without Delegates
Well, without delegates, we'd need some sort of interface with a single method, say eval declared as follows:
// Interface describing real-valued functions of one variable.
interface Integrand {
double eval(double x);
}
Then we'd need to create a whole bunch of classes implementing this interface, as follows:
// Some function
class MyFunc1 : Integrand {
public double eval(double x) {
return /* some_result */ ;
}
}
// Some other function
class MyFunc2 : Integrand {
public double eval(double x) {
return /* some_result */ ;
}
}
// etc
Then to use them in our Gauss3 method, we need to invoke it as follows:
double res1 = Gauss3(new MyFunc1(), -1, 1, 16);
double res2 = Gauss3(new MyFunc2(), 0, Math.PI, 16);
And Gauss3 needs to do the look like the following:
static double Gauss3(Integrand f, double a, double b, int n) {
// Use the integrand passed in:
f.eval(x);
}
So we need to do all that just to use our arbitrary functions in Guass3.
With Delegates
public delegate double Integrand(double x);
Now we can define some static (or not) functions adhering to that prototype:
class Program {
public delegate double Integrand(double x);
// Define implementations to above delegate
// with similar input and output types
static double MyFunc1(double x) { /* ... */ }
static double MyFunc2(double x) { /* ... */ }
// ... etc ...
public static double Gauss3(Integrand f, ...) {
// Now just call the function naturally, no f.eval() stuff.
double a = f(x);
// ...
}
// Let's use it
static void Main() {
// Just pass the function in naturally (well, its reference).
double res = Gauss3(MyFunc1, a, b, n);
double res = Gauss3(MyFunc2, a, b, n);
}
}
No interfaces, no clunky .eval stuff, no object instantiation, just simple function-pointer like usage, for a simple task.
Of course, delegates are more than just function pointers under the hood, but that's a separate issue (function chaining and events).
Delegates are extremely useful when wanting to declare a block of code that you want to pass around. For example when using a generic retry mechanism.
Pseudo:
function Retry(Delegate func, int numberOfTimes)
try
{
func.Invoke();
}
catch { if(numberOfTimes blabla) func.Invoke(); etc. etc. }
Or when you want to do late evaluation of code blocks, like a function where you have some Transform action, and want to have a BeforeTransform and an AfterTransform action that you can evaluate within your Transform function, without having to know whether the BeginTransform is filled, or what it has to transform.
And of course when creating event handlers. You don't want to evaluate the code now, but only when needed, so you register a delegate that can be invoked when the event occurs.
Delegates Overview
Delegates have the following properties:
Delegates are similar to C++ function pointers, but are type safe.
Delegates allow methods to be passed as parameters.
Delegates can be used to define callback methods.
Delegates can be chained together; for example, multiple methods can be called on a single event.
Methods don't need to match the delegate signature exactly. For more information, see Covariance and Contra variance.
C# version 2.0 introduces the concept of Anonymous Methods, which permit code blocks to be passed as parameters in place of a separately defined method.
I've just go my head around these, and so I'll share an example as you already have descriptions but at the moment one advantage I see is to get around the Circular Reference style warnings where you can't have 2 projects referencing each other.
Let's assume an application downloads an XML, and then saves the XML to a database.
I have 2 projects here which build my solution: FTP and a SaveDatabase.
So, our application starts by looking for any downloads and downloading the file(s) then it calls the SaveDatabase project.
Now, our application needs to notify the FTP site when a file is saved to the database by uploading a file with Meta data (ignore why, it's a request from the owner of the FTP site). The issue is at what point and how? We need a new method called NotifyFtpComplete() but in which of our projects should it be saved too - FTP or SaveDatabase? Logically, the code should live in our FTP project. But, this would mean our NotifyFtpComplete will have to be triggered or, it will have to wait until the save is complete, and then query the database to ensure it is in there. What we need to do is tell our SaveDatabase project to call the NotifyFtpComplete() method direct but we can't; we'd get a ciruclar reference and the NotifyFtpComplete() is a private method. What a shame, this would have worked. Well, it can.
During our application's code, we would have passed parameters between methods, but what if one of those parameters was the NotifyFtpComplete method. Yup, we pass the method, with all of the code inside as well. This would mean we could execute the method at any point, from any project. Well, this is what the delegate is. This means, we can pass the NotifyFtpComplete() method as a parameter to our SaveDatabase() class. At the point it saves, it simply executes the delegate.
See if this crude example helps (pseudo code). We will also assume that the application starts with the Begin() method of the FTP class.
class FTP
{
public void Begin()
{
string filePath = DownloadFileFromFtpAndReturnPathName();
SaveDatabase sd = new SaveDatabase();
sd.Begin(filePath, NotifyFtpComplete());
}
private void NotifyFtpComplete()
{
//Code to send file to FTP site
}
}
class SaveDatabase
{
private void Begin(string filePath, delegateType NotifyJobComplete())
{
SaveToTheDatabase(filePath);
/* InvokeTheDelegate -
* here we can execute the NotifyJobComplete
* method at our preferred moment in the application,
* despite the method being private and belonging
* to a different class.
*/
NotifyJobComplete.Invoke();
}
}
So, with that explained, we can do it for real now with this Console Application using C#
using System;
namespace ConsoleApplication1
{
/* I've made this class private to demonstrate that
* the SaveToDatabase cannot have any knowledge of this Program class.
*/
class Program
{
static void Main(string[] args)
{
//Note, this NotifyDelegate type is defined in the SaveToDatabase project
NotifyDelegate nofityDelegate = new NotifyDelegate(NotifyIfComplete);
SaveToDatabase sd = new SaveToDatabase();
sd.Start(nofityDelegate);
Console.ReadKey();
}
/* this is the method which will be delegated -
* the only thing it has in common with the NofityDelegate
* is that it takes 0 parameters and that it returns void.
* However, it is these 2 which are essential.
* It is really important to notice that it writes
* a variable which, due to no constructor,
* has not yet been called (so _notice is not initialized yet).
*/
private static void NotifyIfComplete()
{
Console.WriteLine(_notice);
}
private static string _notice = "Notified";
}
public class SaveToDatabase
{
public void Start(NotifyDelegate nd)
{
/* I shouldn't write to the console from here,
* just for demonstration purposes
*/
Console.WriteLine("SaveToDatabase Complete");
Console.WriteLine(" ");
nd.Invoke();
}
}
public delegate void NotifyDelegate();
}
I suggest you step through the code and see when _notice is called and when the method (delegate) is called as this, I hope, will make things very clear.
However, lastly, we can make it more useful by changing the delegate type to include a parameter.
using System.Text;
namespace ConsoleApplication1
{
/* I've made this class private to demonstrate that the SaveToDatabase
* cannot have any knowledge of this Program class.
*/
class Program
{
static void Main(string[] args)
{
SaveToDatabase sd = new SaveToDatabase();
/* Please note, that although NotifyIfComplete()
* takes a string parameter, we do not declare it,
* all we want to do is tell C# where the method is
* so it can be referenced later,
* we will pass the parameter later.
*/
var notifyDelegateWithMessage = new NotifyDelegateWithMessage(NotifyIfComplete);
sd.Start(notifyDelegateWithMessage );
Console.ReadKey();
}
private static void NotifyIfComplete(string message)
{
Console.WriteLine(message);
}
}
public class SaveToDatabase
{
public void Start(NotifyDelegateWithMessage nd)
{
/* To simulate a saving fail or success, I'm just going
* to check the current time (well, the seconds) and
* store the value as variable.
*/
string message = string.Empty;
if (DateTime.Now.Second > 30)
message = "Saved";
else
message = "Failed";
//It is at this point we pass the parameter to our method.
nd.Invoke(message);
}
}
public delegate void NotifyDelegateWithMessage(string message);
}
I consider delegates to be Anonymous Interfaces. In many cases you can use them whenever you need an interface with a single method, but you don't want the overhead of defining that interface.
A delegate is a simple class that is used to point to methods with a specific signature, becoming essentially a type-safe function pointer. A delegate's purpose is to facilitate a call back to another method (or methods), after one has been completed, in a structured way.
While it could be possible to create an extensive set of code to perform this functionality, you don’t need too. You can use a delegate.
Creating a delegate is easy to do. Identify the class as a delegate with the "delegate" keyword. Then specify the signature of the type.