Questions tagged [delegates]
Delegates can refer to several concepts. An object can rely on another (a delegate) to perform a function. Delegation can also refer to programming language feature making use of the method lookup rules for dispatching self-calls. In C#, a delegate defines which method to call when an event is triggered.
delegates
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Can I call a constructor from another constructor (do constructor chaining) in C++?
As a C# developer I'm used to running through constructors:
class Test {
public Test() {
DoSomething();
}
public Test(int count) : this() {
DoSomethingWithCount(count);
...
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votes
12
answers
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Why would you use Expression<Func<T>> rather than Func<T>?
I understand lambdas and the Func and Action delegates. But expressions
stump me.
In what circumstances would you use an Expression<Func<T>> rather than a plain old Func<T>?
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Pass Method as Parameter using C#
I have several methods all with the same parameter types and return values but different names and blocks. I want to pass the name of the method to run to another method that will invoke the passed ...
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votes
3
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Func vs. Action vs. Predicate [duplicate]
With real examples and their use, can someone please help me understand:
When do we need a Func<T, ..> delegate?
When do we need an Action<T> delegate?
When do we need a Predicate<T>...
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How do I create delegates in Objective-C?
I know how delegates work, and I know how I can use them.
But how do I create them?
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706
votes
7
answers
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Func delegate with no return type
All of the Func<T> delegates return a value. What are the .NET delegates that can be used with methods that return void?
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votes
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answers
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How can I make a weak protocol reference in 'pure' Swift (without @objc)
weak references don't seem to work in Swift unless a protocol is declared as @objc, which I don't want in a pure Swift app.
This code gives a compile error (weak cannot be applied to non-class type ...
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votes
8
answers
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When & why to use delegates? [duplicate]
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 ...
376
votes
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answers
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What are the differences between delegates and events?
What are the differences between delegates and an events? Don't both hold references to functions that can be executed?
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votes
14
answers
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Unsubscribe anonymous method in C#
Is it possible to unsubscribe an anonymous method from an event?
If I subscribe to an event like this:
void MyMethod()
{
Console.WriteLine("I did it!");
}
MyEvent += MyMethod;
I can un-...
- 4,261
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votes
14
answers
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Java Delegates?
Does the Java language have delegate features, similar to how C# has support for delegates?
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delegate keyword vs. lambda notation
Once it is compiled, is there a difference between:
delegate { x = 0; }
and
() => { x = 0 }
?
- 12.3k
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votes
6
answers
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What is a C++ delegate?
What is the general idea of a delegate in C++? What are they, how are they used and what are they used for?
I'd like to first learn about them in a 'black box' way, but a bit of information on the ...
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votes
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answers
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Why are Objective-C delegates usually given the property assign instead of retain?
I'm surfing through the wonderful blog maintained by Scott Stevenson, and I'm trying to understand a fundamental Objective-C concept of assigning delegates the 'assign' property vs 'retain'. Note, the ...
anon
166
votes
13
answers
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What is Delegate? [closed]
I am confused that what is the actual role of a delegate?
I have been asked this question many times in my interviews, but I don't think that interviewers were satisfied with my answer.
Can anyone ...
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Delegates: Predicate vs. Action vs. Func
Can someone provide a good explanation (hopefully with examples) of these 3 most important delegates:
Predicate
Action
Func
Sasha
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votes
4
answers
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What's the best way to communicate between view controllers?
Being new to objective-c, cocoa, and iPhone dev in general, I have a strong desire to get the most out of the language and the frameworks.
One of the resources I'm using is Stanford's CS193P class ...
A Hopeful Soul
164
votes
12
answers
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Function Pointers in Java
This may be something common and trivial, but I seem to be having trouble finding a concrete answer. In C# there is a concept of delegates, which relates strongly to the idea of function pointers from ...
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answer
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Difference Between Invoke and DynamicInvoke
What is the difference between Invoke and DynamicInvoke in delegates? Please give me some code example which explain difference between that two methods.
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votes
12
answers
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views
How to hide the keyboard when I press return key in a UITextField?
Clicking in a textfield makes the keyboard appear.
How do I hide it when the user presses the return key?
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votes
9
answers
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Why can't an anonymous method be assigned to var?
I have the following code:
Func<string, bool> comparer = delegate(string value) {
return value != "0";
};
However, the following does not compile:
var comparer = delegate(string value) {
...
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votes
10
answers
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views
How can I clear event subscriptions in C#?
Take the following C# class:
c1 {
event EventHandler someEvent;
}
If there are a lot of subscriptions to c1's someEvent event and I want to clear them all, what is the best way to achieve this? ...
- 4,372
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votes
15
answers
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Delegates in swift?
How does one go about making a delegate, i.e. NSUserNotificationCenterDelegate in swift?
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votes
9
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143
votes
8
answers
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Super-simple example of C# observer/observable with delegates
I recently started digging into C# but I can't by my life figure out how delegates work when implementing the observer/observable pattern in the language.
Could someone give me a super-simple example ...
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votes
4
answers
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How do I set up a simple delegate to communicate between two view controllers?
I have two UITableViewControllers and need to pass the value from the child view controller to the parent using a delegate. I know what delegates are and just wanted to see a simple to follow example.
...
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votes
9
answers
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Uses of Action delegate in C# [closed]
I was working with the Action Delegates in C# in the hope of learning more about them and thinking where they might be useful.
Has anybody used the Action Delegate, and if so why? or could you give ...
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votes
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Why must a lambda expression be cast when supplied as a plain Delegate parameter
Take the method System.Windows.Forms.Control.Invoke(Delegate method)
Why does this give a compile time error:
string str = "woop";
Invoke(() => this.Text = str);
// Error: Cannot convert lambda ...
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131
votes
15
answers
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Wrap a delegate in an IEqualityComparer
Several Linq.Enumerable functions take an IEqualityComparer<T>. Is there a convenient wrapper class that adapts a delegate(T,T)=>bool to implement IEqualityComparer<T>? It's easy enough ...
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votes
3
answers
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"Delegate subtraction has unpredictable result" in ReSharper/C#?
When using myDelegate -= eventHandler ReSharper (version 6) issues:
Delegate subtraction has unpredictable result
The rational behind this is explained by JetBrains here. The explanation makes ...
user166390
126
votes
7
answers
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views
Are C# events synchronous?
There are two parts to this question:
Does raising an event block the thread, or does it start execution of EventHandlers asynchronously and the thread goes continues on at the same time?
Are the ...
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votes
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C# pattern to prevent an event handler hooked twice [duplicate]
Duplicate of: How to ensure an event is only subscribed to once
and Has an event handler already been added?
I have a singleton that provides some service and my classes hook into some events on it, ...
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votes
22
answers
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How do I know that the UICollectionView has been loaded completely?
I have to do some operation whenever UICollectionView has been loaded completely, i.e. at that time all the UICollectionView's datasource / layout methods should be called. How do I know that?? Is ...
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votes
6
answers
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How to add a delegate to an interface C#
I need to have some delegates in my class.
I'd like to use the interface to "remind" me to set these delegates.
How to?
My class look like this:
public class ClsPictures : myInterface
{
// ...
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What is the difference between Func<string,string> and delegate?
I see delegates in two forms:
A. Func<string, string> convertMethod = lambda
B. public delegate string convertMethod(string value);
I'm uncertain of what actually the difference between ...
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votes
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Where do I use delegates? [closed]
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.
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10
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Difference between events and delegates and its respective applications [closed]
I don't see advantages of using events over delegates, other than being syntactical sugar. Perhaps I am misunderstanding, but it seems that event is just a placeholder for delegate.
Would you explain ...
Sasha
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votes
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answers
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Invoke(Delegate)
Can anybody please explain this statement written on this link
Invoke(Delegate):
Executes the specified delegate on the thread that owns the control's underlying window handle.
Can anybody explain ...
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answers
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104
votes
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answers
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Compiler Ambiguous invocation error - anonymous method and method group with Func<> or Action
I have a scenario where I want to use method group syntax rather than anonymous methods (or lambda syntax) for calling a function.
The function has two overloads, one that takes an Action, the other ...
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votes
10
answers
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views
Wrapping StopWatch timing with a delegate or lambda?
I'm writing code like this, doing a little quick and dirty timing:
var sw = new Stopwatch();
sw.Start();
for (int i = 0; i < 1000; i++)
{
b = DoStuff(s);
}
sw.Stop();
Console.WriteLine(sw....
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votes
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answers
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Proper naming convention for a .NET Delegate type? [closed]
By convention classes are often named like nouns, methods like verbs and interfaces like adjectives.
What is the common naming convention for a delegate? Or what's a good way to differentiate its ...
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votes
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answer
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What's the difference between 'weak' and 'assign' in delegate property declaration
Whats the difference between this:
@property (nonatomic, weak) id <SubClassDelegate> delegate;
and this:
@property (nonatomic, assign) id <SubClassDelegate> delegate;
I want to use ...
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Assigning to 'id<Delegate>' from incompatible type 'ViewController *const_strong'
Throughout my app, I'm getting semantic issue warnings when I set ViewController.delegate = self. I have searched and found similar posts but none were able to solve my problem.
ViewController.m:
...
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votes
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What's the method signature for passing an async delegate?
I've recently moved back to C# from being in Objective-C land, and the async/await keywords in C# 5 look cool. But I'm still trying to get a handle on the proper syntax.
I want to declare a method ...
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votes
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Performance of calling delegates vs methods
Following this question - Pass Method as Parameter using C# and some of my personal experience I'd like to know a little more about the performance of calling a delegate vs just calling a method in C#....
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votes
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Is there a downside to adding an anonymous empty delegate on event declaration?
I have seen a few mentions of this idiom (including on SO):
// Deliberately empty subscriber
public event EventHandler AskQuestion = delegate {};
The upside is clear - it avoids the need to check ...
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85
votes
8
answers
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C# Generics won't allow Delegate Type Constraints
Is it possible to define a class in C# such that
class GenericCollection<T> : SomeBaseCollection<T> where T : Delegate
I couldn't for the life of me accomplish this last night in .NET 3....
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votes
8
answers
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Creating delegates manually vs using Action/Func delegates
Today I was thinking about declaring this:
private delegate double ChangeListAction(string param1, int number);
but why not use this:
private Func<string, int, double> ChangeListAction;
or ...
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votes
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Events - naming convention and style
I'm learning about Events / Delegates in C#. Could I ask your opinion on the naming/coding style I've chosen (taken from the Head First C# book)?
Am teaching a friend about this tomorrow, and am ...
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