Asked  7 Months ago    Answers:  5   Viewed   34 times

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-subscribe like this:

MyEvent -= MyMethod;

But if I subscribe using an anonymous method:

MyEvent += delegate(){Console.WriteLine("I did it!");};

is it possible to unsubscribe this anonymous method? If so, how?

 Answers

26
Action myDelegate = delegate(){Console.WriteLine("I did it!");};

MyEvent += myDelegate;


// .... later

MyEvent -= myDelegate;

Just keep a reference to the delegate around.

Tuesday, June 1, 2021
 
mgraph
answered 7 Months ago
82

Because Invoke/BeginInvoke accepts Delegate (rather than a typed delegate), you need to tell the compiler what type of delegate to create ; MethodInvoker (2.0) or Action (3.5) are common choices (note they have the same signature); like so:

control.Invoke((MethodInvoker) delegate {this.Text = "Hi";});

If you need to pass in parameters, then "captured variables" are the way:

string message = "Hi";
control.Invoke((MethodInvoker) delegate {this.Text = message;});

(caveat: you need to be a bit cautious if using captures async, but sync is fine - i.e. the above is fine)

Another option is to write an extension method:

public static void Invoke(this Control control, Action action)
{
    control.Invoke((Delegate)action);
}

then:

this.Invoke(delegate { this.Text = "hi"; });
// or since we are using C# 3.0
this.Invoke(() => { this.Text = "hi"; });

You can of course do the same with BeginInvoke:

public static void BeginInvoke(this Control control, Action action)
{
    control.BeginInvoke((Delegate)action);
}

If you can't use C# 3.0, you could do the same with a regular instance method, presumably in a Form base-class.

Wednesday, June 2, 2021
 
makadev
answered 6 Months ago
51

There is no way to do this in Objective-C currently. Apple has published some work on their efforts to add blocks (really more like lambda closures than anonymous classes) to the language. You would likely be able to do something similar to the anonymous delegate with those.

In the mean time, most Cocoa programmers add the delegate methods to a separate category on the delegate class. This helps to keep the code more organized. In the .m file for the class in your example, I would do something like this:

@interface MyClass (UIActionSheetDelegate)
- (void)actionSheet:(UIActionSheet*)actionSheet clickedButtonAtIndex:(NSInteger)buttonIndex;
@end

@implementation MyClass
//... normal stuff here
@end

@implementation MyClass (UIActionSheetDelegate)
- (void)actionSheet:(UIActionSheet *)actionSheet clickedButtonAtIndex:(NSInteger)buttonIndex{
    if (buttonIndex == 0){
        [[Settings sharedSettings] removeItemAtIndex:/*need index variable here*/];
        [drinksTable reloadData];
    }
}
@end

Xcode's method popup in the editor window will separate the category's declaration and implementation from the main class'.

Monday, August 9, 2021
 
user336786
answered 4 Months ago
52

To get this behavior, you need to copy the variable locally, not use the iterator:

for (int i = 0; i < 7; i++)
{
    var inneri = i;
    Button newButton = new Button();
    newButton.Text = "Click me!";
    newButton.Click += delegate(Object sender, EventArgs e)
    {
        MessageBox.Show("I am button number " + inneri);
    };
    this.Controls.Add(newButton);
}

The reasoning is discussed in much greater detail in this question.

Monday, August 9, 2021
 
Kzqai
answered 4 Months ago
58

var does not mean "use an anonymous type", it means "Compiler, go figure out the type for me!". In the first three cases, the type is actually a "named" type - System.Int32, System.String, and System.Int32[] (in the last case the type of array's elements is also deduced by the compiler from the type of array elements that you put in the initializer).

The last case is the only one where an anonymous type is used. It is by design that C#'s anonymous types are immutable. The primary case for adding them in the language in the first place has been introduction of LINQ, which does not need mutability in cases when anonymous types are produced. In general, immutable classes tend to give designers less problems, especially when concurrency is involved, so designers of the language decided to go with immutable anonymous types.

Friday, August 13, 2021
 
kmunky
answered 4 Months ago
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