We have some classes where we need to fire events during initialization. The current state is that we use emtpy ctors, then attach all the events we need and then use init() to fully initialize the object.
Id like to get rid of this two-phase-initialization. Its ugly and causes problems when using the objects without initialization (we tried the 'isInitialized' flag, but that ugly too...).
Is there a better way to attach events to an object which are needed during initialization?
Edit:
Its code like this:
public class ExampleClass {
public int value;
public event ErrorHandling OnError;
public ExampleClass(int value) {
this.value = value;
}
void init() {
// do something with that value
OnError?.Invoke(value); // something went wrong
}
}
int main() {
ExampleClass example = new ExampleClass(2);
example.OnError += // some error handling
example.init();
}
Its a very limited example. But in fact I cant get rid of the events during initialization.
TIA
Do you need to use events? Either pass an IErrorHandler into the constructor (ideally with dependency injection), or like pass an Action that will be run instead of the event
Related
Here is the code for custom event args. I am confuse about use of those and also role of those. I can not understand this property public object AddedObject { get; private set; } the code is here :
public class ObjectAddedEventArgs : EventArgs
{
public ObjectAddedEventArgs(object addedObject)
{
AddedObject = addedObject;
}
public object AddedObject { get; private set; }
}
I can not understand use of the get and set property of added object. Please explain to me.
The AddedObject property is what is called an "auto property", which simply means that the C# compiler will generate a private variable to hold the value of the property. The "get" is the mechanism which allows you to read the value of the property. The "set" is the mechanism which allows you to set the value of the property, although in this case since the set is private you aren't able to set the value. This translates to code which would look roughly like this:
private object _AddedObject;
public object AddedObject
{
get { return this._AddedObject; }
private set { this._AddedObject = value; }
}
The class itself (the ObjectAddedEventArgs class) is used to provide additional data (the AddedObject value) to an event handler which, presumably, would access that data and do something with it as part of it's response to the event.
As per Microsoft's documentation:
EventArgs is the base class for classes containing event data.
....
This class contains no event data; it is used by events that do not
pass state information to an event handler when an event is raised. If
the event handler requires state information, the application must
derive a class from this class to hold the data.
That being said, the original developer of that class intended to handle events and at the same time making the object AddedObject available to the event handler method. See the website above for a nice example.
the good thing when you declare your events like this
event EventHandler<ObjectAddedEventArgs> MyObjectAddedEvent;
you can subscribe to it in a weak manner easily
myObjectInstance.MyObjectAddedEvent += new EventHandler<ObjectAddedEventArgs>(MyObjectAddedEventMethod).MakeWeak(eh => d.MyObjectAddedEvent -= eh);
private void MyObjectAddedEventMethod(object sender, ObjectAddedEventArgseventargs)
{
//do something with the event args
}
the weak stuff you find here.
I need to attach an event handler to an object, and I placed this code on a button click event. However, I noticed that this will cause the same event to attach multiple times with each click.
Is there a way to run a piece of code on class creation? The class in question is a static class btw.
I can do something like:
if (bool == false)
{
attach event handler;
bool = true;
}
Just not sure if this is the right way to do it. Thanks.
There are static constructors, that are (in principle) only run once per class.
Something like this:
public static class MyStaticClass
{
public static int MyStaticProperty;
//no accessors required, as this is never explicitly invoked
static MyStaticClass() //no parameters either
{
MyStaticProperty = 100;
}
}
....
//writes: 100
Console.WriteLine(MyStaticClass.MyStaticProperty);
However, if a constructor won't do it, because you have some parameters that need to be set, or there are some prerequisite steps that need to be done, I would indeed recommend a private boolean check, as you have done.
You use a constructor - it will run on class creation.
Constructors are class methods that are executed when an object of a class or struct is created. They have the same name as the class or struct, and usually initialize the data members of the new object.
For static classes, use static constructors:
A static constructor is used to initialize any static data, or to perform a particular action that needs to be performed once only. It is called automatically before the first instance is created or any static members are referenced.
try
if(Button1.Click == null)
Button1.Click += new System.EventHandler(this.myEventHandler);
This is abit difficult to word, so I am going to rely mostly on code.
BTW if you can word the question in a better light please dont hesitate giving your 2c!
class CustomEventArgs : EventArgs
{
public delegate void CustomEventHandler( Object sender, CustomEventArgs args );
public int data;
public CustomEventArgs (int _data)
{
data = _data;
}
}
This is the event that we will be using in this example.
class EventGenerator
{
public event CustomEventArgs.CustomEventHandler WeOccasion;
public EventGenerator ()
{
Task.Factory.StartNew( () =>
{
var index = 1;
// just loop and generate events every now and then
while (true)
{
Thread.Sleep( 1000 );
WeOccasion( this, new CustomEventArgs (++index));
}
});
}
}
This class just loops through firing off CustomEventHandler events.
class EventActivity
{
// EventActivity has an event of the same type as EventGenerator's
public event CustomEventArgs.CustomEventHandler WeOccasion;
// this is the part I cant seem to get right
public event CustomEventArgs.CustomEventHandler Source ( get; set; }
public bool Active {
set
{
if (value)
{
Source += DoWork;
}
else
{
Source -= DoWork;
}
}
}
private void DoWork( Object sender, CustomEventArgs frame);
}
Here is where I really need help. I want almost a pointer to an event in an another class of type CustomEventHandler that I can later assign event handlers to when I activate the activity.
Here is a usage example wrapped in a class;
class EventAssigner
{
EventGenerator Generator;
EventActivity DoSomeThing1;
EventActivity DoSomeThing2;
public EventAssigner ()
{
// init
Generator = new EventGenerator();
DoSomeThing1 = new EventActivity();
DoSomeThing2 = new EventActivity();
// assign sources
DoSomeThing1.Source = Generator.WeOccasion;
DoSomeThing2.Source = DoSomeThing1.WeOccasion;
// activate the first activity
DoSomeThing1.Active = true;
}
public void Activate2()
{
// activate the second activity
DoSomeThing2.Active = true;
}
public void Deactivate2()
{
// deactivate the second activity
DoSomeThing2.Active = false;
}
}
Obiously this code doesnt work, and I suppose thats what I am asking. Can you get this design pattern to work?
What you're asking to do isn't really possible with .NET events, and probably isn't as desirable as you might think. A bit of background should help explain why:
Properties have a basic pattern with get and set operations. These are invoked by accessing the property (for a get) and an assignment to the property (for a set):
var x = instance.Prop1; // access
instance.Prop1 = x; // assignment
When you access an event from outside the class (i.e. instance.Event) you are given the "public" face, which, like properties, has two operations: add handler and remove handler. These are invoked using the += and -= operators.
instance.Event += this.Handler; // add
instance.Event -= this.Handler; // remove
The important thing to notice that it doesn't have a "get" operation - there is no way to get a reference to the event outside the class; you can only modify the handlers registered.
When you access an event from within a class, you are given the "private" face, which is essentially a special collection of delegates (function pointers) to the registered event handlers. When you invoke the delegate, you're actually asking the framework to iterate through the registered event handlers and invoke those.
if(this.Event != null)
{
this.Event.Invoke(e, args); // raise event
}
This separation of public face and private face is what allows you have a nice simple event keyword which magically gives you an event. It is also what stops you passing a reference to the event around.
To pass the event into registration methods, you have to pass the object the event is attached to. If you have multiple classes which implement the same event and you want to register them all in the same way, you should have them implement an interface with the event (yes, events can be on interfaces) and write your method to accept the interface as an argument.
If I'm reading you correct, you want the line
DoSomeThing1.Source = Generator.WeOccasion;
to save the pointer to the WeOccasion event, so that you can add the DoWork call to it later, right?
I don't think that is possible with "normal" code, as the event is not a value, but rather like a property. Consider the following analogous code:
myProp = aPerson.Name; // attempt to save the name property for later
myProp = "Fred"; // intent is to set aPerson.Name = "Fred"
If you want this to work I'd suggest using reflection to find the event, and add to it using the EventInfo.AddEventHandler method (http://msdn.microsoft.com/en-us/library/system.reflection.eventinfo.addeventhandler.aspx)
I have the following class, which has one public event called LengthChanged:
class Dimension
{
public int Length
{
get
{
return this.length;
}
set
{
if (this.length != value)
{
this.length = value;
this.OnLengthChanged ();
}
}
protected virtual void OnLengthChanged()
{
var handler = this.LengthChanged;
if (handler != null)
{
handler (this, System.EventArgs.Empty);
}
}
public event System.EventHandler LengthChanged;
private int length;
}
I would like to be able to register/unregister handlers for this event in a method called Observer, which does not know anything about the Dimension class. I have come up with two scenarios, none of which are really satisfying:
Define an interface ILengthChanged with the LengthChanged event, then make sure Dimension implements ILengthChanged. Then I have to provide one implementation of the Observer method for every interface I define. This by no way generic enough. I'd really want to be able to simply pass in a reference to a System.EventHandler event.
Use System.Action<System.EventHandler> callbacks for registering and unregistering the event handler in the Observer method, just like that:
class Foo
{
public void Observer(System.Action<System.EventHandler> register,
System.Action<System.EventHandler> unregister)
{
register (this.MyEventHandler);
// keep track of the unregister callback, so that we can unregister
// our event handler later on, if needed...
}
private void MyEventHandler(object sender, System.EventArgs e)
{
...
}
}
which would then be invoked like this:
Foo foo = ...;
Dimension dim = ...;
foo.Observer (x => dim.LengthChanged += x, x => dim.LengthChanged -= x);
and which, when executed, will indeed end up wiring the LengthChanged event with the internal event handler MyEventHandler. But this is not very elegant. I would have loved to be able to write this instead:
Foo foo = ...;
Dimension dim = ...;
foo.Observer (dim.LengthChanged);
but I've no idea how this could be achieved. Maybe I am missing something really obvious here? I guess that some dynamic magic could do the trick, somehow, but this would not enforce compile-time type checking: I don't want the users of Observer to pass in references to events which do not satisfy the System.EventHandler event signature.
Unfortunately there isn't really a way of doing this. Events aren't first class citizens in .NET in general - although F# tries to promote them there.
Either pass in the subscribe/unsubscribe delegate or using a string indicating the name of the event. (The latter is often shorter, but obviously less safe at compile-time.)
Those are the approaches which Reactive Extensions takes - if there were a cleaner way of doing it, I'm sure they would be using that :(
You can create a custom accessor.
public event EventHandler NewEvent
{
add { Dimension.LengthChanged += value; }
remove { Dimension.LengthChanged -= value; }
}
Please see the documentation.
Event is not supposed to be passed into another method. However, you can pass delegate into another method. Perhaps, what you are looking for are just a simple public delegate instead of event.
If you change your event to this
public System.EventHandler LengthChanged;
You can simply pass the LengthChanged to Observer like this
Foo foo = ...;
Dimension dim = ...;
foo.Observer (dim.LengthChanged);
I'm currently developing a tiny technical Framework that is independant of any applications. Business code just refers to this Framework.
According this article : http://msdn.microsoft.com/en-us/library/5z57dxz2.aspx (exemple 2), we need to provide a delegate for the custom event.
Problem is, anyone can Invoke my handler (and then raise the event), even in my Business Code and that isn't logical for me, so what is the best way to raise a custom Event with a delegate that is only "internal" and not "public" ?
Thanks for help.
I am not sure if I get it right or not. I think that you feel like if you provide a public Delegate type for your custom event, anyone will be able to Raise that event.
Well, that is not true. Only the class that defines that custom event can raise it. If this is your issue, don't worry.
Not true. It's not allowed to invoke an event outside the class which the event belongs to. Others can only use += and -= operators to your event. Only in the class, you can invoke the event. That is a difference between an event and a normal delegate. That is:
public Data
{
public event EventHandler OnSave
public EventHandler OnLoad;
private void Load()
{
if (OnLoad!=null) OnLoad();
//other operations
}
private void Save()
{
if (OnSave!=null) OnSave();
//other operations
}
}
And outside the class:
Data data = new Data();
data.OnLoad += (s,e) => {};
data.OnSave += (s,e) => {};
data.OnLoad = (s,e)=>{};
//data.OnSave = (s,e)=>{}; //invalid
data.OnLoad();
//data.OnSave(); //invalid
The delegate is just a type declaration describing the "signature" of your event. This has to be public. To actually invoke your event you often implement a method named OnEvent (where you substitute Event with Click or Closed or whatever describes your event). This method should be private (or protected) in your class.
You cannot declare an event using a delegate that is less "visible" than the event.
Problem is, anyone can Invoke my handler (and then raise the event), even in my Business Code
That isn't true. You declare an event as follows:
public event FooEventHandler Foo;
The only thing that external code can do with the event is subscribe to it (+=), or unsubscribe from it (-=). It can't access the actual delegate, which is generated by the compiler as a private field. In other words, this code would be invalid :
SomeClass x = new SomeClass();
x.Foo(x, new FooEventArgs()); // compilation error here
Don't forget that an event is actually a pair of methods (add and remove). The compiler rewrites the event declaration to something along those lines:
private FooEventHandler _foo;
public event FooEventHandler Foo
{
add { _foo += value; }
remove { _foo -= value; }
}
(the generated code is actually a bit more complex, with some locking to ensure thread safety)
As you can see, the _foo field is private, so client code can't access it. Only the event's add and remove accessors are accessible.
One way of doing it:
Instead of public event, create a method that will manually subscribe your desired delegates, and store them in `private List _delegates' field.
Then, from the 'inside', call each of them when you desire.
public class Framework
{
public delegate void Method();
public void AttachEvent(Method M)
{
_methods.Add(M);
}
private List<Method> _methods;
private FireMethods()
{
_methods.Foreach(x=>x.Invoke());
}
}
Or, you can embrace 'by design' feature of the events that they aren't publicly Invoke()-able.
:)