hoping someone could lend a hand here. I have a custom even argument that contains a collection. I can build the event arg but I cannot implement it. I want to put the event handler in the base class and override it in my derived classes.
The even arg:
public class ImportEventArgs<T> : EventArgs
{
public IEnumerable<T> Data { get; }
public ImportEventArgs(IEnumerable<T> data)
{
Data = data;
}
}
The part in my base class that is not resolving:
public virtual void EventHandler<EventArgs> ImportComplete;
The override:
override void EventHandler<WellPathImportEventArgs<WellPath>> ImportComplete;
I figured this is not the correct way of doing this, could anyone point me in the right direction?
You cannot change a method's signature when you override it.
However, since all event args derive from the same type, you don't have to. Just keep the existing method definition and cast the argument, like this:
public override void ImportComplete(object sender, EventArgs e)
{
var myEventArgs = e as ImportEventArgs<WellPath>;
if (myEventArgs != null)
{
foreach (var item in myEventArgs.Data)
{
//Your code here
}
}
}
Related
How can I avoid this warning message in VS2017 (or: what may be happening):
'ObsCol<T>.ObsCol()' contains a call chain that results in a call to a virtual method
defined by the class.
Review the following call stack for unintended consequences:
ObsCol<T>..ctor()
ObservableCollection<T>.add_CollectionChanged(NotifyCollectionChangedEventHandler): Void
this is the code:
[Serializable]
public class ObsCol<T> : ObservableCollection<T>
{
public ObsCol()
{
this.CollectionChanged += new NotifyCollectionChangedEventHandler(ObsCol_CollectionChanged);
}
private void ObsCol_CollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
{
if (e.Action == NotifyCollectionChangedAction.Add)
MainViewModel.IsDirty = e.NewItems != null || e.OldItems != null;
}
}
The purpose of this all is to know in the ViewModel whether or not the calculation output corresponds to the input data.
Upon entering the constructor, CollectionChanged is empty, so my handler is the only thing in the list.
Changing CollectionChanged does not change the collection, does it?
Things seem to work alright, though...
The Problem: some subclass could override the event, leading to unexpected construction behavior
public class Boom<T> : ObsCol<T>
{
public override event NotifyCollectionChangedEventHandler CollectionChanged
{
add { throw new NotImplementedException(); }
remove { }
}
}
Solutions:
Use OnCollectionChanged override
public class Solution1<T> : ObservableCollection<T>
{
protected override void OnCollectionChanged(NotifyCollectionChangedEventArgs e)
{
if (e.Action == NotifyCollectionChangedAction.Add)
DoSomething();
base.OnCollectionChanged(e);
}
...
}
Seal the class
public sealed class Solution2<T> : ObservableCollection<T>
{
public Solution2()
{
this.CollectionChanged += new NotifyCollectionChangedEventHandler(ObsCol_CollectionChanged);
}
...
}
Seal the event
public class Solution3<T> : ObservableCollection<T>
{
public sealed override event NotifyCollectionChangedEventHandler CollectionChanged
{
add { base.CollectionChanged += value; }
remove { base.CollectionChanged -= value; }
}
...
}
What you do depends on your use case... the first solution is most flexible, since you can still create subclasses with all kinds of override.
With the SerializableAttribute in your question, it might be worth to seal the whole class since you probably don't want to serialize sub-types anyway (Solution 2).
If you are ever in a situation, where the OnEvent/RaiseEvent method is not available for whatever reason, Solution 3 allows to seal only the minimal subset of functionality that is used in the constructor.
The following classes
public class PagedItemList<T>
{
public delegate void PageChanged(int newPage);
public event PageChanged PageChangedEvent;
}
public class SomeClass
{
public void SetupWithPagedList<T>(PagedItemList<T> list)
{
list.PageChangedEvent += new PagedItemList<T>.PageChanged(NotifyPageChanged);
}
public void NotifyPageChanged(int newPage) { }
}
Throws the error:
InvalidCastException: Cannot cast from source type to destination type
When I try to add a listener to the event as seen in the example above.
The call to SetupWithPagedList is correctly parametrized:
obj.SetupWithPagedList<Monster>(pagedMonstersList)
I have done events and delegates a lot before, the only difference here is that there is a <T> involved in this class. Has anyone had issues doing events and delegates with templates?
I think your problem in this case is with, the instances. The first question I made when I saw your PagedItemListCode was, why SetupWithPagedList is not an static method, and I checked that you are calling the NotifyPageChanged method in the instance of the class. I do not know the logic behind the problem, but maybe the right way is like this:
public static void SetupWithPagedList<T>(PagedItemList<T> list)
{
list.PageChangedEvent += new PagedItemList<T>.PageChanged(list.NotifyPageChanged);
}
Note that the instance of the class maybe is not the same instance of the object that is passed to the SetupWithPagedList method. Maybe the class is PagedItemList<A> and the method parameter is PagedItemList<Monster>. Maybe the class do not need to be generic. Check it.
EDIT
I tried your code and works perfect:
var p = new PagedItemList<int>();
var sc = new SomeClass();
sc.SetupWithPagedList(p);
p.RaisPageChanged(5);
...
public class PagedItemList<T>
{
public delegate void PageChanged(int newPage);
public event PageChanged PageChangedEvent;
public void RaisPageChanged(int page)
{
if (PageChangedEvent != null)
PageChangedEvent(page);
}
}
public class SomeClass
{
public void SetupWithPagedList<T>(PagedItemList<T> list)
{
list.PageChangedEvent += new PagedItemList<T>.PageChanged(NotifyPageChanged);
}
public void NotifyPageChanged(int newPage)
{
Debug.WriteLine("Page: ",newPage);
}
}
Check it, maybe is something else.
I am bubbling events in my application and so therefore using the bubble events method. As this method handles all sorts of bubbled events their is a switch or if statement within it to determine what sort of event we're dealing with. I was wondering if I could get around this by creating different versions of the event args class. So let me explain, say I have two types of event that are handled differently called X and Y, I create new event args classes for these two events as they store different types of info.
public class EventsArgsX : EventsArgs
public class EventsArgsY : EventsArgs
then when I RaiseBubbleEvent from somewhere in my application I can pass either of the two event arg based types, so..
EventArgsX foox = new EventArgsX();
RaiseBubbleEvent(null,foox);
or
EventArgsY fooy = new EventArgsY();
RaiseBubbleEvent(null,fooy);
then the OnBubbleEvent method picks this up, who's signature is
override OnBubbleEvent(object source, EventArgs e)
now i cant overload this method as its overriden in the first place, so what I thought I could do was have another method with overloads in it to handle this, so
protected override OnBubbleEvent(object source, EventArgs e)
{
DoStuff(e);
}
private void DoStuff(EventArgsY args)
{}
private void DoStuff(EventArgsX args)
{}
but of course the problem is that EventArgs e in the OnBubbleEvent method is of type EventArgs at the time of calling. However we know its not. So how would i case it back to its actual type in order for the method call to work?
Many thanks, hope you can help me with this, its seems really easy like a might be missing something or that it just cant be done
any ideas??
It's simple:
protected override OnBubbleEvent(object source, EventArgs e)
{
if(e is EventArgsX)
DoStuff((EventArgsX)e);
else if(e is EventArgsY)
DoStuff((EventArgsY)e);
}
This, being KISS, is not very extensible. If you're planning on adding more event types, you can try double dispatch:
public abstract class EventArgsBase : EventArgs
{
public abstract void Bubble(IEventBubbler eb);
}
public interface IEventBubbler
{
Bubble(EventArgsX ex);
Bubble(EventArgsY ey);
}
public class EventArgsX : EventArgsBase
{
public virtual void Bubble(IEventBubbler eb)
{
eb.Bubble(this);
}
}
public class EventArgsY : EventArgsBase
{
public virtual void Bubble(IEventBubbler eb)
{
eb.Bubble(this);
}
}
C# - .net 3.5
I have a family of classes that inherit from the same base class.
I want a method in the base class to be invoked any time a property in a derrived class is accessed (get or set). However, I don't want to write code in each and every property to call the base class... instead, I am hoping there is a declarative way to "sink" this activity into the base class.
Adding some spice to the requirement, I do need to determine the name of the property that was accessed, the property value and its type.
I imagine the solution would be a clever combination of a delegate, generics, and reflection. I can envision creating some type of array of delegate assignments at runtime, but iterating over the MemberInfo in the constructor would impact performance more than I'd like. Again, I'm hoping there is a more direct "declarative" way to do this.
Any ideas are most appreciated!
You can't do it automatically, but you can pretty much get 95% for free. This is a classic case for aspect-oriented programming. Check out PostSharp, which has the OnFieldAccessAspect class. Here's how you might solve your problem:
[Serializable]
public class FieldLogger : OnFieldAccessAspect {
public override void OnGetValue(FieldAccessEventArgs eventArgs) {
Console.WriteLine(eventArgs.InstanceTag);
Console.WriteLine("got value!");
base.OnGetValue(eventArgs);
}
public override void OnSetValue(FieldAccessEventArgs eventArgs) {
int i = (int?)eventArgs.InstanceTag ?? 0;
eventArgs.InstanceTag = i + 1;
Console.WriteLine("value set!");
base.OnSetValue(eventArgs);
}
public override InstanceTagRequest GetInstanceTagRequest() {
return new InstanceTagRequest("logger", new Guid("4f8a4963-82bf-4d32-8775-42cc3cd119bd"), false);
}
}
Now, anything that inherits from FieldLogger will get the same behavior. Presto!
I don't believe this is possible to do declaratively, i have never seen it done that way. What you can do though is implement the INotifyPropertyChanged interface on your base class, and have the implementation of the interface in the base class. Something like this:
public class A : INotifyPropertyChanged
{
#region INotifyPropertyChanged Members
public event PropertyChangedEventHandler PropertyChanged;
#endregion
protected virtual void RaiseOnPropertyChanged(object sender, string propertyName)
{
if (this.PropertyChanged != null)
PropertyChanged(sender, new PropertyChangedEventArgs(propertyName);
}
public A()
{
this.PropertyChanged += new PropertyChangedEventHandler(A_PropertyChanged);
}
void A_PropertyChanged(object sender, PropertyChangedEventArgs e)
{
//centralised code here that deals with the changed property
}
}
public class B : A
{
public string MyProperty
{
get { return _myProperty; }
set
{
_myProperty = value;
RaiseOnPropertyChanged(this, "MyProperty");
}
}
public string _myProperty = null;
}
In following code, I want to extend the behaviour of a class by deriving/subclassing it, and make use of an event of the base class:
public class A
{
public event EventHandler SomeEvent;
public void someMethod()
{
if(SomeEvent != null) SomeEvent(this, someArgs);
}
}
public class B : A
{
public void someOtherMethod()
{
if(SomeEvent != null) SomeEvent(this, someArgs); // << why is this not possible?
//Error: The event 'SomeEvent' can only appear on the left hand side of += or -=
//(except when used from within the type 'A')
}
}
Why isn't it possible?
And what is the common solution for this kind of situation?
Others have explained how to get round the issue, but not why it's coming up.
When you declare a public field-like event, the compiler creates a public event, and a private field. Within the same class (or nested classes) you can get at the field directly, e.g. to invoke all the handlers. From other classes, you only see the event, which only allows subscription and unsubscription.
The standard practice here is to have a protected virtual method OnSomeEvent on your base class, then call that method in derived classes. Also, for threading reasons you will want to keep a reference to the handler before checking null and calling it.
For an explanation of the why read Jon Skeet's answer or the C# specification which describes how the compiler automatically creates a private field.
Here is one possible work around.
public class A
{
public event EventHandler SomeEvent;
public void someMethod()
{
OnSomeEvent();
}
protected void OnSomeEvent()
{
EventHandler handler = SomeEvent;
if(handler != null)
handler(this, someArgs);
}
}
public class B : A
{
public void someOtherMethod()
{
OnSomeEvent();
}
}
Edit: Updated code based upon Framework Design Guidelines section 5.4 and reminders by others.
Todd's answer is correct. Often you will see this implemented throughout the .NET framework as OnXXX(EventArgs) methods:
public class Foo
{
public event EventHandler Click;
protected virtual void OnClick(EventArgs e)
{
var click = Click;
if (click != null)
click(this, e);
}
}
I strongly encourage you to consider the EventArgs<T>/EventHandler<T> pattern before you find yourself making all manner of CustomEventArgs/CustomEventHandler for raising events.
The reason the original code doesn't work is because you need to have access to the event's delegate in order to raise it, and C# keeps this delegate private.
Events in C# are represented publicly by a pair of methods, add_SomeEvent and remove_SomeEvent, which is why you can subscribe to an event from outside the class, but not raise it.
My answer would be that you shouldn't have to do this.
C# nicely enforces Only the type declaring/publishing the event should fire/raise it.
If the base class trusted derivations to have the capability to raise its events, the creator would expose protected methods to do that. If they don't exist, its a good hint that you probably shouldn't do this.
My contrived example as to how different the world would be if derived types were allowed to raise events in their ancestors. Note: this is not valid C# code.. (yet..)
public class GoodVigilante
{
public event EventHandler LaunchMissiles;
public void Evaluate()
{
Action a = DetermineCourseOfAction(); // method that evaluates every possible
// non-violent solution before resorting to 'Unleashing the fury'
if (null != a)
{ a.Do(); }
else
{ if (null != LaunchMissiles) LaunchMissiles(this, EventArgs.Empty); }
}
virtual protected string WhatsTheTime()
{ return DateTime.Now.ToString(); }
....
}
public class TriggerHappy : GoodVigilante
{
protected override string WhatsTheTime()
{
if (null != LaunchMissiles) LaunchMissiles(this, EventArgs.Empty);
}
}
// client code
GoodVigilante a = new GoodVigilante();
a.LaunchMissiles += new EventHandler(FireAway);
GoodVigilante b = new TriggerHappy(); // rogue/imposter
b.LaunchMissiles += new EventHandler(FireAway);
private void FireAway(object sender, EventArgs e)
{
// nuke 'em
}
Wrap it with a protected virtual On... method:
public class BaseClass
{
public event EventHandler<MyArgs> SomeEvent;
protected virtual void OnSomeEvent()
{
if(SomeEvent!= null)
SomeEvent(this, new MyArgs(...) );
}
}
Then override this in a derived class
public class DerivedClass : BaseClass
{
protected override void OnSomeEvent()
{
//do something
base.OnSomeEvent();
}
}
You'll set this pattern all over .Net - all form and web controls follow it.
Do not use the prefix Raise... - this is not consistent with MS's standards and can cause confusion elsewhere.