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How to add a PropertyChanged event for a property?

I have created various properties inside of a User Control, and have had great success with accessing and editing them. I'm now trying to set up events for a number of these to be raised when one of these properties is changed. I have tried the MSDN example code for doing this (see here), but it is giving me this error when I try to build the solution:
Severity Code Description Project File Line Suppression State
Error CS0079 The event 'AbilityScoreDisplay.AbilityTitleChanged' can only appear on the left hand side of += or -= DnD Character Sheet C:\Users\bradley beasley\Documents\Visual Studio 2019\Projects\DnD Character Sheet\DnD Character Sheet\AbilityScoreDisplay.Designer.cs 199 Active
Another issue that I am having is that I am struggling to figure out how to get that event to appear in the Visual Studio 2019 Designer Properties window.
Here is the code that I have added to the designer file:
namespace DnD_Character_Sheet
{
partial class AbilityScoreDisplay : UserControl
{
public string AbilityTitle
{
get
{
return AbiltyTitleLabel.Text;
}
set
{
AbiltyTitleLabel.Text = value;
Invalidate();
}
}
public int AbilityModifier
{
get
{
return Convert.ToInt32(AbilityModifierTextBox.Text);
}
private set
{
if (value >= 0) AbilityModifierTextBox.Text = String.Format("+{0}", value);
else AbilityModifierTextBox.Text = value.ToString();
Invalidate();
}
}
public int AbilityScore
{
get
{
return Convert.ToInt32(AbilityScoreLabel.Text);
}
set
{
AbilityModifier = (int)(Math.Floor((double)(value) / 2)) - 5;
Invalidate();
}
}
private EventHandler onAbilityTitleChanged { get; set; }
private EventHandler onAbilityScoreChanged { get; set; }
public event EventHandler AbilityTitleChanged
{
add
{
onAbilityTitleChanged += value;
}
remove
{
onAbilityTitleChanged -= value;
}
}
public event EventHandler AbilityScoreChanged
{
add
{
onAbilityScoreChanged += value;
}
remove
{
onAbilityScoreChanged -= value;
}
}
protected virtual void OnAbilityTitleChanged(EventArgs e)
{
AbilityTitleChanged?.Invoke(this, e);
}
protected virtual void OnAbilityScoreChanged(EventArgs e)
{
AbilityScoreChanged?.Invoke(this, e);
}
}
}
The aim is to enable an event to be raised whenever a property is changed so that it can do other stuff elsewhere in the form that the controls will be in. I'm fairly certain that I am missing some very important stuff, or that my code is not that effective at all, but I am learning this kind of code for the first time, and I have tried many different things that have just not worked.
Any help at all would be greatly appreciated :)

I think you are confusing a few concepts. Let's do it step by step.
First, you need to be able to track event handlers:
private EventHandler _onAbilityTitleChanged;
You expose this event through a public property:
public event EventHandler AbilityTitleChanged
{
add
{
_onAbilityTitleChanged += value;
}
remove
{
_onAbilityTitleChanged -= value;
}
}
Finally, you need to fire the event so that all subscribed handlers can react to it. You can do so when the title changes (setter):
public string AbilityTitle
{
get
{
return AbiltyTitleLabel.Text;
}
set
{
AbiltyTitleLabel.Text = value;
//Raising the event!
_onAbilityTitleChanged?.Invoke(this, new EventArgs());
}
}
Other classes can then subscribe to your event:
var control = new AbilityScoreDisplay();
control.AbilityTitleChanged += SomeHandlerForWhenTitleChanges;
private void SomeHandlerForWhenTitleChanges(object sender, EventArgs e)
{
//....
}
You might want to read up a bit on the INotifyPropertyChanged interface as well.

You typically do this by implementing INotifyPropertyChanged. This allows you to use one single event for all the properties. The property name is passed in the event arguments.
partial class AbilityScoreDisplay : UserControl, INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged;
private void OnPropertyChanged(string propertyName)
{
PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
}
...
}
In the properties do this (with AbilityModifier as an example):
private int _abilityModifier;
public int AbilityModifier
{
get { return _abilityModifier; }
private set {
if (value != _abilityModifier) {
_abilityModifier = value;
AbilityModifierTextBox.Text = value >= 0
? String.Format("+{0}", value)
: value.ToString();
OnPropertyChanged(nameof(AbilityModifier));
}
}
}
Assuming this event handler
private void ScoreDisplay_PropertyChanged(object sender, PropertyChangedEventArgs e)
{
...
}
You can subscribe the event with
PropertyChanged += ScoreDisplay_PropertyChanged;
You need to use the add/remove syntax only in rare cases. Typically, when you create your own event store, because you have a lot of events and don't want to consume space for unsubscribed events.
You can use INotifyPropertyChanged together with data binding to immediately update the UI when changes are made to the data. To do this you would create a class with properties and the INotifyPropertyChanged implementation. In the form you then assign an instance of this class to the DataSource of a BindingSource. The controls are then bound to this BindingSource.
Then you can drop all the code used to read from or to write to text boxes or labels etc., as the binding mechanism does it automatically for you.

C# - Check and notify of boolean value changed

Good day all
I am struggling to familiarize myself with Events, I just can't seem to get my head around the concept, after quite a few tutorials and videos, I am still lost, thus I will be specific about my problem.
In short, a thread pings an IP, if the ping reports a success,
ServerOnline = (ping == success ) ? true : false;
The event listener should "listen" for a variable change, get the variable and process accordingly.
A example based on the code below would be of an immense help.
class tcp_connector
{
bool ServerOnline
void thread_checkServer()
{
//do code
ServerOnline = true;
//notify of variable change
}
}
class tcp_sender
{
//when ServerOnline bool = true
//button.color = color.green;
}

You can declare an event in your tcp_connector that you raise if ServerOnline changes like this:
class tcp_connector
{
// standard event pattern
public event EnventHandler ServerOnlineChanged;
protected virtual void OnServerOnlineChanged
{
EventHandler handler = ServerOnlineChanged; // for thread safety
if (handler != null)
handler(this, EventArgs.Empty);
}
private bool _serverOnline;
public bool ServerOnline // implement as property
{
get { return _serverOnline; }
set {
if (_serverOnline == value) return;
_serverOnline = value;
OnServerOnlineChanged(); // raise event
}
}
void thread_checkServer()
{
//do code
// be sure to use the property ServerOnline, not the
// field _serverOnline!
// the property setter will raise ServerOnlineChangedEvent
ServerOnline = true;
}
}
And then you can consume that event in your tcp_sender like this:
class tcp_sender
{
private tcp_connector _connector;
public tcp_sender()
{
_connector = new tcp_connector();
// subscribe to event
_connector.ServerOnlineChanged += tcp_connector_ServerOnlineChanged;
}
// the event handler for the ServerOnlineChanged event
private void tcp_connector_ServerOnlineChanged(object sender, EventArgs e)
{
if (_connector.ServerOnline)
button.color = color.green;
}
}
This is not really a "listener" that watches your property ServerOnline, but by seperating it into a field and a property you can recognize changes and raise the specified event.

How to avoid multiple registering of events

i have an event where i register event handlers.
event Action OnGameList;
then for example i reach some code like this:
backend.OnGameList += ProcessGameList;
backend.GetGameList(); //this will trigger the above event.
every time i reach this code, the handler is ADDED. that means the second time it will be called twice.
of course i could remove it in the function like this:
backend.OnGameList -= ProcessGameList;
but i have the feeling that there is a better solution for this kind of problem.

I think you should use some kind of backing field to track that you are already subscribed. I.e.
private bool _subscribed = false;
SubscribeToOnGameListEvent();
backend.GetGameList();
private void SubscribeToOnGameListEvent()
{
if (!_subscribed)
{
backend.OnGameList += ProcessGameList;
_subscribed = true;
}
}

You can check the presence of particular delegate in invocation list:
class Foo
{
private EventHandler bar;
public event EventHandler Bar
{
add
{
if (bar == null)
{
bar = value;
}
else
{
if (!bar.GetInvocationList().Contains(value))
{
bar += value;
}
}
}
remove
{
// ...
}
}
public void RaiseBar()
{
if (bar != null)
{
bar(this, EventArgs.Empty);
}
}
}

A design pattern to disable event handling

To simply illustrate my dilemma, let say that I have the following code:
class A
{
// May be set by a code or by an user.
public string Property
{
set { PropertyChanged(this, EventArgs.Empty); }
}
public EventHandler PropertyChanged;
}
class B
{
private A _a;
public B(A a)
{
_a = a;
_a.PropertyChanged += Handler;
}
void Handler(object s, EventArgs e)
{
// Who changed the Property?
}
public void MakeProblem()
{
_a.Property = "make a problem";
}
}
In order to perform its duty, class B have to react on A's PropertyChanged event but also is capable of alternating that property by itself in certain circumstances. Unfortunately, also other objects can interact with the Property.
I need a solution for a sequential flow. Maybe I could just use a variable in order to disable an action:
bool _dontDoThis;
void Handler(object s, EventArgs e)
{
if (_dontDoThis)
return;
// Do this!
}
public void MakeProblem()
{
_dontDoThis = true;
_a.Property = "make a problem";
_dontDoThis = false;
}
Are there a better approaches?
Additional considerations
We are unable to change A.
A is sealed.
There are also other parties connected to the PropertyChanged event and I don't know who their are. But when I update the Property from B, they shouldn't be also notified. But I'm unable to disconnect them from the event because I don't know them.
What if also more threads can interact with the Property in the mean time?
The more bullets solved, the better.
Original problem
My original problem is a TextBox (WPF) that I want to complement depending on its content and focus. So I need to react on TextChanged event and I also need to omit that event if its origin is derived from my complements. In some cases, other listeners of a TextChanged event shouldn't be notified. Some strings in certain state and style are invisible to others.

If it is so important not to handle events you initiated, maybe you should change the way you set Property to include the initiator of the change?
public class MyEventArgs : EventArgs
{
public object Changer;
}
public void SetProperty(string p_newValue, object p_changer)
{
MyEventArgs eventArgs = new MyEventArgs { Changer = p_changer };
PropertyChanged(this, eventArgs);
}
And then in your handler - simply check your are not the initiator.
I find all these changes in registration and members very problematic in terms on multi threading and extensibility.

Well essentially you are trying to break the event delegation mechanism and any "solution" to that is going to be brittle since updates to the BCL might break your code. You could set the backing field using reflection. This of course would require that you do have permissions to do this and seeing the generic framing of the question it might not always be that you have the needed permissions
public void MakeProblem()
{
if (_backingField == null) {
_backingField = = _a.GetType().GetField(fieldname)
}
_backingField.SetValue(_a,"make a problem");
}
but as I started out, you are trying to break the event delegation mechanism. The idea is that the receivers of the event are independent. Disabling might lead to so very hard to find bugs because looking at any given piece of code it looks correct but only when you realize that some devious developer has hack the delegation mechanism do you realize why the information that is shown on screen seems to be a cached version of the actual value. The debugger shows the expected value of the property but because the event was hidden the handler responsible for updating the display was never fired and hence an old version is displayed (or the log shows incorrect information so when you are trying to recreate a problem a user has reported based on the content of the log you will not be able to because the information in the log is incorrect because it was based on no one hacking the event delegation mechanism

To my opinion your solution is possible, though I would have created a nested IDisposable class inside B that does the same thing with 'using', or put the '_dontDoThis = false' inside a 'finally' clause.
class A
{
// May be set by a code or by an user.
public string Property
{
set { if (!_dontDoThis) PropertyChanged(this, EventArgs.Empty); }
}
public EventHandler PropertyChanged;
bool _dontDoThis;
}
class B
{
private class ACallWrapper : IDisposable
{
private B _parent;
public ACallWrapper(B parent)
{
_parent = parent;
_parent._a._dontDoThis = true;
}
public void Dispose()
{
_parent._a._dontDoThis = false;
}
}
private A _a;
public B(A a)
{
_a = a;
_a.PropertyChanged += Handler;
}
void Handler(object s, EventArgs e)
{
// Who changed the Property?
}
public void MakeProblem()
{
using (new ACallWrapper(this))
_a.Property = "make a problem";
}
}
On the other hand, I would've used the 'internal' modifier for these things if those two classes are inside the same assembly.
internal bool _dontDoThis;
That way, you keep a better OOP design.
Moreover, if both classes are on the same assembly, I would've written the following code inside A:
// May be set by a code or by an user.
public string Property
{
set
{
internalSetProperty(value);
PropertyChanged(this, EventArgs.Empty);
}
}
internal internalSetProperty(string value)
{
// Code of set.
}
In this case, B could access internalSetProperty without triggering to PropertyChanged event.
Thread Sync:
NOTE: The next section applies to WinForms - I don't know if it applies to WPF as well.
For thread synchronizations, because we're referring to a control. you could use the GUI thread mechanism for synchronization:
class A : Control
{
public string Property
{
set
{
if (this.InvokeRequired)
{
this.Invoke((Action<string>)setProperty, value);
reutrn;
}
setProperty(value);
}
}
private void setProperty string()
{
PropertyChanged(this, EventArgs.Empty);
}
}

Great question.
As a general case, you can not mess around with event handlers of sealed classes. Normally you could override A's hypothetical OnPropertyChanged and based on some flag either raise the event or not. Alternatively you could provide a setter method that does not raise event, as suggested by #Vadim. However, if A is sealed your best option is to add flag to a lister, just as you did. That will enable you to recognize PropertyChanged event raised by B, but you won't be able to suppress the event for other listeners.
Now, since you provided context... There is a way of doing exactly this in WPF. All that needs to be done is B's handler for TextBox.TextChanged needs to set e.Handled = _dontDoThis. That will supress notifications for all other listeners, provided B's one was added as the first one. How to make sure this happens? Reflection!
UIElement exposes only AddHandler and RemoveHandler methods, there is no InsertHandler that would allow to manually specifiy the priority for the handler. However, a quick peek into .NET source code (either download the whole thing or query what you need) reveals that AddHandler forwards arguments to an interal method EventHandlersStore.AddRoutedEventHandler, which does this:
// Create a new RoutedEventHandler
RoutedEventHandlerInfo routedEventHandlerInfo =
new RoutedEventHandlerInfo(handler, handledEventsToo);
// Get the entry corresponding to the given RoutedEvent
FrugalObjectList<RoutedEventHandlerInfo> handlers = (FrugalObjectList<RoutedEventHandlerInfo>)this[routedEvent];
if (handlers == null)
{
_entries[routedEvent.GlobalIndex] = handlers = new FrugalObjectList<RoutedEventHandlerInfo>(1);
}
// Add the RoutedEventHandlerInfo to the list
handlers.Add(routedEventHandlerInfo);
All this stuff is internal, but can be recreated using reflection:
public static class UIElementExtensions
{
public static void InsertEventHandler(this UIElement element, int index, RoutedEvent routedEvent, Delegate handler)
{
// get EventHandlerStore
var prop = typeof(UIElement).GetProperty("EventHandlersStore", BindingFlags.NonPublic | BindingFlags.Instance);
var eventHandlerStoreType = prop.PropertyType;
var eventHandlerStore = prop.GetValue(element, new object[0]);
// get indexing operator
PropertyInfo indexingProperty = eventHandlerStoreType.GetProperties(BindingFlags.NonPublic | BindingFlags.Instance)
.Single(x => x.Name == "Item" && x.GetIndexParameters().Length == 1 && x.GetIndexParameters()[0].ParameterType == typeof(RoutedEvent));
object handlers = indexingProperty.GetValue(eventHandlerStore, new object[] { routedEvent });
if (handlers == null)
{
// just add the handler as there are none at the moment so it is going to be the first one
if (index != 0)
{
throw new ArgumentOutOfRangeException("index");
}
element.AddHandler(routedEvent, handler);
}
else
{
// create routed event handler info
var constructor = typeof(RoutedEventHandlerInfo).GetConstructors(BindingFlags.NonPublic | BindingFlags.Instance).Single();
var handlerInfo = constructor.Invoke(new object[] { handler, false });
var insertMethod = handlers.GetType().GetMethod("Insert");
insertMethod.Invoke(handlers, new object[] { index, handlerInfo });
}
}
}
Now calling InsertEventHandler(0, textBox, TextBox.TextChangedEvent, new TextChangedEventHandler(textBox_TextChanged)) will make sure your handler will be the first one on the list, enabling you to suppress notifications for other listeners!
public partial class MainWindow : Window
{
public MainWindow()
{
InitializeComponent();
var textBox = new TextBox();
textBox.TextChanged += (o, e) => Console.WriteLine("External handler");
var b = new B(textBox);
textBox.Text = "foo";
b.MakeProblem();
}
}
class B
{
private TextBox _a;
bool _dontDoThis;
public B(TextBox a)
{
_a = a;
a.InsertEventHandler(0, TextBox.TextChangedEvent, new TextChangedEventHandler(Handler));
}
void Handler(object sender, TextChangedEventArgs e)
{
Console.WriteLine("B.Handler");
e.Handled = _dontDoThis;
if (_dontDoThis)
{
e.Handled = true;
return;
}
// do this!
}
public void MakeProblem()
{
try
{
_dontDoThis = true;
_a.Text = "make a problem";
}
finally
{
_dontDoThis = false;
}
}
}
Output:
B.Handler
External handler
B.Handler

I found one solution with regard to third parties, that are connected to the property and we don't want to nofify them when that property changed.
There are though the requirements:
We are capable of override the A.
The A has a virtual method that is invoked when property changed and allows to suspend the event to be raised.
The event is raised immediately when property is being changed.
The solution is to replace the A by MyA, as follows:
class A
{
// May be set by a code or by an user.
public string Property
{
set { OnPropertyChanged(EventArgs.Empty); }
}
// This is required
protected virtual void OnPropertyChanged(EventArgs e)
{
PropertyChanged(this, e);
}
public EventHandler PropertyChanged;
}
// Inject MyA instead of A
class MyA : A
{
private bool _dontDoThis;
public string MyProperty
{
set
{
try
{
_dontDoThis = true;
Property = value;
}
finally
{
_dontDoThis = false;
}
}
}
protected override void OnPropertyChanged(EventArgs e)
{
// Also third parties will be not notified
if (_dontDoThis)
return;
base.OnPropertyChanged(e);
}
}
class B
{
private MyA _a;
public B(MyA a)
{
_a = a;
_a.PropertyChanged += Handler;
}
void Handler(object s, EventArgs e)
{
// Now we know, that the event is not raised by us.
}
public void MakeProblem()
{
_a.MyProperty = "no problem";
}
}
Unfortunately we still use back bool field and we assume a single thread. To rid of the first, we could use a refactored solution suggest by EZSlaver (here). First, create a disposable wrapper:
class Scope
{
public bool IsLocked { get; set; }
public static implicit operator bool(Scope scope)
{
return scope.IsLocked;
}
}
class ScopeGuard : IDisposable
{
private Scope _scope;
public ScopeGuard(Scope scope)
{
_scope = scope;
_scope.IsLocked = true;
}
public void Dispose()
{
_scope.IsLocked = false;
}
}
Then the MyProperty might be refactored to:
private readonly Scope _dontDoThisScope = new Scope();
public string MyProperty
{
set
{
using (new ScopeGuard (_dontDoThisScope))
Property = value;
}
}

How to override method and event in WinForm UserControl in C#?

I have a custom control in C# WinForms called BaseControl and there I have a property called Selected.
I want to have an event SelectedChanged and virtual method OnSelecteChanged in the base control and they should behave in the same manner as we have in Control class for many properties i.e. Click event and OnClick method.
Means anyone who derives from my BaseControl can either bind to the event or can override the OnSelectedChanged method.
So, when the value of Selected property is changed event should be fired and if the method is overridden control should go to that method.
I know how to fire the event but don't know how to do it for method.
Please guide me...

Below is an example of how events should be implemented:
public class BaseControl : Control
{
private object _selected;
public object Selected
{
get { return _selected; }
set
{
if (!Equals(_selected, value))
{
_selected = value;
OnSelectedChanged(EventArgs.Empty);
}
}
}
public event EventHandler SelectedChanged;
protected virtual void OnSelectedChanged(EventArgs e)
{
if (SelectedChanged != null)
SelectedChanged(this, e);
}
}
With this example, you can override OnSelectedChanged in an overriden class, like this:
public class MyControl : BaseControl
{
protected override void OnSelectedChanged(EventArgs e)
{
base.OnSelectedChanged(e);
// My own logic.
}
}

private bool _selected;
public bool Selected
{
get { return _selected; }
set
{
if (value != _selected)
{
_selected = value;
OnSelectedChanged();
}
}
}
public event EventHandler SelectedChanged;
protected virtual void OnSelectedChanged()
{
var handler = SelectedChanged;
if (handler != null)
handler(this, EventArgs.Empty);
}

Basically you don't fire the event from your Selected property setter - you call the method, and make the method call the event. Anyone overriding the method should call base.OnSelectedChanged to make sure the event still fires. So your method should look something like this:
protected virtual void OnSelectedChanged(EventArgs e) {
EventHandler handler = Selected; // Or your own delegate variable
if (handler != null) {
handler(this, e);
}
}