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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.

How do I add and subtract event handlers inside a derived abstract class?

Short version
In my abstract class MyCbo_Abstract (derived from ComboBox class), I want to create a custom property that when set will subtract all the control's event handlers, set the base property value, then re-add all the control's event handlers.
What I have so far
I have a concrete ComboBox class derived from an abstract ComboBox class derived from Microsoft's ComboBox class.
public abstract class MyCbo_Abstract : ComboBox
{
public MyCbo_Abstract() : base()
{
}
}
public partial class MyCboFooList : MyCbo_Abstract
{
public MyCboFooList() : base()
{
}
}
My main Form class subscribes to certain base ComboBox events.
Note: The designer has: this.myCboFooList = new MyCboFooList();
public partial class FormMain : Form
{
public FormMain()
{
myCboFooList.SelectedIndexChanged += myCboFooList_SelectedIndexChanged;
}
private void myCboFooList_SelectedIndexChanged(object sender, EventArgs e)
{
// do stuff
}
}
There are times when I want to suppress the invocation of defined event handlers, e.g., when I programmatically set a ComboBox object's SelectedIndex property.
Instead of having to remember to write the code to subtract and re-add event handlers each time I want to modify the SelectedIndex property and suppress its events, I want to create a custom property SelectedIndex_NoEvents that when set will subtract all the control's event handlers, set the base property value SelectedIndex, then re-add all the control's event handlers.
The problem
My problem is that I don't know how to iterate over a EventHandlerList because it has no GetEnumerator. And, in looking at the list in the debugger, saveEventHandlerList is a weird chained thing that I can't figure out how to otherwise traverse.
public abstract class MyCbo_Abstract : ComboBox
{
int selectedIndex_NoEvents;
public int SelectedIndex_NoEvents
{
get
{
return base.SelectedIndex;
}
set
{
EventHandlerList saveEventHandlerList = new EventHandlerList();
saveEventHandlerList = Events;
//foreach won't work - no GetEnumerator available. Can't use for loop - no Count poprerty
foreach (EventHandler eventHandler in saveEventHandlerList)
{
SelectedIndexChanged -= eventHandler;
}
base.SelectedIndex = value;
//foreach won't work - no GetEnumerator available. Can't use for loop - no Count poprerty
foreach (EventHandler eventHandler in saveEventHandlerList)
{
SelectedIndexChanged += eventHandler;
}
saveEventHandlerList = null;
}
}
//Probably don't need this
public override int SelectedIndex
{
get
{
return base.SelectedIndex;
}
set
{
base.SelectedIndex = value;
}
}
public DRT_ComboBox_Abstract() : base()
{
}
}

Before giving you the solution that I created, let me say that this feels extremely hacky. I urge you to seriously think about another solution. There may be all kinds of crazy edge cases where this code breaks down, I haven't thoroughly tested it beyond the example code shown below.
Add the following utility class:
public class SuspendedEvents
{
private Dictionary<FieldInfo, Delegate> handlers = new Dictionary<System.Reflection.FieldInfo, System.Delegate>();
private object source;
public SuspendedEvents(object obj)
{
source = obj;
var fields = obj.GetType().GetFields(BindingFlags.Static | BindingFlags.Instance | BindingFlags.NonPublic | BindingFlags.Public);
foreach (var fieldInfo in fields.Where(fi => fi.FieldType.IsSubclassOf(typeof(Delegate))))
{
var d = (Delegate)fieldInfo.GetValue(obj);
handlers.Add(fieldInfo, (Delegate)d.Clone());
fieldInfo.SetValue(obj, null);
}
}
public void Restore()
{
foreach (var storedHandler in handlers)
{
storedHandler.Key.SetValue(source, storedHandler.Value);
}
}
}
You can use it like this:
var events = new SuspendedEvents(obj); //all event handlers on obj are now detached
events.Restore(); // event handlers on obj are now restored.
I used the following test setup:
void Main()
{
var obj = new TestObject();
obj.Event1 += (sender, e) => Handler("Event 1");
obj.Event1 += (sender, e) => Handler("Event 1");
obj.Event2 += (sender, e) => Handler("Event 2");
obj.Event2 += (sender, e) => Handler("Event 2");
obj.Event3 += (sender, e) => Handler("Event 3");
obj.Event3 += (sender, e) => Handler("Event 3");
Debug.WriteLine("Prove events are attached");
obj.RaiseEvents();
var events = new SuspendedEvents(obj);
Debug.WriteLine("Prove events are detached");
obj.RaiseEvents();
events.Restore();
Debug.WriteLine("Prove events are reattached");
obj.RaiseEvents();
}
public void Handler(string message)
{
Debug.WriteLine(message);
}
public class TestObject
{
public event EventHandler<EventArgs> Event1;
public event EventHandler<EventArgs> Event2;
public event EventHandler<EventArgs> Event3;
public void RaiseEvents()
{
Event1?.Invoke(this, EventArgs.Empty);
Event2?.Invoke(this, EventArgs.Empty);
Event3?.Invoke(this, EventArgs.Empty);
}
}
It produces the following output:
Prove events are attached
Event 1
Event 1
Event 2
Event 2
Event 3
Event 3
Prove events are detached
Prove events are reattached
Event 1
Event 1
Event 2
Event 2
Event 3
Event 3

There is no way to easily disable event firing of WinForm controls exposed in the .Net framework. However, the Winform controls follow a standard design pattern for events in that all event signatures are based on the EventHandler Delegate and the registered event handlers are stored in an EventHandlerList that is defined in the Control Class. This list is stored in a field (variable) named "events" and is only publicly exposed via the read-only property Events.
The class presented below uses reflection to temporarily assign null to the events field effectively removing all event handlers registered for the Control.
While it may be an abuse of the pattern, the class implements the IDisposable Interface to restore the events field on disposal of the class instance. The reason for this is to facilitate the use of the using block to wrap the class usage.
public class ControlEventSuspender : IDisposable
{
private const string eventsFieldName = "events";
private const string headFieldName = "head";
private static System.Reflection.FieldInfo eventsFieldInfo;
private static System.Reflection.FieldInfo headFieldInfo;
private System.Windows.Forms.Control target;
private object eventHandlerList;
private bool disposedValue;
static ControlEventSuspender()
{
Type compType = typeof(System.ComponentModel.Component);
eventsFieldInfo = compType.GetField(eventsFieldName, System.Reflection.BindingFlags.Instance | System.Reflection.BindingFlags.NonPublic);
headFieldInfo = typeof(System.ComponentModel.EventHandlerList).GetField(headFieldName, System.Reflection.BindingFlags.Instance | System.Reflection.BindingFlags.NonPublic);
}
private static bool FieldInfosAquired()
{
if (eventsFieldInfo == null)
{
throw new Exception($"{typeof(ControlEventSuspender).Name} could not find the field '{ControlEventSuspender.eventsFieldName}' on type Component.");
}
if (headFieldInfo == null)
{
throw new Exception($"{typeof(ControlEventSuspender).Name} could not find the field '{ControlEventSuspender.headFieldName}' on type System.ComponentModel.EventHandlerList.");
}
return true;
}
private ControlEventSuspender(System.Windows.Forms.Control target) // Force using the the Suspend method to create an instance
{
this.target = target;
this.eventHandlerList = eventsFieldInfo.GetValue(target); // backup event hander list
eventsFieldInfo.SetValue(target, null); // clear event handler list
}
public static ControlEventSuspender Suspend(System.Windows.Forms.Control target)
{
ControlEventSuspender ret = null;
if (FieldInfosAquired() && target != null)
{
ret = new ControlEventSuspender(target);
}
return ret;
}
protected virtual void Dispose(bool disposing)
{
if (!this.disposedValue)
{
if (disposing)
{
if (this.target != null)
{
RestoreEventList();
}
}
}
this.disposedValue = true;
}
public void Dispose()
{
Dispose(true);
}
private void RestoreEventList()
{
object o = eventsFieldInfo.GetValue(target);
if (o != null && headFieldInfo.GetValue(o) != null)
{
throw new Exception($"Events on {target.GetType().Name} (local name: {target.Name}) added while event handling suspended.");
}
else
{
eventsFieldInfo.SetValue(target, eventHandlerList);
eventHandlerList = null;
target = null;
}
}
}
Example usage in the button1_Click method:
public partial class Form1 : Form
{
public Form1()
{
InitializeComponent();
}
private void button1_Click(object sender, EventArgs e)
{
using (ControlEventSuspender.Suspend(comboBox1))
{
comboBox1.SelectedIndex = 3; // SelectedIndexChanged does not fire
}
}
private void button2_Click(object sender, EventArgs e)
{
comboBox1.SelectedIndex = -1; // clear selection, SelectedIndexChanged fires
}
private void button3_Click(object sender, EventArgs e)
{
comboBox1.SelectedIndex = 3; // SelectedIndexChanged fires
}
private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
{
Console.WriteLine("index changed fired");
System.Media.SystemSounds.Beep.Play();
}
}
SoapBox Diatribe
Many will say that the use of Reflection to access non-public class members is dirty or some other derogatory term and that it introduces a brittleness to the code as someone may change the underlying code definition such that the code that relies on member names (magic strings) is no longer valid. This is a valid concern, but I view it as no different than code that accesses external databases.
Reflection can be thought of a query of a type (datatable) from an assembly (database) for specific fields (members: fields, properties, events). It is no more brittle than a SQL statement such as Select SomeField From SomeTable Where AnotherField=5. This type of SQL code is prevent in the world and no one thinks twice about writing it, but some external force could easily redefine the database you code relies on an render all the magic string SQL statements invalid as well.
Use of hard coded names is always at risk of being made invalid by change. You have to weigh the risks of moving forward versus the option of being frozen in fear of proceeding because someone wants to sound authoritative (typically a parroting of other such individuals) and criticize you for implementing a solution that solves the current problem.

I was hoping to write code that would programatically locate all event handler method names created using controlObject.Event += EventHandlerMethodName, but as you see in the other answers, code to do this is complicated, limited, and perhaps not able to work in all cases
This is what I came up with. It satisfies my desire to consolidate the code that subtracts and re-adds event handler method names into my abstract class, but at the expense of having to write code to store and manage event handler method names and having to write code for each control property where I want to suppress the event handler, modify the property value, and finally re-add the event handler.
public abstract class MyCbo_Abstract : ComboBox
{
// create an event handler property for each event the app has custom code for
[DesignerSerializationVisibility(DesignerSerializationVisibility.Hidden)]
private EventHandler evSelectedValueChanged;
[DesignerSerializationVisibility(DesignerSerializationVisibility.Hidden)]
public EventHandler EvSelectedValueChanged { get => evSelectedValueChanged; set => evSelectedValueChanged = value; }
public MyCbo_Abstract() : base()
{
}
// Create a property that parallels the one that would normally be set in the main body of the program
public object _DataSource_NoEvents
{
get
{
return base.DataSource;
}
set
{
SelectedValueChanged -= EvSelectedValueChanged;
if (value == null)
{
base.DataSource = null;
SelectedValueChanged += EvSelectedValueChanged;
return;
}
string valueTypeName = value.GetType().Name;
if (valueTypeName == "Int32")
{
base.DataSource = null;
SelectedValueChanged += EvSelectedValueChanged;
return;
}
//assume StringCollection
base.DataSource = value;
SelectedValueChanged += EvSelectedValueChanged;
return;
}
}
}
public partial class MyCboFooList : MyCbo_Abstract
{
public MyCboFooList() : base()
{
}
}
Designer has
this.myCboFooList = new MyCboFooList();
Main form code
public partial class FormMain : Form
{
public FormMain()
{
myCboFooList.SelectedValueChanged += OnMyCboFooList_SelectedValueChanged;
myCboFooList.EvSelectedValueChanged = OnMyCboFooList_SelectedValueChanged;
}
private void OnMyCboFooList_SelectedValueChanged(object sender, EventArgs e)
{
// do stuff
}
}
And now, if I want to set a property and suppress event(s), I can write something like the following and not have to remember to re-add the event handler method name
myCboFooList._DataSource_NoEvents = null;

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);
}
}
}

Is it smelly to raise an event from within another event?

public class Basket
{
private int _unitCount;
public int UnitCount
{
get { return _unitCount; }
set
{
_unitCount = Math.Max(0, value);
OnUnitCountChanged(new EventArgs());
}
}
public event EventHandler UnitCountChanged;
public event EventHandler Depleted;
protected virtual void OnUnitCountChanged(EventArgs args)
{
var handler = UnitCountChanged;
if(handler!=null) { handler(this, args); }
if(_unitCount == 0) { OnDepleted(new EventArgs()); }
}
protected virtual void OnDepleted(EventArgs args)
{
var handler = UnitCountChanged;
if(handler!=null) { handler(this, args); }
}
}
Is there a problem with checking the conditions for Depleted and raising that event if necessary within the UnitCountChanged event, or should I be doing both in the UnitCount setter (and anywhere else in a non-trivial example)?

While I have seen it, I would recommend against it and raise the event in methods that it would occur in, like your UnitCount setter. Since you have the virtual access modifier keyword, someone could override the method and if they don't call the base object it wouldn't work as expected.
I'm not a fan of making it more complicated to use my code.

There are times when it may be useful (for example, if you are extending a base class and do not have the ability to override the methods that are raising the events), but in general, I'd recommend against it.
In this case, I'd say it's better to raise both events in the UnitCount setter:
public int UnitCount
{
get { return _unitCount; }
set
{
_unitCount = value;
OnUnitCountChanged(new EventArgs());
if(_unitCount == 0) { OnDepleted(new EventArgs()); }
}
}

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;
}
}