I am having issue as described below. I am new to .NET/Visual Studio (2013) and I am trying to figure out why code below wont work.
I have following class
public class PropertySettings
{
...
// get single instance of this class
public static PropertySettings Instance
{
get { return thisInstance; }
}
// event declaration
public event EventHandler<MyObj> PropertyChanged;
...
public void SaveProperty(string propertyName, object obj)
{
var oldValue = obj.OldVal;
var newValue = obj.NewVal;
// Why is PropertyChanged event always null?
if (PropertyChanged != null && oldValue != newValue)
{
PropertyChanged(this, obj); // pass reference to itself
}
}
}
The SaveProperty method is checking that PropertyChanged != null and if so, it calls it by passing a reference to itself and obj.
Then the SaveProperty method is called from some other class like this:
PropertySettings.Instance.SaveProperty("Width", Width);
The problem I am having is that the PropertyChanged is always null so PropertyChanged event is never called.
If you have an instance of your class:
var x = new PropertySettings();
Then you need to "wire up" any event handlers like this:
// "wire up" AKA "subscribe to" AKA "register" event handler.
x.PropertyChanged += HandlePropertyChanged;
// e.g. event handler...
void HandlePropertyChanged(object sender, object e)
{
throw new NotImplementedException();
}
Otherwise, PropertyChanged == null will be true.
Related
I have this event in a webservice:
public event FindProductsByCharacteristicsCompletedEventHandler FindProductsByCharacteristicsCompleted
{
[MethodImpl(MethodImplOptions.Synchronized)]
add
{
_findProductsByCharacteristicsCompleted += value;
}
[MethodImpl(MethodImplOptions.Synchronized)]
remove
{
_findProductsByCharacteristicsCompleted -= value;
}
}
And im then checking if the event value is null with this later in the class:
private void OnFindProductsByCharacteristicsOperationCompleted(object arg)
{
var handler = _findProductsByCharacteristicsCompleted;
if (handler == null)
return;
handler(this, new FindProductsByCharacteristicsCompletedEventArgs(completedEventArgs.Results, completedEventArgs.Error, completedEventArgs.Cancelled, completedEventArgs.UserState));
}
Your event implementation looks like it is an endless recursion. You are using the property itself in its implementation.
Change it to this:
private FindProductsByCharacteristicsCompletedEventHandler
_findProductsByCharacteristicsCompleted;
public event FindProductsByCharacteristicsCompletedEventHandler
FindProductsByCharacteristicsCompleted
{
[MethodImpl(MethodImplOptions.Synchronized)]
add
{
_findProductsByCharacteristicsCompleted += value;
}
[MethodImpl(MethodImplOptions.Synchronized)]
remove
{
_findProductsByCharacteristicsCompleted -= value;
}
}
And now, implement your method like this:
var handler = _findProductsByCharacteristicsCompleted;
if(handler == null)
return;
handler(this, new FindProductsByCharacteristicsCompletedEventArgs(...));
This has the advantage that it is thread-safe.
Even if someone detached the last handler from the event after you checked for null but before you actually raised the event, you would not get an exception, because you are operating on the unchanged local variable.
I'm not well versed in event-based programming. Basically, I'm still stumbling around with it. I'm trying to get something set up, but even with the tutorials, I can't wrap my head around it. What I would like to do (in words) is the following:
I have a dataobject where a property changes. I notice this in the setter of the property, and want to raise an event that the property has changed.
Elsewhere (in a different class entirely), I want to know that the property on this object has changed, and take some action.
Now I'm sure this is a common enough scenario, but my google-fu is letting me down. I'm simply not understanding http://msdn.microsoft.com/en-us/library/ms743695.aspx.
I have this:
public class ChattyClass {
private int someMember;
public event PropertyChangedEventHandler PropertyChanged;
public int SomeMember {
get {
return this.someMember;
}
set {
if (this.someMember != value){
someMember = value;
// Raise event/fire handlers. But how?
}
}
}
public class NosyClass{
private List<ChattyClass> myChatters;
public void addChatter(ChattyClass chatter){
myChatters.add(chatter);
// Start listening to property changed events
}
private void listner(){
// I want this to be called when the PropertyChangedEvent is called
Console.WriteLine("Hey! Hey! Listen! A property of a chatter in my list has changed!");
}
}
What do I do to wire this up?
Concerning the comment pointing me back to the link:
In the example I see:
protected void OnPropertyChanged(string name)
{
PropertyChangedEventHandler handler = PropertyChanged;
if (handler != null)
{
handler(this, new PropertyChangedEventArgs(name));
}
}
What I'm not understanding:
Why isn't this just calling PropertyChanged(this, new PropertyCHangedEventArgs(name))
Where does PropertyChanged get assigned?
What does the assignment look like?
You have to fire the event. In the example on MSDN, they made a protected method OnPropertyChanged to handle this easier (and to avoid duplicate code).
// Create the OnPropertyChanged method to raise the event
protected void OnPropertyChanged(string name)
{
PropertyChangedEventHandler handler = PropertyChanged;
if (handler != null)
{
handler(this, new PropertyChangedEventArgs(name));
}
}
What this method does, is look whether there is an event handler assigned or not (if it is not assigned and you just call it, you'll get a NullReferenceException). If there is one assigned, call this event handler. The event handler provided, has to have the signature of the PropertyChangedEventHandler delegate. This signature is:
void MyMethod(object sender, PropertyChangedEventArgs e)
Where the first parameter has to be of the type object and represents the object that fires the event, and the second parameter contains the arguments of this event. In this case, your own class fires the event and thus give this as parameter sender. The second parameter contains the name of the property that has changed.
Now to be able to react upon the firing of the event, you have to assign an event handler to the class. In this case, you'll have to assign this in your addChatter method. Apart from that, you'll have to first define your handler. In your NosyClass you'll have to add a method to do this, for example:
private void chatter_PropertyChanged(object sender, PropertyChangedEventArgs e)
{
Console.WriteLine("A property has changed: " + e.PropertyName);
}
As you can see, this method corresponds to the signature I explained before. In the second parameter, you'll be able to find the information of which parameter has been changed. Last thing to do, is add the event handler. Now in your addChatter method, you'll have to assign this:
public void AddChatter(ChattyClass chatter)
{
myChatters.Add(chatter);
// Assign the event handler
chatter.PropertyChanged += new PropertyChangedEventHandler(chatter_PropertyChanged);
}
I would suggest you to read something about events in .NET / C#: http://msdn.microsoft.com/en-us/library/awbftdfh . I think after reading/learning this, things will be more clear to you.
You can find a console application here on pastebin if you would like to test it quickly (just copy/paste into a new console application).
With newer versions of C#, you can inline the call to the event handler:
// inside your setter
PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(nameof(MyProperty)));
You could also use something like Fody PropertyChanged to automatically generated the necessary code (visit the link to their GitHub page, with samples).
The link that you looked is for the MVVM pattern and WPF. It is not a general C# implementation. You need something like this:
public event EventHandler PropertyChanged;
public int SomeMember {
get {
return this.someMember;
}
set {
if (this.someMember != value) {
someMember = value;
if (PropertyChanged != null) { // If someone subscribed to the event
PropertyChanged(this, EventArgs.Empty); // Raise the event
}
}
}
...
public void addChatter(ChattyClass chatter) {
myChatters.add(chatter);
chatter.PropertyChanged += listner; // Subscribe to the event
}
// This will be called on property changed
private void listner(object sender, EventArgs e){
Console.WriteLine("Hey! Hey! Listen! A property of a chatter in my list has changed!");
}
If you want to know what property has changed you need to change your event definition to:
public event PropertyChangedEventHandler PropertyChanged;
And change the calling to:
public int SomeMember {
get {
return this.someMember;
}
set {
if (this.someMember != value){
someMember = value;
if (PropertyChanged != null) { // If someone subscribed to the event
PropertyChanged(this, new PropertyChangedEventArgs("SomeMember")); // Raise the event
}
}
}
private void listner(object sender, PropertyChangedEventArgs e) {
string propertyName = e.PropertyName;
Console.WriteLine(String.Format("Hey! Hey! Listen! a {0} of a chatter in my list has changed!", propertyName));
}
why isn't this just calling PropertyChanged(this, new
PropertyCHangedEventArgs(name))
Because if no one attached an handler to the event, then the PropertyChanged object returns null. So you'll have to ensure it's not null before calling it.
where does PropertyChanged get assigned?
In the "listener" classes.
For example, you could write in other class:
ChattyClass tmp = new ChattyClass();
tmp.PropertyChanged += (sender, e) =>
{
Console.WriteLine(string.Format("Property {0} has been updated", e.PropertyName));
};
What does the assignment look like?
In C# we use the assignment operators += and -= for events. I recommend reading the following article to understand how to write event handlers using the anonymous method form (example above) and the "old" form.
From taking the original code, and incorporating #Styxxy 's answer, I come out with:
public class ChattyClass : INotifyPropertyChanged
{
private int someMember, otherMember;
public int SomeMember
{
get
{
return this.someMember;
}
set
{
if (this.someMember != value)
{
someMember = value;
OnPropertyChanged("Some Member");
}
}
}
public int OtherMember
{
get
{
return this.otherMember;
}
set
{
if (this.otherMember != value)
{
otherMember = value;
OnPropertyChanged("Other Member");
}
}
}
protected virtual void OnPropertyChanged(string propertyName)
{
PropertyChangedEventHandler handler = PropertyChanged;
if (handler != null) handler(this, new PropertyChangedEventArgs(propertyName));
}
public event PropertyChangedEventHandler PropertyChanged;
}
public class NosyClass
{
private List<ChattyClass> myChatters = new List<ChattyClass>();
public void AddChatter(ChattyClass chatter)
{
myChatters.Add(chatter);
chatter.PropertyChanged+=chatter_PropertyChanged;
}
private void chatter_PropertyChanged(object sender, PropertyChangedEventArgs e)
{
Console.WriteLine("A property has changed: " + e.PropertyName);
}
}
Following is the Code i am using
class ImpersonatedTab : System.Windows.Forms.TabPage
{
Credentials _cred = new Credentials();
public delegate void Savesetting(TabData Tb);
public event Savesetting TriggerSaveSetting;
public ImpersonatedTab(TabData tb)
{
........
}
private void SaveData()
{
TriggerSaveSetting(_tabdata);
}
private Onclick()
{
SaveData();
}
}
When i call Onclick function within ImpersonatedTab class it returns error saying TriggerSaveSetting is null
I initialize this call like
ImpersonatedTab Tab = new ImpersonatedTab(tb);
Tab.TriggerSaveSetting += new ImpersonatedTab.Savesetting(Tab_TriggerSaveSetting);
i have created events earlier, but am not able to figure out whats wrong with this one.. i am sure should be some silly mistake.
One possible case where this could happen is if you try to call the event from within the constructor of the ImpersonatedTab class. There where you put those .... Also it is a good practice to check if the event handler has been initialized before calling it.
Change your code to this:
public delegate void Savesetting(TabData Tb);
private Savesetting saveSettingDlg;
public event Savesetting TriggerSaveSetting {
add { saveSettingDlg += value; }
remove { saveSettingDlg -= value; }
}
private void SaveData() {
var handler = saveSettingDlg;
if (handler != null) handler(_tabdata);
}
You can now set a breakpoint on the add accessor and SaveData() and verify that event subscription is working correctly. If you do see the add accessor getting called but still get a null for handler then there's a problem with the object reference.
You are trying to invoke the TriggerSaveSetting event handlers before any handler was attached to it. Make sure, to check, the event has been some handlers attached:
private void OnTriggerSaveSetting(_tabdata)
{
if (TriggerSaveSetting != null)
TriggerSaveSetting(_tabdata);
}
I'm making custom events for C# and sometimes it isn't working.
This is how I'm making the event happen:
private bool isDoorOpen;
public bool IsDoorOpen {
get { return isDoorOpen;}
private set { isDoorOpen = value; DoorsChangeState(this, null);}
}
And these are the event declarations:
//events
public delegate void ChangedEventHandler(Elevator sender, EventArgs e);
public event ChangedEventHandler PositionChanged;
public event ChangedEventHandler DirectionChanged;
public event ChangedEventHandler BreaksChangeState;
public event ChangedEventHandler DoorsChangeState;
This works as long as there are methods attached to the events, but if there isn't, it throws a null ref exception. What am I doing wrong?
The recommended way to call an event is
var handler = this.DoorsChangeState;
if (handler != null)
handler(this, null);
The reason for copying the handler locally is incase the event handler changes on another thread while you're checking for null.
EDIT: Found the article talking about race conditions.
http://blogs.msdn.com/ericlippert/archive/2009/04/29/events-and-races.aspx
I know this question has been discussed (and answered) several times here on SO.
Also somewhere here i got the following extension methods to make this pattern more easy to use:
public static class EventHandlerExtensions
{
public static void FireEvent<T>(this EventHandler<T> handler, object sender, T args) where T : EventArgs
{
var temp = handler;
if (temp != null)
{
temp(sender, args);
}
}
public static void FireEvent(this EventHandler handler, object sender)
{
var temp = handler;
if (temp != null)
{
temp(sender, EventArgs.Empty);
}
}
}
So in your code you can say:
public bool IsDoorOpen
{
get { return isDoorOpen;}
private set
{
isDoorOpen = value;
DoorsChangeState.FireEvent(this);
}
}
If a event isn't subscribed to when it fires, a NullReferenceException will be thrown. This is correct behaviour, not something you've done wrong.
You should check:
if(DoorsChangeState != null)
{
DoorsChangeState(this, null); // Only fire if subscribed to
}
Before invoking an event you must check if the event is null:
if (DoorsChangeState != null)
DoorsChangeState(this, null);
When DoorsChangeState is null that means there are no listeners on that event.
You need to check to see if the event has been subscribed to.
I use this standard form for throwing all of my events.
var temp = EventName;
if(EventName!= null)
temp(this, null);
I have a class which wraps another class and exposes several events from the class it's wrapping. (The instance it wraps can change)
I used the following code:
public event EventHandler AnEvent;
public OtherClass Inner {
get { /* ... */ }
set {
//...
if(value != null)
value.AnEvent += AnEvent;
//...
}
}
However, the events were raised inconsistently.
What's wrong with this code?
The problem is that Delegates are immutable.
If you add a handler to an event, it creates a new Delegate instance which contains the old handlers and the newly added handler. The old Delegate is not modified and is discarded.
When I write, value.AnEvent += AnEvent, it adds the Delegate containing the current handlers (if any) to the inner class's event. However, changes to the outer class's event are ignored because they don't change the Delegate instance that I added to the inner classes event. Similarly, if I remove a handler after setting the Inner property, the handler isn't removed from the inner class's event.
There are two correct ways to do this.
I can make my own handler that invokes the wrapper's event, like this:
public event EventHandler AnEvent;
public OtherClass Inner {
get { /* ... */ }
set {
if(Inner != null)
Inner.AnEvent -= Inner_AnEvent;
//...
if(value != null)
value.AnEvent += Inner_AnEvent;
//...
}
}
void Inner_AnEvent(object sender, EventArgs e) {
var handler = AnEvent;
if (handler != null) handler(sender, e);
}
The other way is to make a custom event in the wrapper that adds its handlers to the inner class's event, like this:
EventHandler anEventDelegates
public OtherClass Inner {
get { /* ... */ }
set {
//...
if(value != null)
value.AnEvent += anEventDelegates;
//...
}
}
public event EventHandler AnEvent {
add {
anEventDelegates += value;
if (Inner != null) Inner.AnEvent += value;
}
remove {
anEventDelegates -= value;
if(Inner != null) Inner -= value;
}
}
Note that this is not entirely thread-safe.
I solved this problem myself and am posting the question & answer for the benefit of people with similar problems.
The your answer - there are two problems here...
First: in both cases, you are raising the outer event with the wrong sender. Someone subscribing to an event on the outer class would expect those classes to be raised with a sender of that outer class.
This is particularly important in things like winform Controls, or binding-list implementations, where the sender is used to identify the object between many that share a handler.
This should instead be something like:
void Inner_AnEvent(object sender, EventArgs e) {
var handler = AnEvent;
if (handler != null) handler(this, e);
}
The second (much more minor) issue is that you are currently taking out an event on the inner class even if the outer class has no subscribers. You can fix this with a bit more custom handling...
private EventHandler anEvent;
public event EventHandler AnEvent {
add { // note: not synchronized
bool first = anEvent == null;
anEvent += value;
if(first && anEvent != null && inner != null) {
inner.SomeEvent += Inner_AnEvent;
}
}
remove { // note: not synchronized
bool hadValue = anEvent != null;
anEvent -= value;
if(hadValue && anEvent == null && inner != null) {
inner.SomeEvent -= Inner_AnEvent;
}
}
}
(and similar code in the Inner get/set to only subscribe if we have listeners...
if(value != null && anEvent != null)
value.AnEvent += Inner_AnEvent;
This might be a big saver if you have lots of instances of the outer-class, but rarely use the event.