How to trigger event when a variable s value is changed

Question

I m currently creating an application in C  using Visual Studio  I want to create some code so that when a variable has a value of 1 then a certain piece of code is carried out  I know that I can use an if statement but the problem is that the value will be changed in an asynchronous process so technically the if statement could be ignored before the value has changed   Is it possible to create an event handler so that when the variable value changes an event is triggered  If so  how can I do this   It is completely possible that I could have misunderstood how an if statement works  Any help would be much appreciated

User · Answer

You can use a property setter to raise an event whenever the value of a field is going to change.

You can have your own EventHandler delegate or you can use the famous System.EventHandler delegate.

Usually there's a pattern for this:

Define a public event with an event handler delegate (that has an argument of type EventArgs).
Define a protected virtual method called OnXXXXX (OnMyPropertyValueChanged for example). In this method you should check if the event handler delegate is null and if not you can call it (it means that there are one or more methods attached to the event delegation).
Call this protected method whenever you want to notify subscribers that something has changed.

Here's an example

private int _age;

//#1
public event System.EventHandler AgeChanged;

//#2
protected virtual void OnAgeChanged()
{ 
     if (AgeChanged != null) AgeChanged(this,EventArgs.Empty); 
}

public int Age
{
    get
    {
         return _age;
    }

    set
    {
         //#3
         _age=value;
         OnAgeChanged();
    }
 }

The advantage of this approach is that you let any other classes that want to inherit from your class to change the behavior if necessary.

If you want to catch an event in a different thread that it's being raised you must be careful not to change the state of objects that are defined in another thread which will cause a cross thread exception to be thrown. To avoid this you can either use an Invoke method on the object that you want to change its state to make sure that the change is happening in the same thread that the event has been raised or in case that you are dealing with a Windows Form you can use a BackgourndWorker to do things in a parallel thread nice and easy.

User · Answer

The  NET framework actually provides an interface that you can use for notifying subscribers when a property has changed  System ComponentModel INotifyPropertyChanged  This interface has one event PropertyChanged  Its usually used in WPF for binding but I have found it useful in business layers as a way to standardize property change notification   In terms of thread safety I would put a lock under in the setter so that you don t run into any race conditions   Here are my thoughts in code       public class MyClass   INotifyPropertyChanged       private object  lock       public int MyProperty               get                       return  myProperty                    set                       lock  lock                                  The property changed event will get fired whenever                   the value changes  The subscriber will do work if the value is                   1  This way you can keep your business logic outside of the setter                 if value     myProperty                                         myProperty   value                      NotifyPropertyChanged  MyProperty                                                         private NotifyPropertyChanged string propertyName                  Raise PropertyChanged event           public event PropertyChangedEventHandler PropertyChanged      public class MySubscriber       private MyClass  myClass               void PropertyChangedInMyClass object sender  PropertyChangedEventArgs e                switch e PropertyName                        case  MyProperty                   DoWorkOnMyProperty  myClass MyProperty                   break                       void DoWorkOnMyProperty int newValue                if newValue    1                           DO WORK HERE                     Hope this is helpful

User · Answer

you can use generic class  class Wrapped lt T gt         private T  value       public Action ValueChanged       public T Value               get   gt   value           set                        value   value              OnValueChanged                         protected virtual void OnValueChanged     gt  ValueChanged  Invoke        and will be able to do the following  var i   new Wrapped lt int gt      i ValueChanged         gt    Console WriteLine  quot changed  quot        i Value   10  i Value   10  i Value   10  i Value   10   Console ReadKey     result  changed  changed  changed  changed  changed  changed  changed

User · Answer

Seems to me like you want to create a property   public int MyProperty       get   return  myProperty        set                myProperty   value          if   myProperty    1                           DO SOMETHING HERE                    private int  myProperty    This allows you to run some code any time the property value changes  You could raise an event here  if you wanted

User · Answer

A simple method involves using the get and set functions on the variable      using System      public string Name      get       return name                 set       name  value       OnVarChange  Invoke                    private string name           public event System Action OnVarChange

User · Answer

just use a property  int   theVariable  public int TheVariable    get return  theVariable     set       theVariable   value       if    theVariable    1           Do stuff here

[c#] How to trigger event when a variable's value is changed?

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