Raise an event whenever a property's value changed?

Question

There is a property, it's named ImageFullPath1

public string ImageFullPath1 {get; set; }

I'm going fire an event whenever its value changed. I am aware of changing INotifyPropertyChanged, but I want to do it with events.

Answer 1

The INotifyPropertyChanged interface is implemented with events. The interface has just one member, PropertyChanged, which is an event that consumers can subscribe to.

The version that Richard posted is not safe. Here is how to safely implement this interface:

public class MyClass : INotifyPropertyChanged
{
    private string imageFullPath;

    protected void OnPropertyChanged(PropertyChangedEventArgs e)
    {
        PropertyChangedEventHandler handler = PropertyChanged;
        if (handler != null)
            handler(this, e);
    }

    protected void OnPropertyChanged(string propertyName)
    {
        OnPropertyChanged(new PropertyChangedEventArgs(propertyName));
    }

    public string ImageFullPath
    {
        get { return imageFullPath; }
        set
        {
            if (value != imageFullPath)
            {
                imageFullPath = value;
                OnPropertyChanged("ImageFullPath");
            }
        }
    }

    public event PropertyChangedEventHandler PropertyChanged;
}

Note that this does the following things:

• Abstracts the property-change notification methods so you can easily apply this to other properties;

• Makes a copy of the PropertyChanged delegate before attempting to invoke it (failing to do this will create a race condition).

• Correctly implements the INotifyPropertyChanged interface.

If you want to additionally create a notification for a specific property being changed, you can add the following code:

protected void OnImageFullPathChanged(EventArgs e)
{
    EventHandler handler = ImageFullPathChanged;
    if (handler != null)
        handler(this, e);
}

public event EventHandler ImageFullPathChanged;

Then add the line OnImageFullPathChanged(EventArgs.Empty) after the line OnPropertyChanged("ImageFullPath").

Since we have .Net 4.5 there exists the CallerMemberAttribute, which allows to get rid of the hard-coded string for the property name in the source code:

    protected void OnPropertyChanged(
        [System.Runtime.CompilerServices.CallerMemberName] string propertyName = "")
    {
        OnPropertyChanged(new PropertyChangedEventArgs(propertyName));
    }

    public string ImageFullPath
    {
        get { return imageFullPath; }
        set
        {
            if (value != imageFullPath)
            {
                imageFullPath = value;
                OnPropertyChanged();
            }
        }
    }

Answer 2

I use largely the same patterns as Aaronaught, but if you have a lot of properties it could be nice to use a little generic method magic to make your code a little more DRY

public class TheClass : INotifyPropertyChanged {
    private int _property1;
    private string _property2;
    private double _property3;

    protected virtual void OnPropertyChanged(PropertyChangedEventArgs e) {
        PropertyChangedEventHandler handler = PropertyChanged;
        if(handler != null) {
            handler(this, e);
        }
    }

    protected void SetPropertyField<T>(string propertyName, ref T field, T newValue) {
        if(!EqualityComparer<T>.Default.Equals(field, newValue)) {
            field = newValue;
            OnPropertyChanged(new PropertyChangedEventArgs(propertyName));
        }
    }

    public int Property1 {
        get { return _property1; }
        set { SetPropertyField("Property1", ref _property1, value); }
    }
    public string Property2 {
        get { return _property2; }
        set { SetPropertyField("Property2", ref _property2, value); }
    }
    public double Property3 {
        get { return _property3; }
        set { SetPropertyField("Property3", ref _property3, value); }
    }

    #region INotifyPropertyChanged Members

    public event PropertyChangedEventHandler PropertyChanged;

    #endregion
}

Usually I also make the OnPropertyChanged method virtual to allow sub-classes to override it to catch property changes.

Answer 3

Raising an event when a property changes is precisely what INotifyPropertyChanged does. There's one required member to implement INotifyPropertyChanged and that is the PropertyChanged event. Anything you implemented yourself would probably be identical to that implementation, so there's no advantage to not using it.

Answer 4

public event EventHandler ImageFullPath1Changed;

public string ImageFullPath1
{
    get
    {
        // insert getter logic
    }
    set
    {
        // insert setter logic       

        // EDIT -- this example is not thread safe -- do not use in production code
        if (ImageFullPath1Changed != null && value != _backingField)
            ImageFullPath1Changed(this, new EventArgs(/*whatever*/);
    }
}                        

That said, I completely agree with Ryan. This scenario is precisely why INotifyPropertyChanged exists.

Answer 5

If you change your property to use a backing field (instead of an automatic property), you can do the following:

public event EventHandler ImageFullPath1Changed;
private string _imageFullPath1 = string.Empty;

public string ImageFullPath1 
{
  get
  {
    return imageFullPath1 ;
  }
  set
  {
    if (_imageFullPath1 != value)
    { 
      _imageFullPath1 = value;

      EventHandler handler = ImageFullPathChanged;
      if (handler != null)
        handler(this, e);
    }
  }
}

Answer 6

There is already have good answers but some people are still confused

• EventArgs and who they can use it
• And some of them about how to pass custom parameters

class Program
    {
        static void Main(string[] args)
        {
            Location loc = new Location();

            loc.LocationChanged += (obj, chngLoc) =>
            {
                Console.WriteLine("Your LocId Is");
                Console.WriteLine(chngLoc.LocId);
                Console.WriteLine(chngLoc.LocCode);
                Console.WriteLine(chngLoc.LocName);
                Console.ReadLine();
            };

            Console.WriteLine("Default Location Is");
            Console.WriteLine(loc.LocId);

            Console.WriteLine("Change Location");
            loc.LocId = Console.ReadLine();
        }
    }

    public class Location
    {

        private string _locId = "Default Location";
        public string LocId
        {
            get
            {
                return _locId;
            }
            set
            {

                _locId = value;
                if (LocationChanged != null && value != LocId)
                {
                    B1Events b1 = new B1Events();
                    b1.LocCode = "Changed LocCode";
                    b1.LocId = value;
                    b1.LocName = "Changed LocName";
                    LocationChanged(this, b1);
                }
                
            }
        }
         public event EventHandler<B1Events> LocationChanged;
    }

    public class B1Events : EventArgs
    {
        public string LocId { get; set; }
        public string LocCode{ get; set; }
        public string LocName { get; set; }
    }