Simple small INotifyPropertyChanged implementation

Question

Say I have the following class:

public MainFormViewModel
{
    public String StatusText {get; set;}
}

What is the easiest smallest way to get my changes to StatusText to reflect to any controls that bind to it?

Obviously I need to use INotifyPropertyChanged, but is there a cool way to do it that does not clutter up my code? need lots of files? etc?

Note: If this is a dupe then I am sorry. I searched and could not find any thing but using T4 code Generation which does not sound easy (to setup at least).

Answer 1

Unfortunately C# doesn't offer an easy mechanism to do that automatically... It has been suggested to create a new syntax like this :

public observable int Foo { get; set; }

But I doubt it will ever be included in the language...

A possible solution would to use an AOP framework like Postsharp, that way you just need to decorate your properties with an attribute:

public MainFormViewModel : INotifyPropertyChanged
{
    [NotifyPropertyChanged]
    public String StatusText {get; set;}
}

(haven't tried, but I'm pretty sure Postsharp allows you to do that kind of thing...)

---

UPDATE: OK, I managed to make it work. Note that it's a very crude implementation, using reflection on a private field to retrieve the delegate... It could certainly be improved, but I'll leave it to you ;)

[Serializable]
public class NotifyPropertyChangedAttribute : LocationInterceptionAspect
{
    public override void OnSetValue(LocationInterceptionArgs args)
    {
        object oldValue = args.GetCurrentValue();
        object newValue = args.Value;
        base.OnSetValue(args);
        if (args.Instance is INotifyPropertyChanged)
        {
            if (!Equals(oldValue, newValue))
            {
                RaisePropertyChanged(args.Instance, args.LocationName);
            }
        }
    }

    private void RaisePropertyChanged(object instance, string propertyName)
    {
        PropertyChangedEventHandler handler = GetPropertyChangedHandler(instance);
        if (handler != null)
            handler(instance, new PropertyChangedEventArgs(propertyName));
    }

    private PropertyChangedEventHandler GetPropertyChangedHandler(object instance)
    {
        Type type = instance.GetType().GetEvent("PropertyChanged").DeclaringType;
        FieldInfo propertyChanged = type.GetField("PropertyChanged",
                                                  BindingFlags.Instance | BindingFlags.NonPublic);
        if (propertyChanged != null)
            return propertyChanged.GetValue(instance) as PropertyChangedEventHandler;

        return null;
    }
}

Note that your class still need to implement the INotifyPropertyChanged interface. You just don't have to explicitly raise the event in your property setters.

Answer 2

Have a go of this http://code.google.com/p/notifypropertyweaver/

All you need to do is implement INotifyPropertyChanged

So your code will look like

public MainFormViewModel : INotifyPropertyChanged
{
    public String StatusText {get; set;}

    #region INotifyPropertyChanged Implementation
}

The build task will compile this (you never see the below code)

public MainFormViewModel : INotifyPropertyChanged
{
    public String StatusText {get; set;}
    private string statusText;

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

    #region INotifyPropertyChanged Implementation
}