Why can't I invoke PropertyChanged event from an Extension Method?

Question

I've tried to code a class to avoid a method like "RaisePropertyChanged". I know that I can inherit from a class that has that implementation but in some cases I can't. I've tried with a Extension Method but Visual Studio complain.

public static class Extension
{
    public static void RaisePropertyChanged(this INotifyPropertyChanged predicate, string propertyName)
    {
        if (predicate.PropertyChanged != null)
        {
            predicate.PropertyChanged(propertyName, new PropertyChangedEventArgs(propertyName));
        }
    }
}

It said:

"The event 'System.ComponentModel.INotifyPropertyChanged.PropertyChanged' can only appear on the left hand side of += or -="

Answer 1

Reed is right. However, I see what you're trying to do (make your code reusable—good for you); and I'll just point out that this is often easily rectified by accepting the PropertyChangedEventHandler delegate itself and passing it from within the INotifyPropertyChanged implementation:

public static void Raise(this PropertyChangedEventHandler handler, object sender, string propertyName)
{
    if (handler != null)
    {
        handler(sender, new PropertyChangedEventArgs(propertyName));
    }
}

Then from within your class which implements INotifyPropertyChanged, you can call this extension method like so:

PropertyChanged.Raise(this, "MyProperty");

This works because, as Marc said, within the class declaring the event you can access it like a field (which means you can pass it as a delegate argument to a method, including extension methods).