A Better INotifyPropertyChanged Implementation

By gamlerhart November 25, 2009 .NET

The INotifyPropertyChanged-interface and companions are a well know citizens of the .NET-framework. How do you implement this interfaces?. Does it look similar to this?

public class Person : INotifyPropertyChanged
    {
        private string firstname = "";
        /* Other fields for the properties */
        public event PropertyChangedEventHandler PropertyChanged;

        public string Firstname
        {
            get { return firstname; }
            set
            {
                firstname = value;
                OnPropertyChanged("FirstName");
            }
        }
        /** Other properties **/

        private void OnPropertyChanged(string propertyName)
        {
            var property = PropertyChanged;
            if (null != property)
                property(this, new PropertyChangedEventArgs(propertyName));
        }
    }

 

This is a common pattern. Often it’s even uglier because the code in the ‘OnPropertyChanged’-method is copy & pasted into the property-implementation.

Dilema of INotifyPropertyChange

Dilema of INotifyPropertyChange

So what makes me sad about this common implementation?

1. First you need to implement the code in ‘OnPropertyChanged’ over and over. Of course you can build a base-class for this. However I try to be careful with inheritance. Therefore I really don’t like just inherit for this purpose.

2. The property name is a string and that pretty bad here: You have no assistance from the compiler. When you refactor the property-name or make a typo you have introduce a very subtle bug into your program. If your lucky you catch is with unit-test. But honestly, I concentrate my unit test on more important stuff than validating properties.

Now the goal is clear. I want a implementation which has this properties:

1. Reusing the code in the ‘OnPropertyChanged’-method without inheritance

2. Static type safety, my compiler should complain when I make a typo.

My effort resulted in this:

public string Firstname
{
    get { return firstname; }
    set
    {
        firstname = value;
        PropertyChanged.Fire(this,()=>Firstname);
    }
}

Now you can call “Fire” on all PropertyChangedEventHandler-events. The ‘Fire’-Implementation is an extension-method which implements the boiler-plate: Checking for null etc. To avoid the string as property-name you pass in a lambda-expression. This avoid typos and makes refactoring safer.

Of course it’s not perfect. You could still pass something wrong with the lambda. The implementation only checks that it’s a property. But it’s way better than a pure string.

You could extend the implementation to other event-types quite easy. For example for the INotifyPropertyChanging-interface. The source is attached to this post. Have fun with it.

Implementation: NotifyExtensions.cs, TestNotifyExtensions.cs

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