INotifyPropertyChanged and MVVM Light Toolkit

MVVM Light is a framework that helps you build XAML applications according to the Model-View-View-Model architectural pattern. MVVM Light includes several features, and one of those is to simplify the implementation of the INotifyPropertyChanged interface. However, MVVM Light still requires you to write a lot of boilerplate code. This article shows how to use PostSharp together with MVVM Light, whether because you are upgrading an existing project that already uses this framework or because you want to use the other features of MVVM Light.

Without PostSharp

In the example below, there is a View-Model implemented according to the MVVM Light Toolkit specification. The class MainViewModel inherits from the class ViewModelBase, which implements the INotifyPropertyChanged interface and thus is already helping you to use the Notify Property Changed pattern.

On the other hand, there is still part of the boilerplate usually appearing within the NotifyPropertyChanged pattern. For each property, you must have an explicit field, and you must manually notify all dependencies and the property itself. The result is not only more code written, but also a big space for bugs because you must discover and maintain the chain of dependent properties yourself.

using GalaSoft.MvvmLight;
using GalaSoft.MvvmLight.Command;
using System.Windows.Input;
using System.Windows;

namespace MvvmLightTest.ViewModel
{
    public class MainViewModel : ViewModelBase
    {
        int _exampleValue;

        public int ExampleValue
        {
            get
            {
                return _exampleValue;
            }
            set
            {
                if (_exampleValue == value)
                    return;
                _exampleValue = value;
                RaisePropertyChanged("ExampleValue");
            }
        }

    }
}

With PostSharp

With PostSharp, you don’t need to do any of that. You just indicate which classes should have the NotifyPropertyChanged pattern implemented using the NotifyPropertyChangedAttribute attribute and PostSharp does the hard work for you. You can see the difference in the second example, where the previous code got refactored keeping the same functionality.

using GalaSoft.MvvmLight.Command;
using System.Windows.Input;
using System.Windows;
using PostSharp.Patterns.Model;

namespace MvvmLightTest.ViewModel
{
    [NotifyPropertyChanged]
    public class MainViewModel : ViewModelBase
    {
        public int ExampleValue { get; set; }
    }
}

Note that it is no longer necessary to derive your class from the ViewModelBase class. Even if you suppress the inheritance from ViewModelBase class, you can still use other MVVM Light features in your code such as commands. However, if you do keep the inheritance from ViewModelBase, the NotifyPropertyChangedAttribute aspect will invoke the RaisePropertyChanged method of the ViewModelBase class, consistently with the coding practices of MVVM Light.