Strongly Typed INotifyPropertyChanged Event Raisers

INotifyPropertyChanged has been around forever, and so has the nasty string based property names that are passed to raise the events. Its all familiar stuff:

class Program 
{
    static void Main(string[] args)
    {
        
        var streamingPricesService = new StreamingPricesService();

        Expression<Action<Object, PropertyChangedEventArgs>> test = (o, e) => Console.WriteLine("Property '{0}' changed ", e.PropertyName);

        streamingPricesService.PropertyChanged 
            += (o, e) => Console.WriteLine("Property '{0}' changed ", e.PropertyName);
        streamingPricesService.PropertyChanged
            += (o, e) => Console.WriteLine("Property '{0}' changed ", e.PropertyName);
        streamingPricesService.IsOnline = true;

        Console.ReadKey();
    }

    private class StreamingPricesService : INotifyPropertyChanged
    {
        private bool _isOnline;
        public bool IsOnline
        {
            get { return _isOnline; }
            set
            {
                if (_isOnline != value)
                {
                    _isOnline = value;
                    InvokePropertyChanged("IsOnline");
                }
            }
        }


        public event PropertyChangedEventHandler PropertyChanged;

        private void InvokePropertyChanged(string propertyName)
        {
            PropertyChangedEventHandler changed = PropertyChanged;
            if (changed != null) changed(this, new PropertyChangedEventArgs(propertyName));
        }
    }
}

..and its nasty!!

Enter Lambda LambdaExpression, its been around the block by now,  you’ve all seen them, the PropertyChanged wireup below uses the meta data it exposes above, its type-safe and IMO cleaner. You can wrap the functionality up anyway you like, typically I’d put the INotifyPropertyChanged functionality in a class called NotifyingBase and keep the PropertyHelper in a separate class as its usually used in other situations too (however below its just all in 2 classes).

using System;
using System.ComponentModel;
using System.Linq.Expressions;

namespace ConsoleApplication3
{
    class Program 
    {
        static void Main(string[] args)
        {
            var streamingPricesService = new StreamingPricesService();

            streamingPricesService.PropertyChanged += (o, e) => Console.WriteLine("Property '{0}' changed ", e.PropertyName);
            streamingPricesService.PropertyChanged += (o, e) => Console.WriteLine("Property '{0}' changed ", e.PropertyName);
            streamingPricesService.IsOnline = true;

            Console.ReadKey();
        }

        private class StreamingPricesService : INotifyPropertyChanged
        {
            private bool _isOnline;
            public bool IsOnline
            {
                get { return _isOnline; }
                set
                {
                    if (_isOnline != value)
                    {
                        _isOnline = value;
                        InvokePropertyChanged(() => IsOnline);
                    }
                }
            }

            public event PropertyChangedEventHandler PropertyChanged;

            private void InvokePropertyChanged<T>(Expression<Func<T>> expression)
            {
                PropertyChangedEventHandler changed = PropertyChanged;
                if (changed != null)
                {
                    var propertyName = PropertyHelper.GetPropertyName(expression);
                    changed(this, new PropertyChangedEventArgs(propertyName));
                }
            }
        }
    }

    public class PropertyHelper
    {
        public static string GetPropertyName<T>(Expression<Func<T>> expression)
        {
            return GetPropertyNameFromLambda(expression);
        }

        public static string GetPropertyName<T>(Expression<Func<T, Object>> expression)
        {
            return GetPropertyNameFromLambda(expression);
        }

        private static string GetPropertyNameFromLambda(LambdaExpression lambda)
        {
            MemberExpression memberExpression;
            if (lambda.Body is UnaryExpression)
            {
                var unaryExpression = lambda.Body as UnaryExpression;
                memberExpression = unaryExpression.Operand as MemberExpression;
            }
            else
                memberExpression = lambda.Body as MemberExpression;

            if (memberExpression == null)
                throw new ArgumentException(String.Format("Property expression '{0}' did not provide a property name.", lambda));

            return memberExpression.Member.Name;
        }
    }
}