Refactoring PropertyChangedEventHandler

Question

In my UI code I have a lot classes with the same basic skeleton:

• derives from INotifyPropertyChanged

• contains the following code:

  void NotifyPropertyChanged(String info)
  {
      if (PropertyChanged != null)
      {
          PropertyChanged(this, new PropertyChangedEventArgs(info));
      }
  }
  
  
  public event PropertyChangedEventHandler PropertyChanged;
  

It seems like a perfect chance to factor into a class and to derive from that instead of INotifyPropertyChanged, but sadly C# doesn't support multiple inheritance so it's not really going to work. Any ideas on how to refactor this kind of code?

Answer 1

Maybe you can use something like this:

class A1 : INotifyPropertyChanged
{
    private string _myProperty;
    private static Expression<Func<A1, string>> myProperty = _ => _.MyProperty;

    public string MyProperty
    {
        get { return _myProperty; }
        set
        {
            _myProperty = value;
            InvokePropertyChanged(myProperty);
        }
    }

    public event PropertyChangedEventHandler PropertyChanged;

    private void InvokePropertyChanged<T>(Expression<Func<A1, T>> property)
    {
        PropertyChangedEventHandler Handler = PropertyChanged;
        if (Handler != null)
        {
            MemberExpression expression = (MemberExpression)property.Body;
            Handler(this, new PropertyChangedEventArgs(expression.Member.Name));
        }
    }
}

This significally reduce future code changes;)

Or you can use Postsharp plug-in that automatically implements INotifyPropertyChanged.

Answer 2

Can't you just put this code into your superclass's superclass?

• Object
  • Your concrete NotifyPropertyChanged class <-- Insert here
    • Whatever your viewmodel inherited from (and stopped you using multiple inheritance
      • Your concrete viewmodel
      • Another concrete viewmodel

Most MVVM Frameworks provide such a class for you.

Because of the access rules around events, you can't factor this out into an extension method without reflection, unfortunately.

Answer 3

It is 4 lines of code that will never change. Create a snippet!

Answer 4

A common practice is to have a base class implementing INotifyPropertyChanged, like this:

public abstract class ViewModelBase : INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged;

    protected void OnPropertyChanged(string propertyName)
    {
        PropertyChangedEventHandler handler = PropertyChanged;

        if (handler != null)
        {
            handler(this, new PropertyChangedEventArgs(propertyName));
        }
    }
}

Then your classes will derive from this one, and you can call OnPropertyChanged when you wish to notify changes on the value of a property :

public class PersonViewModel : ViewModelBase
{

    public PersonViewModel(Person person)
    {
        this.person = person;
    }

    public string Name
    {
        get
        {
            return this.person.Name;
        }
        set
        {
            this.person.Name = value;
            OnPropertyChanged("Name");
        }
    }
}