Implementing INotifyPropertyChanged - does a bette

Question

Microsoft should have implemented something snappy for INotifyPropertyChanged, like in the automatic properties, just specify {get; set; notify;} I think it makes a lot of sense to do it. Or are there any complications to do it?

Can we ourselves implement something like 'notify' in our properties. Is there a graceful solution for implementing INotifyPropertyChanged in your class or the only way to do it is by raising the PropertyChanged event in each property.

If not can we write something to auto-generate the piece of code to raise PropertyChanged event?

Answer 1

I haven't actually had a chance to try this myself yet, but next time I'm setting up a project with a big requirement for INotifyPropertyChanged I'm intending on writing a Postsharp attribute that will inject the code at compile time. Something like:

[NotifiesChange]
public string FirstName { get; set; }

Will become:

private string _firstName;

public string FirstName
{
   get { return _firstname; }
   set
   {
      if (_firstname != value)
      {
          _firstname = value;
          OnPropertyChanged("FirstName")
      }
   }
}

I'm not sure if this will work in practice and I need to sit down and try it out, but I don't see why not. I may need to make it accept some parameters for situations where more than one OnPropertyChanged needs to be triggered (if, for example, I had a FullName property in the class above)

Currently I'm using a custom template in Resharper, but even with that I'm getting fed up of all my properties being so long.

---

Ah, a quick Google search (which I should have done before I wrote this) shows that at least one person has done something like this before here. Not exactly what I had in mind, but close enough to show that the theory is good.

Answer 2

I have just found ActiveSharp - Automatic INotifyPropertyChanged, I have yet to use it, but it looks good.

To quote from it's web site...

---

Send property change notifications without specifying property name as a string.

Instead, write properties like this:

public int Foo
{
    get { return _foo; }
    set { SetValue(ref _foo, value); }  // <-- no property name here
}

Note that there is no need to include the name of the property as a string. ActiveSharp reliably and correctly figures that out for itself. It works based on the fact that your property implementation passes the backing field (_foo) by ref. (ActiveSharp uses that "by ref" call to identify which backing field was passed, and from the field it identifies the property).

Answer 3

I introduce a Bindable class in my blog at http://timoch.com/blog/2013/08/annoyed-with-inotifypropertychange/ Bindable uses a dictionary as a property bag. It's easy enough to add the necessary overloads for a subclass to manage its own backing field using ref parameters.

• No magic string
• No reflection
• Can be improved to suppress the default dictionary lookup

The code:

public class Bindable : INotifyPropertyChanged {
    private Dictionary<string, object> _properties = new Dictionary<string, object>();

    /// <summary>
    /// Gets the value of a property
    /// </summary>
    /// <typeparam name="T"></typeparam>
    /// <param name="name"></param>
    /// <returns></returns>
    protected T Get<T>([CallerMemberName] string name = null) {
        Debug.Assert(name != null, "name != null");
        object value = null;
        if (_properties.TryGetValue(name, out value))
            return value == null ? default(T) : (T)value;
        return default(T);
    }

    /// <summary>
    /// Sets the value of a property
    /// </summary>
    /// <typeparam name="T"></typeparam>
    /// <param name="value"></param>
    /// <param name="name"></param>
    /// <remarks>Use this overload when implicitly naming the property</remarks>
    protected void Set<T>(T value, [CallerMemberName] string name = null) {
        Debug.Assert(name != null, "name != null");
        if (Equals(value, Get<T>(name)))
            return;
        _properties[name] = value;
        OnPropertyChanged(name);
    }

    public event PropertyChangedEventHandler PropertyChanged;

    protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null) {
        PropertyChangedEventHandler handler = PropertyChanged;
        if (handler != null) {
            handler(this, new PropertyChangedEventArgs(propertyName));
        }
    }
}

It can be used like this:

public class Contact : Bindable {
    public string FirstName {
        get { return Get<string>(); }
        set { Set(value); }
    }
}

Answer 4

I realize this question already has a gazillion answers, but none of them felt quite right for me. My issue is I don't want any performance hits and am willing to put up with a little verbosity for that reason alone. I also don't care too much for auto properties either, which led me to the following solution:

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

    public void OnPropertyChanged(string propertyName)
    {
        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
    }

    protected virtual bool SetValue<TKind>(ref TKind Source, TKind NewValue, params string[] Notify)
    {
        //Set value if the new value is different from the old
        if (!Source.Equals(NewValue))
        {
            Source = NewValue;

            //Notify all applicable properties
            foreach (var i in Notify)
                OnPropertyChanged(i);

            return true;
        }

        return false;
    }

    public AbstractObject()
    {
    }
}

In other words, the above solution is convenient if you don't mind doing this:

public class SomeObject : AbstractObject
{
    public string AnotherProperty
    {
        get
        {
            return someProperty ? "Car" : "Plane";
        }
    }

    bool someProperty = false;
    public bool SomeProperty
    {
        get
        {
            return someProperty;
        }
        set
        {
            SetValue(ref someProperty, value, "SomeProperty", "AnotherProperty");
        }
    }

    public SomeObject() : base()
    {
    }
}

Pros

• No reflection
• Only notifies if old value != new value
• Notify multiple properties at once

Cons

• No auto properties (you can add support for both, though!)
• Some verbosity
• Boxing (small performance hit?)

Alas, it is still better than doing this,

set
{
    if (!someProperty.Equals(value))
    {
        someProperty = value;
        OnPropertyChanged("SomeProperty");
        OnPropertyChanged("AnotherProperty");
    }
}

For every single property, which becomes a nightmare with the additional verbosity ;-(

Note, I do not claim this solution is better performance-wise compared to the others, just that it is a viable solution for those who don't like the other solutions presented.

Answer 5

I created an Extension Method in my base Library for reuse:

public static class INotifyPropertyChangedExtensions
{
    public static bool SetPropertyAndNotify<T>(this INotifyPropertyChanged sender,
               PropertyChangedEventHandler handler, ref T field, T value, 
               [CallerMemberName] string propertyName = "",
               EqualityComparer<T> equalityComparer = null)
    {
        bool rtn = false;
        var eqComp = equalityComparer ?? EqualityComparer<T>.Default;
        if (!eqComp.Equals(field,value))
        {
            field = value;
            rtn = true;
            if (handler != null)
            {
                var args = new PropertyChangedEventArgs(propertyName);
                handler(sender, args);
            }
        }
        return rtn;
    }
}

This works with .Net 4.5 because of CallerMemberNameAttribute. If you want to use it with an earlier .Net version you have to change the method declaration from: ...,[CallerMemberName] string propertyName = "", ... to ...,string propertyName, ...

Usage:

public class Dog : INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged;
    string _name;

    public string Name
    {
        get { return _name; }
        set
        {
            this.SetPropertyAndNotify(PropertyChanged, ref _name, value);
        }
    }
}

Answer 6

Based on the answer by Thomas which was adapted from an answer by Marc I've turned the reflecting property changed code into a base class:

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

    protected void OnPropertyChanged(string propertyName)
    {
        PropertyChangedEventHandler handler = PropertyChanged;
        if (handler != null) 
            handler(this, new PropertyChangedEventArgs(propertyName));
    }

    protected void OnPropertyChanged<T>(Expression<Func<T>> selectorExpression)
    {
        if (selectorExpression == null)
            throw new ArgumentNullException("selectorExpression");
        var me = selectorExpression.Body as MemberExpression;

        // Nullable properties can be nested inside of a convert function
        if (me == null)
        {
            var ue = selectorExpression.Body as UnaryExpression;
            if (ue != null)
                me = ue.Operand as MemberExpression;
        }

        if (me == null)
            throw new ArgumentException("The body must be a member expression");

        OnPropertyChanged(me.Member.Name);
    }

    protected void SetField<T>(ref T field, T value, Expression<Func<T>> selectorExpression, params Expression<Func<object>>[] additonal)
    {
        if (EqualityComparer<T>.Default.Equals(field, value)) return;
        field = value;
        OnPropertyChanged(selectorExpression);
        foreach (var item in additonal)
            OnPropertyChanged(item);
    }
}

Usage is the same as Thomas' answer except that you can pass additional properties to notify for. This was necessary to handle calculated columns which need to be refreshed in a grid.

private int _quantity;
private int _price;

public int Quantity 
{ 
    get { return _quantity; } 
    set { SetField(ref _quantity, value, () => Quantity, () => Total); } 
}
public int Price 
{ 
    get { return _price; } 
    set { SetField(ref _price, value, () => Price, () => Total); } 
}
public int Total { get { return _price * _quantity; } }

I have this driving a collection of items stored in a BindingList exposed via a DataGridView. It has eliminated the need for me to do manual Refresh() calls to the grid.