how to override set in C# of automatic properties

Question

I want to use auto-implemented properties, but at the same time I want to call a method every time the property is changed.

public float inverseMass
{
    get;
    set
    {
        onMassChanged();
        inverseMass = value;
    }
}

Does the previous code make a recursive call to the property? If it does, how to implement this?

Answer 1

If you provide your own get/set then you need to provide your own storage for the variable.

private float _inverseMass;

public float inverseMass
{
    get { return _inverseMass; }
    set
    {
        _inverseMass = value;
        onMassChanged();
    }
}

Answer 2

Use a backing field instead:

public float inverseMass
{
    get
    {
        return _inverseMass;
    }
    set
    {
        _inverseMass = value;
        onMassChanged();
    }
}

private float _inverseMass;

I once read somewhere that the C# guys decided that they wanted to cover the broad of the usage scenarios with those automatic properties, but for special cases you have to go back to good old backing fields.

(To be honest, I believe that Jon Skeet told them how to implement them)

Answer 3

You would need to break the above out in to private/public members so you don't get recursive problems:

private float _inverseMass;
public float inverseMass
{
  get { return this._inverseMass; }
  set { this._inverseMass = value; onMassChanged(); }
}

However, have you looked in to the INotifyPropertyChanged interface? It does pretty much what you're looking for, and depending on what you're writing it could be supported natively.

public class MyClass : INotifyPropertyChanged
{
  public event PropertyChangedEventHandler PropertyChanged;
  private void NotifyPropertyChanged(String property)
  {
    var event = this.PropertyChanged;
    if (event != null)
    {
      event(this, new PropertyChangedEventArgs(property));
    }
  }

  private float _inverseMass;
  public float inverseMass
  {
    get { return this._inverseMass; }
    set { this._inverseMass = value; NotifyPropertyChanged("inverseMass"); }
  }
}

Answer 4

You can't do that in C#.

Its either automatic property or property with backing field.

Answer 5

That doesn't compile, so it's hard to say if it's recursive or not.

You cannot specify only one part of an automatic property. You will need to use a backing-field in this case:

private float _inverseMass;
public float inverseMass 
{ 
    get { return _inverseMass; }
    set 
    { 
        onMassChanged(); 
        _inverseMass = value; 
    } 
}

Though, an aspected-oriented programming framework like PostSharp may be able to help you with this.

Answer 6

The correct answers are given above, but in case you wanted to do this quickly in Visual Studio, you can use Code Snippets instead to generate the property for you.

Have a read of this article which should explain it a bit better: http://www.roelvanlisdonk.nl/?p=1007

If you want, you can even create your own code snippet to add the repetitive code. You could even use it to quickly generate the INotifyPropertyChanged implementation as mentioned above.

Answer 7

Use a private variable as your back up field for the public property.

private float inverseMass;

public float InverseMass{

   set
   {
        onMassChanged();
        inverseMass=value;
   }
   get
   {
      return inverseMass;
   }

}

Answer 8

public float inverseMass
{
    get;
    set
    {
        if (inverseMass != value)
        {
            inverseMass = value;
            onMassChanged();
        }
    }
}