Why is my event handler null?

Question

I've got quite simple problem but gives me a headache and I'm wasting too much time. I've ran out of ideas anyway:) For some reason I can't pass the value of my property variable to my event handler. Here is what I've got , an for me everything is fine but it won't work:(

Any idea why it's not passing the actual value of the variable? Thanks in advance:)

Answer 1

Just as the name suggests, propertyName should contain the property's name, not value. Also see PropertyChangedEventHandler and PropertyChangedEventArgs on MSDN for more details.

As to why your event handler null, I suspect you haven't subscribed to it. You should have something like the following somewhere in your program:

obj.PropertyChanged += new PropertyChangedEventHandler(obj_PropertyChanged);

private void obj_PropertyChanged(object sender, PropertyChangedEventArgs e)
{
  ....
}

then when obj.Moves changes obj_PropertyChanged will be called.

I understand your confusion, so let me give a little more explanations. Suppose you have two classes A and B, and you want B to be notified when a property of A is changed. Then you can make A implement INotifyPropertyChanged, and make B subscribe to the PropertyChanged event of A, like the following:

public class A: INotifyPropertyChanged
{
  private int moves;
  public int Moves
  {
    get { return moves; }
    set
    {
      if (value != moves) {
        moves = value;
        OnPropertyChanged("Moves");
      }
    }
  }

  public event PropertyChangedEventHandler PropertyChanged;

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

public class B
{
  private A a = new A();

  public B()
  {
    a.PropertyChanged += new PropertyChangedEventHandler(a_PropertyChanged);
  }

  private void a_PropertyChanged(object sender, PropertyChangedEventArgs e)
  {
    ....
  }
}

With the above, B.a_PropertyChanged will be called whenever A.Moves is changed. More precisely, if b is an instance of B, then b.a_PropertyChanged will be called whenever b.a.Moves is changed. Also please note how my implementation of the setter of Moves differs from yours.