Firing an event / function on a property? (C#)

Question

I am using a class that I cannot edit, it has a property (a boolean) of which it would be nice to be informed when it changes, I can't edit the properties get or set as I am importing the class from a .dll (which I don't have the code for).

How do I create an event/function that is fired when the property is changed?

Additional
It is only changed within its own class, directly to the underlying private variable.

E.g.

private bool m_MyValue = false;

public bool MyValue
  {
  get { return m_MyValue; }
  }

private void SomeFunction()
  {
  m_MyValue = true;
  }

Answer 1

You can't, basically... not without using something like the debugger API to inject code at execution time and modifying the IL of the original library (and I'm not recommending either of those solutions; aside from anything else it may violate the licence of the library).

Basically if a property doesn't support notification, it doesn't support notification. You should look for a different way of approaching your problem. (Would polling work, for example?)

Answer 2

Arguably, the only real way to do this is to create some kind of "watcher" component, running in a separate thread, whose job is to read the property at intervals and raise an event when the property's value changes. Of course this solution sails in the murky waters of threading and synchronization.

On the assumption that your application is single-threaded in respect to this object, your cleanest solution is to make method calls to this object via a proxy object. It would have the job of checking the before and after state of the property and raising an event in the case it has changed.

Here's a simple example of what I'm talking about:

public class SomeProxy
{
    public SomeProxy(ExternalObject obj)
    {
         _obj = obj;
    }

    public event EventArgs PropertyChanged;

    private bool _lastValue;

    private ExternalObject _obj;

    protected virtual void OnPropertyChanged()
    {
        if(PropertyChanged != null)
            PropertyChanged();
    }

    protected virtual void PreMethodCall()
    {
        _lastValue = _obj.SomeProperty;
    }

    protected virtual void PostMethodCall()
    {
        if(_lastValue != _obj.SomeProperty)
            OnPropertyChanged();
    }

    // Proxy method.
    public SomeMethod(int arg)
    {
        PreMethodCall();
        _obj.SomeMethod(arg); // Call actual method.
        PostMethodCall();
    }
}

Obviously you can build this proxy pattern into a suitable object - you just have to be aware that all calls have to go through the proxy for the event to be raised when you expect it to be.