class MyClass {

  ManualResetEvent mre;

  public MyClass() {
    mre = new ManualResetEvent(false);
    Data = null;
  }

  public object Data { get; set; }

  void DataSet(object sender, EventArgs e) {
    Console.WriteLine("object Data has been set.");
    mre.Set();
  }

}

MyClass mc;

void processA() {
  mc = new MyClass();
  mc.Data = GetDataFromLongProcess();
}

private object data;
public object Data {
    get { return data;}
    set {
        if(value != data) {
            data = value;
            OnDataChanged();
        }
    }
}
protected virtual void OnDataChanged() {
    EventHandler handler = DataChanged;
    if(handler != null) handler(this, EventArgs.Empty);
}
public event EventHandler DataChanged;

MyClass mc = ...
mc.DataChanged += delegate {
    Console.WriteLine("new data! wow!");
};

public event Action OnDataChanged;

 protected object _data = null;
 public object Data
 {
     get { return _data; }
     set
     {
         _data = value;
         if(OnDataChanged != null)
            OnDataChanged();
     }
 }

mc = new MyClass();
 mc.OnDataChanged += delegate() { Console.WriteLine("It changed!"); };
 mc.Data = SomeValue();

public event SomeDelegateThatTakesIntAsParameter myEvent;
 void SetData(int data)
 {
   if(myEvent!= null)
     myEvent(data)
 }