How to raise an event on Property Change?

Question

When I use DataSet, there is possiblity to raise events on RowChanging, RowChanged, ColumnChanging, ColumnChanged, etc...

How to do the same with an entity from Enti

Answer 1

You can do the following to raise an event on property changed in Entity Framework: Suppose you have the Pubs database - it has a table employee with the following table structure:

Now we want to track any changes of the property hire_date. You can do it the following way (this example can be used easily in LinqPad - you just need to define a EF datasource and then you can run the example):

void Main()
{
    var test=new employee();
    test.PropertyChanged += HandleSomethingHappening;
    test.hire_date = DateTime.Now;
}

public void HandleSomethingHappening(object sender, EventArgs e)
{
    var propName=((System.ComponentModel.PropertyChangedEventArgs)e).PropertyName;
    var empObj=(employee)sender;
    if (propName=="hire_date")
    {
        Console.WriteLine(propName+" changed to: " + empObj.hire_date.Date.ToString("d"));
    }
}

If you run it, it will show

Hire date changed: 17.09.2015

on the console, because in the main method we changed the property via:

test.hire_date = DateTime.Now;

---

N.B.

• To remove the event registration, you can use:
  test.PropertyChanged -= HandleSomethingHappening;
• As shown here, Lambdas are also allowed; e.g. you could use:
  test2.PropertyChanged +=
(c, a) => Console.WriteLine(((System.ComponentModel.PropertyChangedEventArgs)a).PropertyName + " property has changed in employee entity");
  which would handle the same event as the example above. But in this case, de-registration is not possible since you cannot refer to the anonymous function implicitly created
• You can use the PropertyChanging event as well, which will trigger before the change is happening
• This is not limited to the Entity Framework, you can use it in every class as this SO article shows.

---

Advanced hints:

If you want to understand better what is going on behind the scenes, I am providing a simplified code of the employee class (just the property and event needed to run the example above):

public class employee //: EntityObject
{


    #region Primitive Properties

    public global::System.DateTime hire_date
    {
        get
        {
            return _hire_date;
        }
        set
        {
            //Onhire_dateChanging(_hire_date);
            _hire_date=value;
            Onhire_dateChanged();
        }
    }
    private DateTime _hire_date;


    void Onhire_dateChanged()
    {
            var handler = this.PropertyChanged; // copy before access (to aviod race cond.)
            if (handler != null)
            {                   
                var args=new PropertyChangedEventArgs() { PropertyName="hire_date" };
                handler(this, (System.EventArgs)args);
            }
    }

    public event EventHandler PropertyChanged;

    #endregion

}


public class PropertyChangedEventArgs: System.EventArgs
{
    public string PropertyName  { get; set; }
}

You can see how the event is wired up - it gets triggered in the property's set method.