Return a value from an Event — is there a Good Pra

Question

I'm doing a small multi-threaded app that uses asynchronous TCP sockets, but I will get to the point: I'm using a custom event to read a value from a form and the delegate used by the event returns a string when finished.

My question here is: is that correct? is it OK to return values from the events? or is there a better way to do this? (like using a simple delegate to the form to read the values)

Answer 1

The closest example I can think of is the FormClosing event in WinForms. It lets the form cancel the event by setting the eventArgs.Cancel property to true. For you to do something similar, you would define your own event args class with the return value as a property on that class. Then pass an event args object whenever you raise the event. Whoever raised the event can inspect the event args object for the return value. Others who are receiving the event can also inspect or change the event args object.

Update: I just ran across the AppDomain.AssemblyResolve event, and it appears to be an event that returns a value. It seems you just need to declare a delegate type that returns a value, and then define your event with that delegate type. I haven't tried creating my own event like this, though. One advantage to using a property on the event argument is that all subscribers to the event can see what previous subscribers have returned.

Answer 2

It's often awkward to return values from events. In practice, I've found it much easier to include a writable property on a set of custom EventArgs that is passed to the event, and then checked after the event fires -- similar to Cancel property of the WinForms FormClosing event.

Answer 3

I looped the properties of the EventArgs like this and pulled out its X and Y values.

private void navBarControl1_Click(object sender, EventArgs e) { int _x = 0; int _y = 0;

        Type t = e.GetType();
        IList<PropertyInfo> props = new List<PropertyInfo>(t.GetProperties());

        foreach (PropertyInfo prop in props)
        {
            if (prop.Name == "X")
            {
                object propValue = prop.GetValue(e, null);
                _x = Convert.ToInt32(propValue);
            }
            if (prop.Name == "Y")
            {
                object propValue = prop.GetValue(e, null);
                _y = Convert.ToInt32(propValue);
            }
        }

Answer 4

void method()
{
    list<string> strings = new list<string>();

    dostuff += stuff;
    dostuff += stuff;

    dostuff(this, new EventHandlerArgs(){ Parameter = strings })

    foreach(string currString in strings)
    {
          //....
    }
}

void stuff(object sender, EventHandlerArgs e)
{
    list<string> strings = e.Parameter as list<string>;

    if (strings != null)
    {
        strings.Add(MyString)
    }
}

Answer 5

If event returns a value and there are multiple handlers registered the event returns the result value of the last called handler. Look for an example at http://blogs.msdn.com/b/deviations/archive/2008/11/27/event-handlers-returning-values.aspx

Answer 6

I don't think it's a good idea... events are basically multicast delegates, so there can be multiple handlers. Which return value will you take in that case ?