Question

Problem: I have a document class which contains a list of objects. These objects raise events such as SolutionExpired, DisplayExpired etc. The document needs to respond to this.

Documents can sometimes exchange objects, but a single object should never be ‘part’ of more than one document.

My document class contains a bunch of methods which serve as event handlers. Whenever an object enters the document, I use AddHandler to set up the events, and whenever an object is removed from the document I use RemoveHandler to undo the damage. However, there are cases where it’s difficult to make sure all the steps are properly taken and I might thus end up with rogue event handlers.

Long story short; how do I remove all the handlers that are pointing to a specific method? Note, I don’t have a list of potential event sources, these could be stored anywhere.

Something like:

RemoveHandler *.SolutionExpired, AddressOf DefObj_SolutionExpired

Answer 1

You can use Delegate.RemoveAll(). (The part you’re interested in is in button2_Click)

public void Form_Load(object sender, EventArgs e) 
{ 
    button1.Click += new EventHandler(button1_Click);
    button1.Click += new EventHandler(button1_Click);
    button2.Click += new EventHandler(button2_Click);
    TestEvent += new EventHandler(Form_TestEvent);
}
event EventHandler TestEvent;
void OnTestEvent(EventArgs e)
{
    if (TestEvent != null)
        TestEvent(this, e);
}
void Form_TestEvent(object sender, EventArgs e)
{
    MessageBox.Show("TestEvent fired");
}
void button2_Click(object sender, EventArgs e)
{
    Delegate d = TestEvent as Delegate;
    TestEvent = Delegate.RemoveAll(d, d) as EventHandler;
}
void button1_Click(object sender, EventArgs e)
{
    OnTestEvent(EventArgs.Empty);
}

You should note that it doesn’t alter the contents of the delegates you pass in to it, it returns an altered delegate. Consequently, you won’t be able to alter the events on a button you’ve dropped on a form from the form, as button1.Click can only have += or -= used on it, not =. This won’t compile:

button1.Click = Delegate.RemoveAll(d, d) as EventHandler;

Also, be sure that wherever you’re implementing this you’re watching out for the potential of race conditions. You could end up with some really strange behavior if you’re removing handlers from an event that is being called by another thread!