Best way to suspend control events in Windows Forms?

Question

This seems like a very simple and a very common problem. The simplest example I can think of is this:

The form has five checkboxes with a "check all/check none" checkbox above them. When a user selects checking all checkboxes, I toggle the states of the "children" - obviously I don't want to fire the check events of all the children until I am done setting all of the checkboxes.

I can't find a form-wide suspend control event. If I'm simply missing it then great simple answer. Barring a simple solution that I am just missing, what is the best way (best practice? accepted solution?) to suspend form control events?

Answer 1

I've come across this before and usually seen people do this:

/*SNIP*/

private bool isMassUpdate;

public void Check1_Check(object sender, EventArgs e)
{
   if(!isMassUpdate)
   {
       do some stuff
   }
}

/*SNIP*/

You can also detach and reattach the event handlers, however, I'm told this can be a source of memory leaks.

Information on memory leaks and event handlers: They're not directly linked to attaching and detaching, but we've seen in one of our applications that bad referencing of event handlers down inheritance trees can cause it.

• .NET Memory Leak Case Study: The Event Handlers That Made The Memory Baloon

• On event handlers and memory leaks

Answer 2

What I do in these cases instead of having a boolean value that suspends events, I use a counter. When the count is > 0, then suspend events, when the count = 0, then resume events. This helps with the problem if I have multiple things that could request a suspension of events.

The other useful thing is if I need to suspend events in a block, I create a little helper class that is IDisposable that I can use in a "using" block (in C#) so I don't forget to decrement the counter once I'm out of scope.

Answer 3

From your other question, I'm going to guess you're using VB .NET. So, RemoveHandler is your best bet. Normally in VB people set up event handlers using the Handles clause. But you can also do it this way:

AddHandler chk1.CheckedChanged, AddressOf DoSomething

where DoSomething might look like this:

Private Sub DoSomething(ByVal sender As Object, ByVal e As EventArgs)
    ' whatever
End Sub

AddHandler wires up the event, so it'll fire. To get it not to fire, use RemoveHandler:

RemoveHandler chk1.CheckedChanged, AddressOf DoSomething

Before updating the Checked property of your child checkboxes, call RemoveHandler on each of them; then when you're done, call AddHandler to put the event handlers back. If all your checkboxes use the same handler, you can put them in a collection and loop through the collection to add or remove the handlers.

Answer 4

You could also consider handling 'click' events for the buttons, rather than check-changed. That might be nearer to your intent.

Answer 5

In my Visual Basic 6.0 application I had to handle users double-clicking everything, so on all my event handlers I have one line that checks a global variable

Private bSuspendEvents as Boolean

Private Sub Button1_Click()

    On Error Goto ErrorHandler

    If bSuspendEvents then Exit Sub

    bSuspendEvents = True

    'Do stuff

    NormalExit:
        bSuspendEvents = False
        Exit Sub

    ErrorHandler:
        'Handle Error
        Resume NormalExit

End Sub

Answer 6

See Stack Overflow question How do I ignore simple events firing when changing control states in C# Windows Forms?

For the more general case, if you have complex relations between the controls, you could check which control fired the event by checking its own Focused property... each object is only dependent on itself.