My C# application has a comboBox with a SelectedIndexChanged event. Usually, I want this event to fire, but but sometimes I need the event to not fire. My comboBox is an MRU file list. If a file in the list is found to not exist, the item is removed from the comboBox, and the comboBox SelectedIndex is set to zero. However, setting the comboBox SelectedIndex to zero causes the SelectedIndexChanged event to fire, which in this case is problematic because it causes some UIF code to be run in the event handler. Is there a graceful way to disable/enable events for C# form controls? Thanks.
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5 Answers
Start the eventhandler method with
ComboBox combo = sender as ComboBox;
if (combo.SelectedIndex == 0)
{
return;
}
If you're issue is with a different eventhandler you could remove the eventhandler's event registration first.
combo.SelectedIndexChanged -= EventHandler<SelectedIndexChangedEventArgs> SomeEventHandler;
combo.SelectedIndex = 0;
combo.SelectedIndexChanged += EventHandler<SelectedIndexChangedEventArgs> SomeEventHandler;
1 Comment
John Grabanski
Is this still the broadly accepted way to prevent the event? [by removing and then re-adding after the 'triggering' action] Just seems like there should be a better way.
I have encountered this many times over the years. My solution is to have a class level variable called _noise and if I know I am about to change the index of combo or any other similiar control that fires when the selected index changes, I do the following in code.
private bool _noise;
Here is the code for the control event handler
private void cbTest_SelectedIndexChange(object sender, EventArgs e)
{
if (_noise) return;
// process the events code
...
}
Then when I know I am going to change the index, I do the following:
_noise = true; // cause the handler to ignore the noise...
cbTest.Index = value;
_noise = false; // let the event process again
Comments
I'm surprised there isn't a better way of doing this, but this is the way I do it. I actually use the Tag field of most controls so I don't have to subclass the control. And I use true/null as the values, since null is the default.
Of course, if you are actually using Tag, you'll need to do it differently...
In handler:
private void control_Event(object sender, EventArgs e)
{
if (control.Tag != null ) return;
// process the events code
...
}
In main code
try
{
control.Tag = true;
// set the control property
control.Value = xxx;
or
control.Index = xxx;
or
control.Checked = xxx;
...
}
finally
{
control.Tag = null;
}
1 Comment
Joe DF
I like this solution more than a class scoped variable as this value is directly tied to the control in question making it a more robust solution considering if you had to do similar things with different controls (or even the same one) in multiple event handlers.
There's a better way!
combo_box = QComboBox() # your combobox
combo_box.blockSignals(True)
combo_box.setCurrentIndex(self, ix)
combo_box.blockSignals(False)
1 Comment
ElRojo
This does seem good. Can you add a little explanation of what this does better or what it is doing? For people who don't understand "blockSignals."
One (fairly ugly) way would be to set a flag in the code that deletes the entry and then check that in the SelectedIndexChanged handler:
if (!deletedEntry)
{
// Do stuff
}
deletedEntry = false;
A better way might be to remove your SelectedIndexChanged event handler at the start of the delete method and reinstate it at the end. This way you code won't know the index has changed.
