WinForms, .NET 3.5
I have a ComboBox that needs to display a growing list of items — there is a long running process fetching items, and I want to be able to display items to the user as they build up over time.
Setting the DatSource property to a naive Array/List doesn’t work since subsequent additions are not registered by the ComboBox.
Now I can set the DatSource to a BindingList — but it seems that this becomes tragically slow. When I throttle additions things go fine; but I’m adding maybe a thousand items every couple of seconds, and this rate just hangs the form.
I’m entirely willing to believe this is an inherent shortcoming of binding directly to a BindingList, but would like to be better understand what is going on. The MSDN articles on DatSource, BindingList and BindingSource discuss concepts like and BindingContexts, CurrencyManagers but don’t help me understand the specific events, redraws and what-not that are being fired that might cause this slow down.
I suspect the BindingSource registers an item-add event to the ComboBox, which then goes through a series of Item-additions, DropDown-checks and invalidation. But MSDN browsing and Googling hasn’t helped me answer this.
Can anyone point me in the right direction?
Clarification
I’m aware of the design problems, and ways to work around this problem. So I’m not really looking for a solution — but I do want to understand the deeper mechanics of what is going on.
The default control is not made for such operations, since all entries are cached on the control as far as I know.
You need to use a control which properly supports a “virtual” mode, which means that it only loads the few items to be displayed currently from the list.