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Editorial Team
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Asked: 2026-05-31T04:57:34+00:00

I have a form with 2 tabs. The first tab is boring, the second

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I have a form with 2 tabs. The first tab is boring, the second tab (unseen as default to the user) contains many comboboxes.

Using the FormLoad() event I populate a combobox on the second tab (with around 11,000 items/strings) on a background thread. The sql command to do this is also asynchronous.

Now, in theory this should mean that when the user finally gets round to clicking the second tab (whether that be in 10 seconds or 10 hours) they should be instantly presented with a nicely populated control – but there is ALWAYS a 4 second delay. I just don’t get it! If all the heavy lifting is done via the background thread (the whole reason for using them in the first place!), why the heck is my application still slow and unresponsive when the user clicks that darn second tab??!!?!

*Bearing in mind you have to give the application a chance to fill the combo in the first place, plus I know when it’s finished populating as the backgroundWorker1_RunWorkerComplete() method fires and sends a debug message to tell me all work has finished.*

Any help will be much appreciated….

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1 Answer

  1. Editorial Team

    11,000 is a lot! The work has been done to populate the comboBox items on the background thread, but the form still has to show all those items. This means the UI thread has to render a proportion of them (or all of them) to the UI (into memory) ready for scrolling; this is what is taking time.

    I would suggest overriding the ComboBox control and handling the scroll event yourself. This way you can load a subset of the entire list sequentially when you need them (if that is possible in your case). This will prevent the four second delay you speak of.

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