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Editorial Team
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Asked: 2026-05-20T10:29:49+00:00

I have a background worker that is started on the main thread as shown

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I have a background worker that is started on the main thread as shown
alt text
(source: developingtrends.co.uk)

It executes on a worker thread as expected
alt text
(source: developingtrends.co.uk)

but for some reason it completes on the worker thread which causes me issues if I try and update anything on the gui thread.
alt text
(source: developingtrends.co.uk)

I have tried a simplified setup on a test app and in this app the thread does end correctly on the main thread
alt text
(source: developingtrends.co.uk)

Can you think of any reason this might have happened?

Thanks

Ross

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1 Answer

  1. Editorial Team

    This behavior was seen in older versions of Windows Forms. Even though the BGW is started on the main thread, the value of SynchronizationContext.Current was null. This bug was fixed sometime in the last year.

    From my blog:

    Windows Forms would only install the WindowsFormsSynchronizationContext when the first Win32 window handle for that thread was created. In particular, SynchronizationContext.Current was null through the main form’s constructor and Load event. It would be set, however, by the time the Show event was invoked. One common workaround was to force the creation of the Win32 window handle (by reading the Handle property), which installed the proper SynchronizationContext as a side-effect.

    Fortunately, that hack is no longer necessary. Sometime in the last year, Microsoft released an update that fixes that issue all the way back to .NET 2.0 Windows Forms projects. I’m not sure which update that was.

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