{"id":833,"date":"2011-06-27T21:39:16","date_gmt":"2011-06-28T01:39:16","guid":{"rendered":"http:\/\/www.ikriv.com\/blog\/?p=833"},"modified":"2011-06-27T21:39:16","modified_gmt":"2011-06-28T01:39:16","slug":"backgroundworker-and-ui-threads","status":"publish","type":"post","link":"https:\/\/ikriv.com\/blog\/?p=833","title":{"rendered":"BackgroundWorker and UI threads"},"content":{"rendered":"

Popular belief (reinforced indirectly by MSDN) is that BackgroundWorker<\/code> class will marshal the “progress” and “completed” events back to the calling thread. This turned out to be most certainly not true. A more restricted (and closer to reality) form of this belief is that if BackgroundWorker<\/code> is spawned by a WPF UI thread, the events will be marshaled back to the UI thread. This is almost true. Read on.<\/p>\n

BackgroundWorker<\/code> uses SynchronizationContext.Post()<\/code> to marshal progress and completed events. It captures the synchronization context of the current thread when you invoke RunorkerAsync()<\/code> method. If current thread has no synchronization context, a default one will be created.<\/p>\n

For WPF UI threads the synchronization context (of type DispatcherSynchronizationContext<\/code>) will indeed post the events back to the UI thread.<\/p>\n\n\n\n\n
UI thread<\/td>\nRunWorkerAsync()<\/code><\/td>\n<\/tr>\n
Thread pool thread<\/td>\nDoWork()<\/code><\/td>\n<\/tr>\n
UI thread<\/td>\nRunWorkerCompleted<\/code> and ProgressChanged<\/code> events<\/td>\n<\/tr>\n<\/table>\n

In background thread, or in main thread of a console application the synchronization context will schedule event calls on a thread pool. So, typically you will end up with 3 or more threads:<\/p>\n\n\n\n\n
Calling thread<\/td>\nRunWorkerAsync()<\/code><\/td>\n<\/tr>\n
Thread pool thread #1<\/td>\nDoWork()<\/code><\/td>\n<\/tr>\n
Thread pool thread #2…n<\/td>\nRunWorkerCompleted<\/code> and ProgressChanged<\/code> events<\/td>\n<\/tr>\n<\/table>\n

An interesting twist occurs if you run BackgroundWorker<\/code> on a soon-to-be-come-UI-thread thread. Typically it’s a main thread of your application before you called Application.Run()<\/code>. Windows is not psychic, and it does not know what you are about to do. So, until you actually started a WPF application, your main thread is considered a regular thread, and default synchronization context will be used instead of DispatcherSynchronizationContext<\/code>. Thus, your background worker events will not<\/b> be marshalled back to the UI thread. Instead, they will be executed on a thread pool. If you touch UI in those callbacks, bad things will happen.<\/p>\n

The workarounds are:
\n– don’t use background workers before Application.Run()<\/code>
\n– use Dispatcher.BeginInvoke()<\/code> inside suspect callbacks (however be prepared that proponents of the myth may remove them as redundant)
\n– manually install DispatcherSynchronizationContext<\/code> (don’t do this it at home; I did not try it, but I think it should work).
\nCodeProject<\/a><\/p>\n","protected":false},"excerpt":{"rendered":"

Popular belief (reinforced indirectly by MSDN) is that BackgroundWorker class will marshal the “progress” and “completed” events back to the calling thread. This turned out to be most certainly not […]<\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"Layout":"","footnotes":""},"categories":[3,4],"tags":[],"class_list":["entry","author-ikriv","post-833","post","type-post","status-publish","format-standard","category-dotnet","category-hack"],"_links":{"self":[{"href":"https:\/\/ikriv.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/833","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/ikriv.com\/blog\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/ikriv.com\/blog\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/ikriv.com\/blog\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/ikriv.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=833"}],"version-history":[{"count":0,"href":"https:\/\/ikriv.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/833\/revisions"}],"wp:attachment":[{"href":"https:\/\/ikriv.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=833"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/ikriv.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=833"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/ikriv.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=833"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}