{"id":109434,"date":"2024-02-22T07:00:11","date_gmt":"2024-02-22T15:00:11","guid":{"rendered":"https:\/\/devblogs.microsoft.com\/oldnewthing\/?p=109434"},"modified":"2024-02-22T09:04:22","modified_gmt":"2024-02-22T17:04:22","slug":"gotcha-dont-forget-to-shut-down-your-dispatcher-queues","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/oldnewthing\/20240222-11\/?p=109434","title":{"rendered":"Gotcha: Don’t forget to shut down your dispatcher queues"},"content":{"rendered":"

If you need a Dispatcher\u00adQueue<\/code>, you can create a Dispatcher\u00adQueue\u00adController<\/code> and then read the Dispatcher\u00adQueue<\/code> property to obtain the associated Dispatcher\u00adQueue<\/code>.<\/p>\n

But don’t throw away that Dispatcher\u00adQueue\u00adController<\/code>!<\/p>\n

\/\/ C#\r\nDispatcherQueueController controller =\r\n    DispatcherQueueController.CreateOnDedicatedThread();\r\nDispatcherQueue queue = controller.DispatcherQueue;\r\n\r\n\/\/ C++\/WinRT\r\nDispatcherQueueController controller =\r\n    DispatcherQueueController::CreateOnDedicatedThread();\r\nDispatcherQueue queue = controller.DispatcherQueue();\r\n\r\n\/\/ C++\/CX\r\nDispatcherQueueController^ controller =\r\n    DispatcherQueueController::CreateOnDedicatedThread();\r\nDispatcherQueue^ queue = controller->DispatcherQueue;\r\n\r\n\/\/ C++\/WRL\r\nComPtr<IDispatcherQueueControllerStatics> statics;\r\nTHROW_IF_FAILED(\r\n    GetActivationFactory(HStringReference(\r\n    RuntimeClass_Windows_System_DispatcherQueueController).Get(),\r\n    &statics));\r\n\r\nComPtr<IDispatcherQueueController> controller;\r\nTHROW_IF_FAILED(\r\n    statics->CreateOnDedicatedThread(&controller));\r\n\r\nComPtr<IDispatcherQueue> queue;\r\nTHROW_IF_FAILED(\r\n    controller->get_DispatcherQueue(&queue));\r\n<\/pre>\n
The Dispatcher\u00adQueue<\/code> runs until it is shut down by a call to Dispatcher\u00adQueue\u00adController.Shutdown\u00adQueue\u00adAsync<\/code>. If you throw away the Dispatcher\u00adQueue\u00adController<\/code>, then you have no way of shutting down the Dispatcher\u00adQueue<\/code>, and it will run forever, long after the destruction of any objects which posted work to the Dispatcher\u00adQueue<\/code>. It just sits there patiently waiting for new work which will never arrive, until it finally gets the rug pulled out from under it at process termination.<\/p>\n
If you create a dispatcher queue each time some action occurs, make sure to shut down that dispatcher queue when you are finished. Otherwise, you’re going to leak dispatcher queues and eventually run out of window manager resources.<\/p>\n
\/\/ C#\r\n\r\nclass MyThing\r\n{\r\n    public MyThing()\r\n    {\r\n        \/\/ Oops\r\n        m_queue =\r\n            DispatcherQueueController.\r\n            CreateOnDedicatedThread().\r\n            DispatcherQueue;\r\n    }\r\n\r\n    DispatcherQueue m_queue;\r\n}\r\n\r\n\/\/ C++\/WinRT\r\n\r\nclass MyThing\r\n{\r\npublic:\r\n    MyThing()\r\n    {\r\n        \/\/ Oops\r\n        m_queue =\r\n            DispatcherQueueController::\r\n            CreateOnDedicatedThread().\r\n            DispatcherQueue();\r\n    }\r\n\r\nprivate:\r\n    DispatcherQueue m_queue{ nullptr };\r\n};\r\n\r\n\/\/ C++\/CX\r\n\r\nclass MyThing\r\n{\r\npublic:\r\n    MyThing()\r\n    {\r\n        \/\/ Oops\r\n        m_queue =\r\n            DispatcherQueueController::\r\n            CreateOnDedicatedThread()->\r\n            DispatcherQueue;\r\n    }\r\n\r\nprivate:\r\n    DispatcherQueue^ m_queue;\r\n};\r\n\r\n\/\/ C++\/WRL\r\n\r\nclass MyThing\r\n{\r\npublic:\r\n    MyThing()\r\n    {\r\n        \/\/ Oops\r\n        ComPtr<IDispatcherQueueControllerStatics> statics;\r\n        THROW_IF_FAILED(\r\n            GetActivationFactory(HStringReference(\r\n            RuntimeClass_Windows_System_DispatcherQueueController).Get(),\r\n            &statics));\r\n\r\n        ComPtr<IDispatcherQueueController> controller;\r\n        THROW_IF_FAILED(\r\n            statics->CreateOnDedicatedThread(&controller));\r\n\r\n        THROW_IF_FAILED(\r\n            controller->get_DispatcherQueue(&m_queue));\r\n    }\r\n\r\nprivate:\r\n    ComPtr<IDispatcherQueue> m_queue;\r\n};\r\n<\/pre>\n
In the above examples, we create a dispatcher queue and throw away the controller, leaving us no way to shut down the queue when the MyThing<\/code> destructs.<\/p>\n
You have to save the Dispatcher\u00adQueue\u00adController<\/code> so that you can call Shutdown\u00adQueue\u00adAsync<\/code>\u00b9 to clean up the dispatcher queue.<\/p>\n
One way to do this is to have an explicit Shut\u00adDown\u00adAsync<\/code> method that must be called prior to destruction.<\/p>\n
\/\/ C#\r\n\r\nclass MyThing\r\n{\r\n    public MyThing()\r\n    {\r\n        m_controller =\r\n            DispatcherQueueController.\r\n            CreateOnDedicatedThread();\r\n        m_queue = m_controller. DispatcherQueue;\r\n    }\r\n\r\n    public Task ShutDownAsync()\r\n    {\r\n        return m_controller.ShutdownQueueAsync();\r\n    }\r\n\r\n    DispatcherQueueController m_controller;\r\n    DispatcherQueue m_queue;\r\n}\r\n\r\n\/\/ C++\/WinRT\r\n\r\nclass MyThing\r\n{\r\npublic:\r\n    MyThing()\r\n    {\r\n        m_controller =\r\n            DispatcherQueueController::\r\n            CreateOnDedicatedThread();\r\n        m_queue = m_controller.DispatcherQueue();\r\n    }\r\n\r\n    IAsyncAction ShutDownAsync()\r\n    {\r\n        return m_controller.ShutdownQueueAsync();\r\n    }\r\n\r\nprivate:\r\n    DispatcherQueueController m_controller{ nullptr };\r\n    DispatcherQueue m_queue{ nullptr };\r\n};\r\n\r\n\/\/ C++\/CX\r\n\r\nclass MyThing\r\n{\r\npublic:\r\n    MyThing()\r\n    {\r\n        m_controller =\r\n            DispatcherQueueController::\r\n            CreateOnDedicatedThread();\r\n        m_queue = m_controller->DispatcherQueue;\r\n    }\r\n\r\n    IAsyncAction ShutDownAsync()\r\n    {\r\n        return m_controller->ShutdownQueueAsync();\r\n    }\r\n\r\nprivate:\r\n    DispatcherQueueController^ m_controller;\r\n    DispatcherQueue^ m_queue;\r\n};\r\n\r\n\/\/ C++\/WRL\r\n\r\nclass MyThing\r\n{\r\npublic:\r\n    MyThing()\r\n    {\r\n        ComPtr<IDispatcherQueueControllerStatics> statics;\r\n        THROW_IF_FAILED(\r\n            GetActivationFactory(HStringReference(\r\n            RuntimeClass_Windows_System_DispatcherQueueController).Get(),\r\n            &statics));\r\n\r\n        THROW_IF_FAILED(\r\n            statics->CreateOnDedicatedThread(&m_controller));\r\n\r\n        THROW_IF_FAILED(\r\n            controller->get_DispatcherQueue(&m_queue));\r\n    }\r\n\r\n    HRESULT ShutDownAsync(IAsyncAction** operation)\r\n    {\r\n        return m_controller->ShutdownQueueAsync(operation);\r\n    }\r\n\r\nprivate:\r\n    ComPtr<IDispatcherQueueController> m_controller;\r\n    ComPtr<IDispatcherQueue> m_queue;\r\n};\r\n<\/pre>\n
This sounds great, but there’s a catch. We’ll look into it next time.<\/p>\n
\u00b9 The method name is spelled incorrectly. It should be Shut\u00adDown\u00adQueue\u00adAsync<\/code>, because “shut down” (two words) is a verb, whereas “shutdown” (one word) is a noun.<\/p>\n","protected":false},"excerpt":{"rendered":"
Keep that dispatcher queue controller around, or you’ll never be able to clean up.<\/p>\n","protected":false},"author":1069,"featured_media":111744,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[1],"tags":[25],"class_list":["post-109434","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-oldnewthing","tag-code"],"acf":[],"blog_post_summary":"
Keep that dispatcher queue controller around, or you’ll never be able to clean up.<\/p>\n","_links":{"self":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/109434","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/users\/1069"}],"replies":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/comments?post=109434"}],"version-history":[{"count":0,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/109434\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/media\/111744"}],"wp:attachment":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/media?parent=109434"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/categories?post=109434"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/tags?post=109434"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}