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p u b l i c ove r r i d e v o i d Post ( Se ndOrPostCa l l b a c k d , o b j e c t state )
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m_cont rol Be g i n l nvok e ( d , n ew object [ ]
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p u b l i c override void Send ( SendOrPostC a l l b a c k d , obj ect state )
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The context uses Beg i n I n vo k e and I n vo k e to implement asynchronous post and synchronous send, respectively, both of which we reviewed ear lier In reality, there's a bit more going on in the implementation-things such as validating that the target GUI thread is still running (since it may have since exited) and so on-but this is immaterial to the discussion WPF also has its own D i s p a t c h e rSyn c h ro n i z at i o n Cont ext in the S y s t e m W i n dows T h r e a d i n g namespace Its implementation looks nearly identical to the Windows Forms one, except that it uses a D i s p a t c h e r object instead of a Cont rol for invoking callbacks
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p u b l i c ove r r i d e void Post ( SendOrPostC a l l b a c k d, object stat e )
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In addition to implementing P o st and S e n d , WPF also overrides the
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alertable wait logic in key areas of WPF' s internal logic where reentrancy would cause serious problems Both ASPNET and the Windows Communication Foundation (WCF) have their own internal Sy n c h ro n i z a t i o n Co n t ext implementations that aren't public They both have to do with internals of the respective sys tems For example, ASPNET sometimes invokes callbacks under a lock and also tracks the number of outstanding callbacks And WCF has its own Com P l u s S y n c h ro n i z a t i o n C o n t e xt that marshals work across COM apartments
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Asynchronous Operations If you need to use the Syn c h ro n i z a t i o n C o n t ext facilities for posting and sending, you' ll need to deal with some boilerplate to capture the current context, check whether it's n u l l or not (since the runtime doesn't automat ically place one there), flow it around properly, and so on Instead of doing that, you can use the AsyncOpe r a t i o nMa n a g e r class, which automates all of this for you It resides in the System Compo n e n tMod e l namespace The amount of boilerplate this saves you is miniscule (a half dozen lines of code can be expressed in a couple), but given that the point of Syn c h ro n i z a t i onContext is to allow a simple and common way of marshaling work across the NET Framework-to enable things like the event-based APM-it makes usage convenient enough to reach the tipping point The Asy n c O p e r a t i o nMa n a g e r just offers two static members
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p u b l i c s t a t i c c l a s s AsyncOperat ionMa n a g e r { p u b l i c s t a t i c Syn c h ro n i z a t ionContext { get ; set ; } p u b l i c s t a t i c AsyncOperation C reateOperat ion ( obj ect u se r S u p p l iedState );
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The Syn c h ro n i z a t ionContext property offers an accessor that lazily ini tializes a default context if none exists at the time of the call Its setter just passes the value you supply to the Sy n c h ro n i z a t ionCont ext SetSyn c h ro
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n i z a t ionCont ext method (The fact that Syn c h ro n i z at ionCont ext is static definitely makes these classes a whole lot less useful But you'll typically not need to change it) And the C re a t eOpe r a t i o n method is just a factory
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for Asy n cOpe r a t i o n objects, passing the state you've supplied so that it's available Each such object represents an operation that can be used to issue posts This is the only way to construct one
p u b l i c sea led c l a s s AsyncOperation { -AsyncOperation ( ) ; I I Met hods public void Operat ionCompleted ( ) ; p u b l i c void Post ( SendOrPostCa l l b a c k d , object a rg ) ; p u b l i c void PostOperat ionCompleted C a l l ba c k (