Cleanest Way to Invoke Cross-Thread Events

Question

I find that the  NET event model is such that I ll often be raising an event on one thread and listening for it on another thread   I was wondering what the cleanest way to marshal an event from a background thread onto my UI thread is   Based on the community suggestions  I ve used this      earlier in the code mCoolObject CoolEvent               new CoolObjectEventHandler mCoolObject CoolEvent      then private void mCoolObject CoolEvent object sender  CoolObjectEventArgs args        if  InvokeRequired                CoolObjectEventHandler cb               new CoolObjectEventHandler                  mCoolObject CoolEvent           Invoke cb  new object     sender  args             return               do the dirty work of my method here

User · Answer

Use the synchronisation context if you want to send a result to the UI thread  I needed to change the thread priority so I changed from using thread pool threads  commented out code  and created a new thread of my own  I was still able to use the synchronisation context to return whether the database cancel succeeded or not        region SyncContextCancel      private SynchronizationContext  syncContextCancel            lt summary gt          Gets the synchronization context used for UI-related operations           lt  summary gt           lt value gt The synchronization context  lt  value gt      protected SynchronizationContext SyncContextCancel               get   return  syncContextCancel                endregion   SyncContextCancel      public void CancelCurrentDbCommand                  syncContextCancel   SynchronizationContext Current             ThreadPool QueueUserWorkItem CancelWork  null            Thread worker   new Thread new ThreadStart CancelWork            worker Priority   ThreadPriority Highest          worker Start               SQLiteConnection  connection      private void CancelWork    object state               bool success   false           try                       if   connection    null                                log Debug  call cancel                     connection Cancel                    log Debug  cancel complete                     connection Close                    log Debug  close complete                    success   true                  log Debug  long running query cancelled    DateTime Now ToLongTimeString                                     catch  Exception ex                        log Error ex Message  ex                      SyncContextCancel Send CancelCompleted  new object     success                public void CancelCompleted object state                object   args    object   state          bool success    bool args 0            if  success                        log Debug  long running query cancelled    DateTime Now ToLongTimeString

User · Answer

I think the cleanest way is definitely to go the AOP route  Make a few aspects  add the necessary attributes  and you never have to check thread affinity again

User · Answer

A couple of observations    Don t create simple delegates explicitly in code like that unless you re pre-2 0 so you could use        BeginInvoke new EventHandler lt CoolObjectEventArgs gt  mCoolObject CoolEvent                   sender                  args      Also you don t need to create and populate the object array because the args parameter is a  params  type so you can just pass in the list  I would probably favor Invoke over BeginInvoke as the latter will result in the code being called asynchronously which may or may not be what you re after but would make handling subsequent exceptions difficult to propagate without a call to EndInvoke  What would happen is that your app will end up getting a TargetInvocationException instead

User · Answer

You can try to develop some sort of a generic component that accepts a SynchronizationContext as input and uses it to invoke the events

User · Answer

I ve always wondered how costly it is to always assume that invoke is required     private void OnCoolEvent CoolObjectEventArgs e      BeginInvoke  o e    gt    do work here   this  e

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I shun redundant delegate declarations   private void mCoolObject CoolEvent object sender  CoolObjectEventArgs args        if  InvokeRequired                Invoke new Action lt object  CoolObjectEventArgs gt  mCoolObject CoolEvent   sender  args           return               do the dirty work of my method here     For non-events  you can use the System Windows Forms MethodInvoker delegate or System Action   EDIT  Additionally  every event has a corresponding EventHandler delegate so there s no need at all to redeclare one

User · Answer

I have some code for this online  It s much nicer than the other suggestions  definitely check it out   Sample usage   private void mCoolObject CoolEvent object sender  CoolObjectEventArgs args           You could use       gt   in place of  delegate   it s a style choice      this Invoke delegate                  Do the dirty work of my method here

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I made the following  universal  cross thread call class for my own purpose  but I think it s worth to share it   using System  using System Collections Generic  using System Text  using System Windows Forms   namespace CrossThreadCalls     public static class clsCrossThreadCalls         private delegate void SetAnyPropertyCallBack Control c  string Property  object Value       public static void SetAnyProperty Control c  string Property  object Value              if  c GetType   GetProperty Property     null                    The given property exists         if  c InvokeRequired                      SetAnyPropertyCallBack d   new SetAnyPropertyCallBack SetAnyProperty             c BeginInvoke d  c  Property  Value                     else                     c GetType   GetProperty Property  SetValue c  Value  null                                private delegate void SetTextPropertyCallBack Control c  string Value       public static void SetTextProperty Control c  string Value              if  c InvokeRequired                  SetTextPropertyCallBack d   new SetTextPropertyCallBack SetTextProperty           c BeginInvoke d  c  Value                 else                 c Text   Value                      And you can simply use SetAnyProperty   from another thread   CrossThreadCalls clsCrossThreadCalls SetAnyProperty lb Speed   Text   KvaserCanReader GetSpeed ToString       In this example the above KvaserCanReader class runs its own thread and makes a call to set the text property of the lb Speed label on the main form

User · Answer

As an interesting side note  WPF s binding handles marshaling automatically so you can bind the UI to object properties that are modified on background threads without having to do anything special   This has proven to be a great timesaver for me   In XAML    lt TextBox Text   Binding Path Name    gt

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