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Threading questionsI have a simple need that I can't seem to locate the answer to in the docs.
Most examples show how a worker thread can pass data back to the thread that created it. I need to do the opposite. Suppose the worker thread displays a form and the thread that created it wants to change that form's text occasionally. Can't seem to locate a simple way of doing that. Thanks in advance Hope this helps....
The example is in C++ but the concept should be the same in VB http://www.codeproject.com/managedcpp/mcppthreads01.asp **Developer** wrote: Show quoteHide quote > I have a simple need that I can't seem to locate the answer to in the docs. > Most examples show how a worker thread can pass data back to the thread that > created it. > > I need to do the opposite. > Suppose the worker thread displays a form and the thread that created it > wants to change that form's text occasionally. > > > > Can't seem to locate a simple way of doing that. > > > > Thanks in advance > > > I don't have a problem starting a thread and passing it parameters.
It's after the thread is started I need to pass data to it - that's what I don't know how to do. Thanks for replying Show quoteHide quote "Ernie Otero" <eoteroNOSPAM@dsli.com> wrote in message news:8_-dnZ0_I_O7ukDeRVn-tg@dsli.com... > Hope this helps.... > The example is in C++ but the concept should be the same in VB > > http://www.codeproject.com/managedcpp/mcppthreads01.asp > > **Developer** wrote: >> I have a simple need that I can't seem to locate the answer to in the >> docs. >> Most examples show how a worker thread can pass data back to the thread >> that created it. >> >> I need to do the opposite. >> Suppose the worker thread displays a form and the thread that created it >> wants to change that form's text occasionally. >> >> >> >> Can't seem to locate a simple way of doing that. >> >> >> >> Thanks in advance >> >> | Suppose the worker thread displays a form and the thread that created it If the worker thread is showing a Form, then you can use Control.Invoke on | wants to change that form's text occasionally. that form or one of its control to transfer information to the worker thread. Alternatively if the worker thread does not have a Form, I have used a System.Collections.Queue to send information to a thread. The thread would look for information in the queue & operate on it. Something like (VS 2003 syntax): ' untested, typed from memory. Public Class ThreadRequestQueue Private Readonly m_padlock As New Object Private Readonly m_queue As New Queue Private Readonly m_event As New AutoResetEvent(False) ' called from the Main thread Public Sub AddRequest(ByVal request As Object) SyncLock m_padlock m_queue.Enqueue(Object) End SyncLock m_event.Set() End Sub ' called from the Worker thread Public Function GetRequest() As Object ' Check to see if there are already items available SyncLock m_padlock If m_queue.Count() > 0 Then Return m_queue.DeQueue() End If End SyncLock ' Cannot block worker thread ' while waiting for the main thread to add requests m_event.WaitOne() ' There must be an item SyncLock m_padlock Return m_queue.Dequeue() End SyncLock End Function End Class The Queue is used to send the requests from the Main thread to the Worker thread. The m_padlock is used to protect the Queue.Enqueue & Queue.Dequeue methods. The worker thread "goes to sleep" if there are no items in the queue to work on, the AutoResetEvent is used to notify (wake up) the worker thread there are more items available. In VS 2005 (.NET 2.0) I would consider using a System.Collections.Generic.Queue(Of T) instead of the object based Queue above: http://msdn2.microsoft.com/en-us/library/7977ey2c.aspx -- Show quoteHide quoteHope this helps Jay [MVP - Outlook] ..NET Application Architect, Enthusiast, & Evangelist T.S. Bradley - http://www.tsbradley.net " **Developer**" <REMOVEdevelo***@a-znet.com> wrote in message news:OejJJ4QJGHA.344@TK2MSFTNGP11.phx.gbl... |I have a simple need that I can't seem to locate the answer to in the docs. | Most examples show how a worker thread can pass data back to the thread that | created it. | | I need to do the opposite. | Suppose the worker thread displays a form and the thread that created it | wants to change that form's text occasionally. | | | | Can't seem to locate a simple way of doing that. | | | | Thanks in advance | | | There may be a subtle race condition in the ThreadRequestQueue class.
It depends on the level of thread-safety offered. As written it is perfectly safe for one adder and one getter simultaneously. And that's fine if that is it's intended use case. But, for those who may want a solution that offers complete thread-safety (multiple adders and getters) I recommend the following implementation. Public Class ThreadRequestQueue Private Readonly m_padlock As New Object Private Readonly m_queue As New Queue Private Readonly m_event As New AutoResetEvent(False) ' Can be called from any thread Public Sub AddRequest(ByVal request As Object) SyncLock m_padlock m_queue.Enqueue(Object) m_event.Set() End SyncLock End Sub ' Can be called from any thread Public Function GetRequest() As Object Do While True m_event.WaitOne() SyncLock m_padlock If m_queue.Count > 0 Then Return m_queue.Dequeue() End If End SyncLock Loop End Function End Class Brian Jay B. Harlow [MVP - Outlook] wrote: Show quoteHide quote > | Suppose the worker thread displays a form and the thread that created it > | wants to change that form's text occasionally. > If the worker thread is showing a Form, then you can use Control.Invoke on > that form or one of its control to transfer information to the worker > thread. > > Alternatively if the worker thread does not have a Form, I have used a > System.Collections.Queue to send information to a thread. The thread would > look for information in the queue & operate on it. > > Something like (VS 2003 syntax): > > ' untested, typed from memory. > Public Class ThreadRequestQueue > > Private Readonly m_padlock As New Object > Private Readonly m_queue As New Queue > Private Readonly m_event As New AutoResetEvent(False) > > ' called from the Main thread > Public Sub AddRequest(ByVal request As Object) > SyncLock m_padlock > m_queue.Enqueue(Object) > End SyncLock > m_event.Set() > End Sub > > ' called from the Worker thread > Public Function GetRequest() As Object > ' Check to see if there are already items available > SyncLock m_padlock > If m_queue.Count() > 0 Then > Return m_queue.DeQueue() > End If > End SyncLock > > ' Cannot block worker thread > ' while waiting for the main thread to add requests > m_event.WaitOne() > > ' There must be an item > SyncLock m_padlock > Return m_queue.Dequeue() > End SyncLock > End Function > > End Class > > The Queue is used to send the requests from the Main thread to the Worker > thread. The m_padlock is used to protect the Queue.Enqueue & Queue.Dequeue > methods. The worker thread "goes to sleep" if there are no items in the > queue to work on, the AutoResetEvent is used to notify (wake up) the worker > thread there are more items available. > > In VS 2005 (.NET 2.0) I would consider using a > System.Collections.Generic.Queue(Of T) instead of the object based Queue > above: > http://msdn2.microsoft.com/en-us/library/7977ey2c.aspx > > -- > Hope this helps > Jay [MVP - Outlook] > .NET Application Architect, Enthusiast, & Evangelist > T.S. Bradley - http://www.tsbradley.net > > > " **Developer**" <REMOVEdevelo***@a-znet.com> wrote in message > news:OejJJ4QJGHA.344@TK2MSFTNGP11.phx.gbl... > |I have a simple need that I can't seem to locate the answer to in the docs. > | Most examples show how a worker thread can pass data back to the thread > that > | created it. > | > | I need to do the opposite. > | Suppose the worker thread displays a form and the thread that created it > | wants to change that form's text occasionally. > | > | > | > | Can't seem to locate a simple way of doing that. > | > | > | > | Thanks in advance > | > | > | Brian,
| But, for those who may want a The hazards of cutting & pasting a post I made a year or so ago...| solution that offers complete thread-safety (multiple adders and | getters) | Public Function GetRequest() As Object Your loop is superfluous! I would reduce GetRequest to:| Do While True | m_event.WaitOne() | SyncLock m_padlock | If m_queue.Count > 0 Then | Return m_queue.Dequeue() | End If | End SyncLock | Loop | End Function | Public Function GetRequest() As Object Your loop is superfluous, as the m_event.WaitOne will only allow a single | m_event.WaitOne() | SyncLock m_padlock | If m_queue.Count > 0 Then | Return m_queue.Dequeue() | End If | End SyncLock | End Function thread to pass (as its an AutoResetEvent). The event being set indicates that at least a single item is in the queue. The return statement will exit the loop. Ergo the loop itself is superfluous. Of course! this also means that the "If m_queue.Count > 0 Then" is superfluous , so we can reduce it further to: | Public Function GetRequest() As Object The "If m_queue.Count > 0 Then" is superfluous again, as the m_event.WaitOne | m_event.WaitOne() | SyncLock m_padlock | Return m_queue.Dequeue() | End SyncLock | End Function will only allow a single thread to pass (as its an AutoResetEvent)... However! (loop or no loop, if or no if) you just introduced a severe bug I was trying to avoid, all the threads block waiting for the next event, although there are multiple entries in the queue. Consider the case where one or more threads add 2 or more items to the Queue before any of the readers have a chance to remove the next item. There is only a single AutoResetEvent event, it is either set or reset. 2 items go in, its set, one item comes out, its reset. All the threads block for the third item, the third item goes in, the second item comes out, all the thread block for the four item, items 4 to 10 goes in, item 3 comes out... The way I setup GetRequest was: - If there are items in the queue, return the next item, only allow a single thread to do this! (prevent the above mentioned blockage). - If there are no items in the queue, wait for the event, be certain to allow threads to add to the queue - The event was set, must be an item, return the next item. *** red flag *** Ah! There's the rub, the first Dequeue might return the item before the second Dequeue has a chance to see it: For now I will use a variation of both of ours: Show quoteHide quote | > ' called from the Worker thread As this allows for the case where reader 1 gets the first Dequeue, while | > Public Function GetRequest() As Object | > ' Check to see if there are already items available | > SyncLock m_padlock | > If m_queue.Count() > 0 Then | > Return m_queue.DeQueue() | > End If | > End SyncLock | > | Do While True | m_event.WaitOne() | SyncLock m_padlock | If m_queue.Count > 0 Then | Return m_queue.Dequeue() | End If | End SyncLock | Loop | > End Function reader 2 gets the WaitOne there by missing the second Dequeue, that the first Dequeue just got... I wasn't considering that scenario as the original code was not intended for multiple readers... Thanks! for pointing it out. -- Show quoteHide quoteHope this helps Jay [MVP - Outlook] ..NET Application Architect, Enthusiast, & Evangelist T.S. Bradley - http://www.tsbradley.net "Brian Gideon" <briangid***@yahoo.com> wrote in message news:1138639794.059282.287610@g14g2000cwa.googlegroups.com... | There may be a subtle race condition in the ThreadRequestQueue class. | It depends on the level of thread-safety offered. As written it is | perfectly safe for one adder and one getter simultaneously. And that's | fine if that is it's intended use case. But, for those who may want a | solution that offers complete thread-safety (multiple adders and | getters) I recommend the following implementation. | | Public Class ThreadRequestQueue | | Private Readonly m_padlock As New Object | Private Readonly m_queue As New Queue | Private Readonly m_event As New AutoResetEvent(False) | | ' Can be called from any thread | Public Sub AddRequest(ByVal request As Object) | SyncLock m_padlock | m_queue.Enqueue(Object) | m_event.Set() | End SyncLock | End Sub | | ' Can be called from any thread | Public Function GetRequest() As Object | Do While True | m_event.WaitOne() | SyncLock m_padlock | If m_queue.Count > 0 Then | Return m_queue.Dequeue() | End If | End SyncLock | Loop | End Function | | End Class | | Brian | | Jay B. Harlow [MVP - Outlook] wrote: | > | Suppose the worker thread displays a form and the thread that created it | > | wants to change that form's text occasionally. | > If the worker thread is showing a Form, then you can use Control.Invoke on | > that form or one of its control to transfer information to the worker | > thread. | > | > Alternatively if the worker thread does not have a Form, I have used a | > System.Collections.Queue to send information to a thread. The thread would | > look for information in the queue & operate on it. | > | > Something like (VS 2003 syntax): | > | > ' untested, typed from memory. | > Public Class ThreadRequestQueue | > | > Private Readonly m_padlock As New Object | > Private Readonly m_queue As New Queue | > Private Readonly m_event As New AutoResetEvent(False) | > | > ' called from the Main thread | > Public Sub AddRequest(ByVal request As Object) | > SyncLock m_padlock | > m_queue.Enqueue(Object) | > End SyncLock | > m_event.Set() | > End Sub | > | > ' called from the Worker thread | > Public Function GetRequest() As Object | > ' Check to see if there are already items available | > SyncLock m_padlock | > If m_queue.Count() > 0 Then | > Return m_queue.DeQueue() | > End If | > End SyncLock | > | > ' Cannot block worker thread | > ' while waiting for the main thread to add requests | > m_event.WaitOne() | > | > ' There must be an item | > SyncLock m_padlock | > Return m_queue.Dequeue() | > End SyncLock | > End Function | > | > End Class | > | > The Queue is used to send the requests from the Main thread to the Worker | > thread. The m_padlock is used to protect the Queue.Enqueue & Queue.Dequeue | > methods. The worker thread "goes to sleep" if there are no items in the | > queue to work on, the AutoResetEvent is used to notify (wake up) the worker | > thread there are more items available. | > | > In VS 2005 (.NET 2.0) I would consider using a | > System.Collections.Generic.Queue(Of T) instead of the object based Queue | > above: | > http://msdn2.microsoft.com/en-us/library/7977ey2c.aspx | > | > -- | > Hope this helps | > Jay [MVP - Outlook] | > .NET Application Architect, Enthusiast, & Evangelist | > T.S. Bradley - http://www.tsbradley.net | > | > | > " **Developer**" <REMOVEdevelo***@a-znet.com> wrote in message | > news:OejJJ4QJGHA.344@TK2MSFTNGP11.phx.gbl... | > |I have a simple need that I can't seem to locate the answer to in the docs. | > | Most examples show how a worker thread can pass data back to the thread | > that | > | created it. | > | | > | I need to do the opposite. | > | Suppose the worker thread displays a form and the thread that created it | > | wants to change that form's text occasionally. | > | | > | | > | | > | Can't seem to locate a simple way of doing that. | > | | > | | > | | > | Thanks in advance | > | | > | | > | | Jay B. Harlow [MVP - Outlook] wrote:
> However! (loop or no loop, if or no if) you just introduced a severe bug I Yep. I totally missed that! It demonstrates just how difficult> was trying to avoid, all the threads block waiting for the next event, > although there are multiple entries in the queue. > synchronization can be. I think your modified solution works well. Brian,
Looking at it, we may be able to reduce it to: | > ' called from the Worker thread If there's an item return it, otherwise wait for the event.| > Public Function GetRequest() As Object | Do While True | > ' Check to see if there are already items available | SyncLock m_padlock | If m_queue.Count > 0 Then | Return m_queue.Dequeue() | End If | End SyncLock | m_event.WaitOne() | Loop | > End Function Which avoids some of the duplication in my duplicate... -- Show quoteHide quoteHope this helps Jay [MVP - Outlook] ..NET Application Architect, Enthusiast, & Evangelist T.S. Bradley - http://www.tsbradley.net "Brian Gideon" <briangid***@yahoo.com> wrote in message news:1138719457.667529.35890@g44g2000cwa.googlegroups.com... | | Jay B. Harlow [MVP - Outlook] wrote: | > However! (loop or no loop, if or no if) you just introduced a severe bug I | > was trying to avoid, all the threads block waiting for the next event, | > although there are multiple entries in the queue. | > | | Yep. I totally missed that! It demonstrates just how difficult | synchronization can be. I think your modified solution works well. | Jay B. Harlow [MVP - Outlook] wrote:
Show quoteHide quote > Brian, I took a good at it and I *think* it's okay. Jon updated his article> Looking at it, we may be able to reduce it to: > > | > ' called from the Worker thread > | > Public Function GetRequest() As Object > | Do While True > | > ' Check to see if there are already items available > | SyncLock m_padlock > | If m_queue.Count > 0 Then > | Return m_queue.Dequeue() > | End If > | End SyncLock > | m_event.WaitOne() > | Loop > | > End Function > > If there's an item return it, otherwise wait for the event. > > Which avoids some of the duplication in my duplicate... > that uses Monitor.Pulse and Monitor.Wait as a solution to this problem yesterday. The update was to fix a subtle bug. <http://www.yoda.arachsys.com/csharp/threads/deadlocks.shtml> Brian Brian,
Monitor might be better. Just thought of a problem with my code. If there are 4 threads waiting for the queue, 3 items in the queue, when the 4th item is added, only 1 thread will wake up, that single thread will process all the items... I'm not sure that Jon's code solves the problem, as Monitor.Pulse only releases a single thread. As I stated before, my code was really intended for one or more producer, single consumer.... -- Show quoteHide quoteHope this helps Jay [MVP - Outlook] ..NET Application Architect, Enthusiast, & Evangelist T.S. Bradley - http://www.tsbradley.net "Brian Gideon" <briangid***@yahoo.com> wrote in message news:1138891238.272466.139120@z14g2000cwz.googlegroups.com... | | Jay B. Harlow [MVP - Outlook] wrote: | > Brian, | > Looking at it, we may be able to reduce it to: | > | > | > ' called from the Worker thread | > | > Public Function GetRequest() As Object | > | Do While True | > | > ' Check to see if there are already items available | > | SyncLock m_padlock | > | If m_queue.Count > 0 Then | > | Return m_queue.Dequeue() | > | End If | > | End SyncLock | > | m_event.WaitOne() | > | Loop | > | > End Function | > | > If there's an item return it, otherwise wait for the event. | > | > Which avoids some of the duplication in my duplicate... | > | | I took a good at it and I *think* it's okay. Jon updated his article | that uses Monitor.Pulse and Monitor.Wait as a solution to this problem | yesterday. The update was to fix a subtle bug. | | <http://www.yoda.arachsys.com/csharp/threads/deadlocks.shtml> | | Brian | Jay B. Harlow [MVP - Outlook] wrote:
> Brian, Yep. I suppose that isn't necessarily a problem though. It just> Monitor might be better. > > Just thought of a problem with my code. If there are 4 threads waiting for > the queue, 3 items in the queue, when the 4th item is added, only 1 thread > will wake up, that single thread will process all the items... I'm not sure > that Jon's code solves the problem, as Monitor.Pulse only releases a single > thread. > depends on what behavior you want. You could use the ManualResetEvent instead which does wake all threads. Jon mentioned that problem in a post recently and he basically said that you could change that behavior in his solution by using PulseAll instead Pulse, but for most scenarios he saw no compelling reason to do so. I was comparing the execution flow with your revised solution and Jon's solution. Functionally, they are very similar. AutoResetEvent.Set and AutoResetEvent.WaitOne appear to have the same semantics as Monitor.Pulse and Monitor.Wait. ManualResetEvent.Set and ManualResetEvent.WaitOne appear to have the same semantics as Monitor.PulseAll and Monitor.Wait. Hello,
As a basic question, doesn't Control.BeginInvoke() work by placing messages in a queue associated with the target Thread? Bill Jay B. Harlow [MVP - Outlook] wrote: Show quoteHide quote > Brian, > Looking at it, we may be able to reduce it to: > > | > ' called from the Worker thread > | > Public Function GetRequest() As Object > | Do While True > | > ' Check to see if there are already items available > | SyncLock m_padlock > | If m_queue.Count > 0 Then > | Return m_queue.Dequeue() > | End If > | End SyncLock > | m_event.WaitOne() > | Loop > | > End Function > > If there's an item return it, otherwise wait for the event. > > Which avoids some of the duplication in my duplicate... > > -- > Hope this helps > Jay [MVP - Outlook] > .NET Application Architect, Enthusiast, & Evangelist > T.S. Bradley - http://www.tsbradley.net > > > "Brian Gideon" <briangid***@yahoo.com> wrote in message > news:1138719457.667529.35890@g44g2000cwa.googlegroups.com... > | > | Jay B. Harlow [MVP - Outlook] wrote: > | > However! (loop or no loop, if or no if) you just introduced a severe bug > I > | > was trying to avoid, all the threads block waiting for the next event, > | > although there are multiple entries in the queue. > | > > | > | Yep. I totally missed that! It demonstrates just how difficult > | synchronization can be. I think your modified solution works well. > | swartzbill2***@yahoo.com wrote:
> Hello, Yes. But, it only works if the target thread is running a message> As a basic question, doesn't Control.BeginInvoke() work by placing > messages in a queue associated with the target Thread? > Bill > loop. Worker threads do not typically have message loops. Brian Attached is a file containing an attempt at Threading.
There are two uppercase questions in the file. Besides that I would appreciate any suggestions at all about the code - anything at all that might be considered better. It's a short, and I believe instructive because it is so short, example of the main thread changing state of a worker thread, and of the worker thread changing state of the main thread. Thanks in advance for
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" **Developer**" <REMOVEdevelo***@a-znet.com> wrote in message [attached file: Threading Example.zip]news:eCk4$BdJGHA.740@TK2MSFTNGP12.phx.gbl... > Attached is a file containing an attempt at Threading. > There are two uppercase questions in the file. > Besides that I would appreciate any suggestions at all about the code - > anything at all that might be considered better. > > It's a short, and I believe instructive because it is so short, example of > the main thread changing state of a worker thread, and of the worker > thread > changing state of the main thread. > > > Thanks in advance for > > 
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