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PONeed help debugging why property is not set/null
 

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  1. PONeed help debugging why property is not set/null
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    <p>I have a class <code>WorkQueue</code>, just take note about the <code>DoNext()</code> for now, the rest are mainly helpers. Basically, <code>WorkQueue</code> is just a Queue of <code>WorkItems</code>. <code>DoNext()</code> is responsible for "Starting a Pending WorkItem with a Free Background Worker". Also note that it will set the <code>WorkItem.Worker</code> property. </p> <pre><code>public class WorkQueue&lt;Tin, Tout&gt; : INotifyCollectionChanged, IEnumerable&lt;WorkItem&lt;Tin, Tout&gt;&gt; { public bool DoNext() { // check if any work left in queue WorkItem&lt;Tin, Tout&gt; item = GetWork(); if (item != null) { // check if any free workers BackgroundWorker worker = GetWorker(); Debug.WriteLine( "[WorkQueue.DoNext] Setting Worker to WorkItem: " + worker); item.Worker = worker; if (worker != null) { worker.RunWorkerAsync(item); return true; } } return false; } public void AddWork(WorkItem&lt;Tin, Tout&gt; item) { _queue.Add(item); RaiseCollectionChanged( new NotifyCollectionChangedEventArgs( NotifyCollectionChangedAction.Add, item)); } public WorkItem&lt;Tin, Tout&gt; GetWork() { return (from i in _queue where i.Status == WorkStatus.Pending select i).FirstOrDefault();; } public BackgroundWorker GetWorker() { return (from worker in _workers where worker.IsBusy == false select worker).FirstOrDefault(); } } </code></pre> <p>The problem I am having is when I do something like below, </p> <pre><code>foreach (string filename in fileNames) { UploadQueue.AddWork(new WorkItem&lt;string, UploadedImage&gt;(filename)); UploadQueue.DoNext(); } </code></pre> <p>Where <code>UploadQueue</code> is a <code>WorkQueue&lt;string, UploadedImage&gt;</code>. On the 1st (first only) <code>DoNext()</code>, the <code>WorkItem.Worker</code> is null. I know that because my Cancel button bound to <code>WorkItem.CancelCommand</code> is disabled. When debugging, I discovered the reason was because worker is null. </p> <pre><code>_cancelCommand = new RelayCommand(... () =&gt; { // Returns true if WorkItem is being processed with a worker that supports // cancellation or if the WorkItem is still Pending // False if otherwise, eg. already completed, cancelled etc if (Status == WorkStatus.Processing) { if (_worker != null &amp;&amp; _worker.WorkerSupportsCancellation) return true; } else if (Status == WorkStatus.Pending) { return true; } return false; }); </code></pre> <p>The solution is to move the <code>DoNext()</code> out of the loop, </p> <pre><code>foreach (string filename in fileNames) UploadQueue.AddWork(new WorkItem&lt;string, UploadedImage&gt;(filename)); UploadQueue.DoNext(); </code></pre> <p>but whats the problem with it inside, why is worker set to null? If its null, from the if clause, the <code>BackgroundWorker</code> should not start? </p> <pre><code>if (worker != null) worker.RunWorkerAsync(item); </code></pre> <p><a href="http://screenr.com/akF" rel="nofollow">Video Demonstrating the Problem</a></p>
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    1. USJon Seigel
    1. USJiew Meng
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      1. PONeed help debugging why property is not set/null
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      1. VTUpMod
    1. COthat's not how I would use background workers...
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      1. PONeed help debugging why property is not set/null
      1. USMitch Wheat
    2. COWhen is the check for the button status? And where does the _cancelCommand fit in?
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      1. PONeed help debugging why property is not set/null
      1. USDoggett
    3. COJust a suggestion, have you thought about using Tasks instead?
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      1. PONeed help debugging why property is not set/null
      1. USPaw Baltzersen
 

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