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    <p>You are correct it is a deadlock issue.</p> <p><code>Parallel.ForEach</code> blocks till the all of the items in <code>list</code> have been processed, and you made that run on the UI thread, so now the UI thread is blocked.</p> <p>You then call <code>Form1.setText(test + "done!");</code> and <code>setText</code> (the <code>s</code> really should be capital btw) calls <code>this.Invoke</code> which will block till the invoke call has finished being processed. However because we blocked the UI thread on the <code>Parallel.ForEach</code> it will never be processed <strong>and you just deadlocked!</strong></p> <p>The way to solve this is put the <code>Parallel.ForEach</code> call on it's own thread too so you do not block the UI thread.</p> <pre><code>public void run() { Task.Factory.StartNew(() =&gt; Parallel.ForEach(list, test =&gt; { DoSomething(test); Form1.setText(test + "done!"); }), TaskCreationOptions.LongRunning); } </code></pre> <p>You also could likely improve performance by changing <code>this.Invoke</code> to <code>this.BeginInvoke</code><sup>1</sup> so the text updating is non blocking too. However, If you only make the <code>BeginInvoke</code> change to your program and leave <code>Parallel.ForEach</code> on the UI thread you may not deadlock anymore but your UI will still freeze and say <code>Not Responding</code> until <code>Parallel.ForEach</code> completes.</p> <hr> <p><sup>1. Everywhere else if you call <code>BeginXxxxx</code> you <strong><em>MUST</em></strong> call <code>EndXxxxx</code>, however <code>BeginInvoke</code> on a class derived from <code>Control</code> <a href="http://msdn.microsoft.com/en-us/library/0b1bf3y3%28v=vs.110%29.aspx#remarksToggle" rel="nofollow">is except from that rule</a> and you are not required to call <code>EndInvoke</code>.</sup></p>
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    1. USScott Chamberlain
    1. USScott Chamberlain
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    1. COThank you so much!
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      1. USKimchiMan
    2. COOh actually I got error msg: Cannot convert lambda expression to type 'System.Threading.Tasks.ParallelOptions' because it is not a delegate type
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    3. COon variable 'test'. What should I do?
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