threading: fixed shutdown bug.

src/Numerics/Threading/ThreadQueue.cs

@@ -137,6 +137,11 @@ namespace MathNet.Numerics.Threading
             _isInWorkerThread = true;
 
             while (_running)
+            {
+                // Get the job...
+                Task task = null;
+
+                lock (_queueSync)
                 {
                     // Check whether we should shut down
                     if (!_running)
@@ -144,10 +149,6 @@ namespace MathNet.Numerics.Threading
                         break;
                     }
 
-                // Get the job...
-                Task task = null;
-                lock (_queueSync)
-                {
                     if (_queue.Count > 0)
                     {
                         task = _queue.Dequeue();
@@ -166,6 +167,8 @@ namespace MathNet.Numerics.Threading
                 // ...and run it
                 task.Compute();
             }
+
+            _isInWorkerThread = false;
         }
 
         /// <summary>

src/UnitTests/ThreadingTests/ParallelForTests.cs

@@ -108,7 +108,7 @@ namespace MathNet.Numerics.UnitTests.ThreadingTests
             }
         }
 
-        [Test, ApartmentState(ApartmentState.MTA), Timeout(15)]
+        [Test, MultipleAsserts, ApartmentState(ApartmentState.MTA), Timeout(15)]
         public void DoesNotGetConfusedByMultipleStartShutdown()
         {
             ThreadQueue.Shutdown();
@@ -132,8 +132,8 @@ namespace MathNet.Numerics.UnitTests.ThreadingTests
             Assert.AreEqual(2, ThreadQueue.ThreadCount);
 
             var items = new int[50];
-
             Parallel.For(0, items.Length, i => items[i]++);
+
             Parallel.For(0, items.Length, i => items[i] += 1000);
 
             ThreadQueue.Shutdown();