Friday, February 26, 2010

Testing waithandles in MSTest

For a client I needed to write a parallel execution mechanism that executes multiple tasks at the same time on different threads but further execution had to wait before all threads signals complete. In this situation, synchronization becomes very important,  so I used the WaitHandle classes to implement this functionality. As no code is complete without good unit tests, I added a test to validate the behavior of the waithandles:

   1:  [TestMethod]
   2:  public void Waithandle_Should_Wait_Until_All_Tasks_Complete()
   3:  {    
   4:     Action task1 = ()=> DoSomething();
   5:     Action task2 = ()=> DoSomething();
   6:     Action task3 = ()=> DoSomething();
   7:   
   8:     IAsyncResult result1= task1.BeginInvoke(null, null, null);   
   9:     IAsyncResult result2= task2.BeginInvoke(null, null, null);   
  10:     IAsyncResult result3= task3.BeginInvoke(null, null, null);      
  11:   
  12:     WaitHandle.WaitAll(new WaitHandle[]{result1.AsyncWaitHandle, result2.AsyncWaitHandle, result3.AsyncWaitHandle});     
  13:     // Asserts go here...
  14:  }


In this example,  WaitHandle.WaitAll stops any further execution until all tasks complete.


The only problem is that when you try to run this test, it fails with the following error message: Test method Ordina.ParallelRunner.Waithandle_Should_Wait_Until_All_Tasks_Completethrew exception:  System.NotSupportedException: WaitAll for multiple handles on a STA thread is not supported..


This is actually documented behavior on the part of WaitHandle.WaitAll. It seems that MSTest runs in an STA by default.


Fortunately, this is configurable and can be edited in your .testrunconfig file. There's no user interface for this setting, so you will have to open the file and  add this line to make MSTest run in an MTA.

   1:  <ExecutionThread apartmentState="1" />

By default, the value of this element is 0 (which is equivalent to ApartmentState.STA - the default value of the enumeration), but as you can see, I've changed it to 1 (which is equivalent to ApartmentState.MTA). After saving this change, the test now succeeds.

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