Question

I want to handle a ManagementException exception for a specific ErrorCode only and am having trouble writing the unit test for it. Ordinarily, I would write the test so that it is something like the following:

Searcher search = MockRepository.GenerateMock<Searcher>(); 
// wrapper for ManagementObjectSearcher

...

search.Expect(s => s.Get()).Throw(new ManagementException());

...

However, this doesn’t set the ErrorCode to the one that I want in particular, indeed ManagementException doesn’t have a constructor which sets this value.

How can this be done?

(Note that I am using RhinoMocks as my mocking framework but I am assuming that this is framework independent; all I need to know here is how to create a ManagementException which has a specific ErrorCode value. Also I have found some references to a System.Management.ManagementException.ThrowWithExtendedInfo(ManagementStatus errorCode) method online but this doesn’t appear to be publicly accessible).

Answer 1

The least effort to get over this hurdle would be a static helper / utility method that uses reflection to hack-slot in the required error code. Using the most excellent Reflector, I see there is a private “errorCode” field, which is only set via internal ctors defined in ManagementException. So 🙂

public static class EncapsulationBreaker
   {
      public static ManagementException GetManagementExceptionWithSpecificErrorCode(ManagementStatus statusToBeStuffed)
      {
         var exception = new ManagementException();
         var fieldInfo = exception.GetType().GetField("errorCode", 
            BindingFlags.NonPublic | BindingFlags.Instance | BindingFlags.GetField | BindingFlags.DeclaredOnly);
         fieldInfo.SetValue(exception, statusToBeStuffed);
         return exception;
      }
   }

Verified that it works

[Test]
      public void TestGetExceptionWithSpecifiedErrorCode()
      {
         var e = EncapsulationBreaker.GetManagementExceptionWithSpecificErrorCode(ManagementStatus.BufferTooSmall);
         Assert.AreEqual(ManagementStatus.BufferTooSmall, e.ErrorCode);
      }

Although I generally frown upon reflection in tests, this is one of the rare cases where it is needed / useful.
HTH