Question

Presently I am attempting to create unit tests using MSTest.exe for multithreading functionality. When I run the tests, here are the errors I’m getting:

A first chance exception of type 'System.NullReferenceException' occurred in ApplicationManagement.exe
The thread 'Agent: adapter run thread for test 'UpdateDirectoryExceptionTest' with id 'a78d3e8e-e859-43aa-87aa-cf006f736dee'' (0x1150) has exited with code 0 (0x0).
The thread '<No Name>' (0x1bec) has exited with code 0 (0x0).
E, 6788, 86, 2011/10/20, 14:02:36.771, WKSTVMC0006\QTAgent32.exe, Unhandled Exception Caught, reporting through Watson: System.NullReferenceException: Object reference not set to an instance of an object.
   at CommonObjects4.clsThread.StartThreadMethod.Execute() in D:\DevProjects\CommonObjects4\classes\clsThread.cs:line 23
   at System.Threading.ThreadHelper.ThreadStart_Context(Object state)
   at System.Threading.ExecutionContext.runTryCode(Object userData)
   at System.Runtime.CompilerServices.RuntimeHelpers.ExecuteCodeWithGuaranteedCleanup(TryCode code, CleanupCode backoutCode, Object userData)
   at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state)
   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean ignoreSyncCtx)
   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
   at System.Threading.ThreadHelper.ThreadStart()
The program '[6788] QTAgent32.exe: Managed (v4.0.30319)' has exited with code -2 (0xfffffffe).
The program '[1120] ApplicationManagement.vshost.exe: Managed (v4.0.30319)' has exited with code 0 (0x0).

If I run the UpdateDirectoryExceptionTest by itself in Visual Studio, it passes with no problem, but when I run the entire suite of unit tests, I receive the above error. Here is the function that I recently started testing, which I think is related to the error:

    public Thread StartClassThread(ref Thread ThreadObject, ThreadMethod MethodObject)
    {
        System.Threading.Thread startClassThreadReturn = null;
        try
        {
            if (ThreadObject == null && MethodObject == null)
            {
                throw new ApplicationException("Cannot have both ThreadObject and MethodObject as null parameters in call to StartClassThread.");
            }
            StartThreadMethod objMethod = new StartThreadMethod();
            objMethod.objThreadMethod = MethodObject;
            if (!(ThreadObject == null))
            {
                if (ThreadObject.ThreadState != System.Threading.ThreadState.Stopped)
                {
                    // Do nothing
                }
            }
            if (!(ThreadObject == null))
            {
                if (ThreadObject.ThreadState == System.Threading.ThreadState.Stopped 
                    | ThreadObject.ThreadState == System.Threading.ThreadState.Aborted
                    | ThreadObject.ThreadState == System.Threading.ThreadState.Unstarted) 
                { 
                    ThreadObject = null; 
                    ThreadObject = new Thread( new System.Threading.ThreadStart( objMethod.Execute ) ); 
                    ThreadObject.Start(); 
                } 
            } 
            else 
            { 
                ThreadObject = new Thread( new System.Threading.ThreadStart( objMethod.Execute ) ); 
                ThreadObject.Start(); 
            } 
            return ThreadObject; 
        } 
        catch (Exception excException) 
        { 
            ZEGApp.clsMain.objApplicationAudit.AuditMethodError(excException, System.Reflection.MethodInfo.GetCurrentMethod().DeclaringType.FullName + "." + System.Reflection.MethodBase.GetCurrentMethod().Name + " - " + excException.StackTrace);
        }
        finally 
        { 
            // Do nothing
        } 
        return startClassThreadReturn;
    } 

Here is the output I receive from CruiseControl.NET, as opposed to testing within Visual Studio:

  <message>Passed                UnitTests.clsThreadTest.StartClassThreadTest2</message>
  <message>Process 'QTAgent32' [PID 3932] has finished profiling.</message>
  <message>Process 'QTAgent32' [PID 5388] has begun profiling.</message>
  <message>Error                 UnitTests.clsUltraCalendarTest.clsUltraCalendarConstructorTest</message>

Does anyone have any ideas as to how I can resolve these errors? TIA.

UPDATE: Here is the full source code for clsThread.cs (please note that line 23 is objThreadMethod(); ):

namespace CommonObjects4
{
    public class clsThread
    {

    #region '" Sub Class "'

    public delegate void ThreadMethod();

    private class StartThreadMethod  
    { 
        public ThreadMethod objThreadMethod; 
        public void Execute() 
        { 
            objThreadMethod(); // this is line 23
        }
    }

    #endregion

    #region '" Enumerator Declaration "'
    #endregion

    #region '" Variable Declaration "'
    #endregion

    #region '" Property Declaration "'
    #endregion

    #region '" Function Declaration "'

    public Thread StartClassThread(Thread ThreadObject, ThreadMethod MethodObject)
    {
        System.Threading.Thread startClassThreadReturn = null;
        try
        {
            if (ThreadObject == null && MethodObject == null)
            {
                throw new ApplicationException("Cannot have both ThreadObject and MethodObject as null parameters in call to StartClassThread.");
            }
            StartThreadMethod objMethod = new StartThreadMethod();
            objMethod.objThreadMethod = MethodObject;
            if (!(ThreadObject == null))
            {
                if (ThreadObject.ThreadState != System.Threading.ThreadState.Stopped)
                {
                    // Do nothing
                }
            }
            if (!(ThreadObject == null))
            {
                if (ThreadObject.ThreadState == System.Threading.ThreadState.Stopped | ThreadObject.ThreadState == System.Threading.ThreadState.Aborted ) 
                { 
                    ThreadObject = null; 
                    ThreadObject = new Thread( new System.Threading.ThreadStart( objMethod.Execute ) ); 
                    ThreadObject.Start(); 
                } 
            } 
            else 
            { 
                ThreadObject = new Thread( new System.Threading.ThreadStart( objMethod.Execute ) ); 
                ThreadObject.Start(); 
            } 
            return ThreadObject; 
        } 
        catch (Exception excException) 
        { 
            ZEGApp.clsMain.objApplicationAudit.AuditMethodError(excException, System.Reflection.MethodInfo.GetCurrentMethod().DeclaringType.FullName + "." + System.Reflection.MethodBase.GetCurrentMethod().Name + " - " + excException.StackTrace);
        }
        finally 
        { 
            // Do nothing
        } 
        return startClassThreadReturn;
    } 

    #endregion 

} 
}

Answer 1

The problem would appear to be that objThreadMethod is null at line 23. The syntax objThreadMethod() is actually shorthand for objThreadMethod.Invoke(), so you’ll get a NullReferenceException if objThreadMethod is null when the line is executed.

As for why it’s null, I’m guessing that you’ve invoked the StartClassThread method with a non-null ThreadObject and a null MethodObject, which resulted in the following block being executed after setting objMethod.objThreadMethod to the null MethodObject:

ThreadObject = null;
ThreadObject = new Thread( new System.Threading.ThreadStart( objMethod.Execute ) );
ThreadObject.Start();