Asynch crash when e.Cancel set?

[David Veeneman]

Does anyone knmow why an asynchronous worker method would crash if the
e.Cancel method is set to true, but not otherwise?

I'm working on a demo app with an asynchronous call, using the .NET 2.0
BackgroundWorke r component. The app works fine if I let the worker method
proceed to completion. But if I cancel the worker method, it crashes.

Specifically, if I set the DoWorkEventArgs .Cancel property to true, the
worker thread crashes at the point where the worker method returns control
to the caller. If I set the Cancel flag to false, the operation is
cancelled, but the RunWorkerComple tedEventArgs.Ca ncelled property is wrong.

My worker method is called by the BackgroundWorke r_DoWork event handler,
which passes in a WidgetList, a reference to the background worker (which
DoWork cast from its sender argument), and the DoWork event arguments. Here
is the worker method code:

private WidgetList PopulateList(Wi dgetList sourceList, BackgroundWorke r
backgroundWorke r, DoWorkEventArgs e)
{
// Create deep copy (by serialization)
WidgetList resultList = sourceList.Deep Copy();

// Populate result list
for (int i = 0; i < 100; i++)
{
// Add new widget to the list
WidgetItem widget = new WidgetItem(i);
resultList.Add( widget);

// Pause to simulate slow process
Thread.Sleep(10 0);

// Check cancellation
if (backgroundWork er.Cancellation Pending)
{
// Set DoWork event args
e.Cancel = false;

// Return unmodified list
return sourceList;
}

// Report progress
backgroundWorke r.ReportProgres s(i);
}

// Set return value
return resultList;
}

And here is the exception I'm getting when the app stops:

System.Reflecti on.TargetInvoca tionException was unhandled
Message="Except ion has been thrown by the target of an invocation."
Source="mscorli b"
StackTrace:
at System.RuntimeM ethodHandle._In vokeMethodFast( Object target,
Object[] arguments, SignatureStruct & sig, MethodAttribute s methodAttribute s,
RuntimeTypeHand le typeOwner)
at System.RuntimeM ethodHandle.Inv okeMethodFast(O bject target,
Object[] arguments, Signature sig, MethodAttribute s methodAttribute s,
RuntimeTypeHand le typeOwner)
at System.Reflecti on.RuntimeMetho dInfo.Invoke(Ob ject obj,
BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo
culture, Boolean skipVisibilityC hecks)
at System.Delegate .DynamicInvokeI mpl(Object[] args)
at
System.Windows. Forms.Control.I nvokeMarshaledC allbackDo(Threa dMethodEntry
tme)
at System.Windows. Forms.Control.I nvokeMarshaledC allbackHelper(O bject
obj)
at System.Threadin g.ExecutionCont ext.runTryCode( Object userData)
at
System.Runtime. CompilerService s.RuntimeHelper s.ExecuteCodeWi thGuaranteedCle anup(TryCode
code, CleanupCode backoutCode, Object userData)
at System.Threadin g.ExecutionCont ext.RunInternal (ExecutionConte xt
executionContex t, ContextCallback callback, Object state)
at System.Threadin g.ExecutionCont ext.Run(Executi onContext
executionContex t, ContextCallback callback, Object state)
at
System.Windows. Forms.Control.I nvokeMarshaledC allback(ThreadM ethodEntry tme)
at System.Windows. Forms.Control.I nvokeMarshaledC allbacks()
at System.Windows. Forms.Control.W ndProc(Message& m)
at
System.Windows. Forms.Control.C ontrolNativeWin dow.OnMessage(M essage& m)
at System.Windows. Forms.Control.C ontrolNativeWin dow.WndProc(Mes sage&
m)
at System.Windows. Forms.NativeWin dow.DebuggableC allback(IntPtr hWnd,
Int32 msg, IntPtr wparam, IntPtr lparam)
at System.Windows. Forms.UnsafeNat iveMethods.Disp atchMessageW(MS G&
msg)
at
System.Windows. Forms.Applicati on.ComponentMan ager.System.Win dows.Forms.Unsa feNativeMethods .IMsoComponentM anager.FPushMes sageLoop(Int32
dwComponentID, Int32 reason, Int32 pvLoopData)
at
System.Windows. Forms.Applicati on.ThreadContex t.RunMessageLoo pInner(Int32
reason, ApplicationCont ext context)
at
System.Windows. Forms.Applicati on.ThreadContex t.RunMessageLoo p(Int32 reason,
ApplicationCont ext context)
at System.Windows. Forms.Applicati on.Run(Form mainForm)
at BackgroundWorke rDemo.Program.M ain() in C:\Documents and
Settings\David Veeneman\My Documents\Visua l Studio
2005\Demos\Back groundWorkerDem o\Program.cs:li ne 17
at System.AppDomai n.nExecuteAssem bly(Assembly assembly, String[]
args)
at System.AppDomai n.ExecuteAssemb ly(String assemblyFile, Evidence
assemblySecurit y, String[] args)
at Microsoft.Visua lStudio.Hosting Process.HostPro c.RunUsersAssem bly()
at System.Threadin g.ThreadHelper. ThreadStart_Con text(Object state)
at System.Threadin g.ExecutionCont ext.Run(Executi onContext
executionContex t, ContextCallback callback, Object state)
at System.Threadin g.ThreadHelper. ThreadStart()

Any ideas as to what's going on here? Thanks.

--
David Veeneman
Foresight Systems

[David Veeneman]

I found my answer. In the DoWorkEventArgs , e.Result can't be set if e.Cancel
is set to true. In my worker method, I was setting e.Cancel to true, then
setting e.Cancel to my unmodified source list, as an easy way of providing a
rollback in a cancellation.

The error doesn't show up until .NET creates the RunWorkerComple tedEventArgs
for the RunWorkerComple ted event. When it tries to set the e.Result
property, an InvalidOperatio nException will be thrown, with the error
message "Operation has been cancelled". If you then try to read e.Result in
the RunWorkerComple ted event handler, the app blows up. The moral of the
story is to check for cancellation before reading e.Result in a
RunWorkerComple ted event handler.

--
David Veeneman
Foresight Systems