C# Background worker setting e.Result in DoWork and getting value back in WorkCompleted

Question

C# 2008 SP1

I am using the background worker

If one of the conditions fails I will set e.cancel to true, and assign the string to the e.result. Everything works there.

However, when the workCompleted fires, I test for the e.Result and I get an exception "e.result throw an exception of type systeminvalidoperation".

I guess I could use a global variable to set in the DoWork and compare in the work completed. But this might not be threadsafe.

Can anyone tell me why I am getting this with the e.Result, and how can I solve this?

Many thanks,

    private void bgwPrepareCall_DoWork(object sender, DoWorkEventArgs e)
    {
        // Check to see if the called number is blocked. 
        // If the nubmer is blocked display error message.
        if (this.IsNoGateway(sipUsername, statusDisplay1.PhoneNumber))
        {
            // this.SetIdleState(DialerErrorMessage.GetErrorDescription(12));
            e.Cancel = true;
            e.Result = "NoGateway";
            return;
        }

        if (!this.IsInvalidTelephoneNumber(sipUsername, statusDisplay1.PhoneNumber))
        {
            // this.SetIdleState(DialerErrorMessage.GetErrorDescription(13));
            e.Cancel = true;
            e.Result = "InvalidNumber";
            return;
        }
    }

    private void bgwPrepareCall_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
    {
        if (e.Error == null)
        {
            if (e.Cancelled)
            {
                // Failed
                switch (e.Result.ToString())
                {
                    case "NoGateway":
                        Console.WriteLine("NoGateway: [ Failed ]");
                        break;

                    case "InvalidNumber":
                        Console.WriteLine("InvalidNumber: [ Failed ]");
                        break;

                    default:
                        Console.WriteLine("Unknown");
                        break;
                }
            }
            if (!e.Cancelled)
            {
                // Successfull
                Console.WriteLine("NoGateway: [ ok ]");    
                Console.WriteLine("InvalidNumber: [ ok ]");
            }
        }
        else
        {
            Console.WriteLine("PrepareCall Failed: [ " + e.Error.Message + " ]");
        }
    }

Answer 1

From MSDN:

If the operation completes successfully and its result is assigned in the DoWork event handler, you can access the result through the RunWorkerCompletedEventArgs.Result property.

and:

Your RunWorkerCompleted event handler should always check the Error and Cancelled properties before accessing the Result property. If an exception was raised or if the operation was canceled, accessing the Result property raises an exception.

So if it doesn't complete successfully (i.e. you cancel it) it looks like it won't work. Perhaps consider returning your cancellation-details as the result (as success) for your abort case, an detecting the difference in the completion handler?

Answer 2

The Result property is meant to represent the result of a completed operation. You've set Cancel to true, meaning that the operation was cancelled, therefore there shouldn't be a result.

It sounds like you should encode the "I aborted because something was wrong" into your result, or throw an exception which will be set as the Error property in the result instead - Cancel is meant to be set if the worker noticed that the call was cancelled externally.

The docs for RunWorkerCompletedEventArgs.Result state:

Your RunWorkerCompleted event handler should always check the Error and Cancelled properties before accessing the Result property. If an exception was raised or if the operation was canceled, accessing the Result property raises an exception.

The "Exceptions" part of the documentation also states that it will throw an exception if Cancelled is true.