Async Method throws Exception

Question

I'm currently having some problems with a method that throws an exception but I'm not sure why. The exception makes my application crash.

System.NullReferenceException: Object reference not set to an instance of an object.  
   at Myapp.AutoProcess.<ToRead>d__36.MoveNext()  
--- End of stack trace from previous location where exception was thrown ---  
   at System.Runtime.CompilerServices.AsyncMethodBuilderCore.<ThrowAsync>b__1(Object state)  
   at System.Threading.QueueUserWorkItemCallback.WaitCallback_Context(Object state)   
   at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext,    ContextCallback callback, Object state, Boolean preserveSyncCtx)  
   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback   callback, Object state, Boolean preserveSyncCtx)  
   at   System.Threading.QueueUserWorkItemCallback.System.Threading.IThreadPoolWorkItem.ExecuteWorkItem()  
   at System.Threading.ThreadPoolWorkQueue.Dispatch() 
   at System.Threading._ThreadPoolWaitCallback.PerformWaitCallback()

I run the method on a separate thread

Thread listenerThread = new Thread(() => ToRead());
listenerThread.Start();

The method that throws the exception looks like this:

private async void ToRead()
{
    while (true)
    {
        if (this.toRead.Count != 0)
        {
            string protocol = this.toRead[0];
            string[] temp = protocol.Split(',');
            string message = temp[0];
            string UserName = temp[1];
            Process(message, UserName);
            this.toRead.RemoveAt(0);
        }
        await Task.Delay(200);
    }
}

It takes incoming messages from a List and filters out the Username and Message to send it to the Process method. I would appreciate if someone could help me out.

Note: The exception occurs about once a day running it on a Windows R2 2008 Server. Therefore I cant really debug it in Visual Studio

Debug the program. Where does it crash? Find out what was null. Did you do any investigation? — usr
– usr, Commented Jan 30, 2014 at 20:12
Why are you threading an async method? Make it return a Task and then you can await it. — Daniel Mann
– Daniel Mann, Commented Jan 30, 2014 at 20:12
Just as Daniel mentioned your ToRead() is just a another thread.so take a look about async and await little bit. — loop
– loop, Commented Jan 30, 2014 at 20:13

Stephen Cleary · Accepted Answer · 2014-01-30 20:20:47Z

14

You're seeing that exception crash your process because you're using an async void method.

Instead of using a thread, use a task, as such:

private async Task ToReadAsync()

Task listenerTask = Task.Run(() => ToReadAsync());

And now you'll be able to nicely retrieve the exception in listenerTask.Exception. But it probably won't give you much more detail.

What's probably happening is that your toRead variable is set to null at some point. The entire concept of the current code is wrong; polling like this is most definitely not the way to send data from one thread to another. Check out BlockingCollection or something like that for a proper approach.

answered Jan 30, 2014 at 20:20

Stephen Cleary

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