code throws Object synchronization method was called from an unsynchronized block of code

Question

I'm writing quite simple synchronized code and I'm really stuck on this error I get. What could cause the error?

Program throws error at last line on PulseAll(processingText)

UPDATE: there is also other function in my monitoring class which is called by main thread ProcessingText(), could it cause the issue?

public void doWork(int threadTaskNumber)
        {
            while (!(bool)WorkDone)
            {
                lock (processingText)
                {
                    while (vowelCounter != 3 && threadTaskNumber != 0)
                    {
                        Monitor.Wait(processingText);
                    }

                    if (!(bool)workDone)
                    {

                        switch (threadTaskNumber)
                        {

                            case 0:
                                processingText += 'A';

                                if (vowelCounter != 3)
                                    vowelCounter++;
                                break;
                            case 1:
                                processingText += 'B';
                                break;
                            case 2:
                                processingText += 'C';
                                break;
                        }

                        if (++threadCounter == 15)
                            WorkDone = true;
                    }

                    Monitor.PulseAll(processingText);
                }
            }
        }

public void ProcessingText()
        {
            lock (processingText)
            {
                Console.WriteLine(processingText);
            }
        }

Answer 1

The SynchronizationLockException is thrown because between these two lines:

Monitor.Wait(processingText);
//...
Monitor.PulseAll(processingText);

...the value of the variable processingText has changed. So the Monitor.PulseAll tries to send a pulse using a lock that is not owned by the current thread, hence the exception you observe.