Question

I’m running heavvy background work with Parallel.Invoke, after all processing has completed I return the method, return again, call a next method to utilize the calculated data I get the error:
Cross-thread operation not valid: Control ” accessed from a thread other than the thread it was created on.

But I already returned from the threads that where created by Parallel.Invoke to the one that called it in the first place. Is it normal that control does not revert to the thread where it started? And how can I assure that this does happen?

Code:

public void TopMethod()
        {
            Calculate(4);
            UpdateGui();
        }

        public void Calculate(int depth)
        {
            Recursive(depth);
        }

        public void Recursive(int depth)
        {
            if (depth > 0)
                System.Threading.Tasks.Parallel.Invoke(
                            delegate { Recursive(depth - 1); });
        }

        public void UpdateGui()
        {
            CalculateOhter(); // Works fine.
            controlElement.Focus(); // Causes exception
        }

Edits:
I know about Control.Invoke But this would be an ugly solution (don’t want to store a delegate in every control) and the program needs to wait for all computations to complete before it can continue. So it would be better if I could somehow force control to return to the thread that I started out with in the first place.

Answer 1

You need to access a control/window from the thread that created that control. Use Control.Invoke and Control.InvokeRequired.