Multithreading and function recursion

Question

I haven't done multi-threading before, so I need a bit of help with this.
I have a form with a button. When user clicks on a button for the first time, a recursive function is called. When a certain condition in the function is true, I want the function to wait where it's at for the next button click.

An example:

void ButtonClick(object sender, EventArgs e)
{
    if (running)
    {
        //notify the thread (the recursive function) to continue
    }
    else functionCall(0);
}

void functionCall(int num)
{
    if (num % 2 == 0)
    {
        displaySomething();
        //wait
    }
    functionCall(num + 1);
}

How would I do this?

Benjamin Scharbach · Answer

I would use the ManualResetEvent to communicate between the thread calls. In C# the ManualResetEvent is like a true/false variable which can talk between threads. So, create a ManualResetEvent(false) with a false setting to begin with. Then use a While() loop to pause and wait until the ManualResetEvent is set to True by your button click. Once the button is clicked and you set the MRE to true, then your While() loop in the recursive function will be notified accross the UI>Thread, and the recursion will then continue successfully. Ben Scharbach https://www.youtube.com/channel/UCih2aBHyGR5cJHApqkwAXBA

Sam Hobbs · Answer

It is normal for the main thread to wait; it normally waits for the user to do something in the window, so I don't understand the problem with the main thread waiting. The main purpose of use of a separate thread is so that the main thread can wait for interaction with the UI. Normally the BackgroundWorker uses ReportProgress to update the form.

Jure · Answer

Thanks for the reply. I managed to get it to work with BackgroundWorker and AutoResetEvent. But the way I did it isn't really that great, because I set the STA (main) thread to wait until the 2nd thread, which is running the recursive function, reaches a certain point. That's because only the STA thread has control over my form (and I want to update the form). I tried by creating a separate class that holds the data and creating an event (->displaying the data on the form) for that class. But the event still executes in the 2nd thread. So, in short, rather than making the main thread wait (GUI-freeze), I want the 2nd thread to send the 1st thread to a function. OR, I want the 2nd thread to have control over the GUI (if that's even possible, I assume it's not). I have been programming for only two months now (started with C++), so all this may be a bit too much for me. But I'm the type of person that learns better from his own projects rather than just by studying tutorials. Can you point me in the right direction? Thanks in advance!

Sam Hobbs · Answer

First read the articles about BackgroundWorker. Look at the articles listed at the right side of this window. Be sure especially to read about synchronization. Note that .Net version 4 has a Task class that might make threads easier but BackgroundWorker is not real difficult. Threading does take time to learn and fortunately there are many articles and samples. If it were me, I would sure read articles and documentation and I would try out the samples. Threads can be fun; use this as an opportunity to have some fun. If you manage to get someone else to do all the programming for you, you will deny yourself the fun.

Jure · Answer

I have tried this and it gave me this error: An unhandled exception of type 'System.Threading.SynchronizationLockException' occurred in mscorlib.dll Additional information: Object synchronization method was called from an unsynchronized block of code. using System; using System.Collections.Generic; using System.ComponentModel; using System.Data; using System.Drawing; using System.Text; using System.Windows.Forms; using System.Threading; namespace C_Sharp_Multi_Threading_with_Form { public partial class Window : Form { bool running = false; public SecondThread SecThread; public Recursion recursion; public Window() { InitializeComponent(); this.button.Click += new EventHandler(ButtonClick); } void ButtonClick(object sender, EventArgs e) { if (running) { SecThread.wait(); } else { SecThread = new SecondThread(); running = true; SecThread.wait(); } } } public class SecondThread : Window { Thread thread; public SecondThread() { thread = new Thread(new ThreadStart(run)); thread.Start(); } void run() { recursion = new Recursion(); } public void wait() { Monitor.Pulse(this); Monitor.Wait(recursion); label.Text = recursion.data.ToString(); } } public class Recursion : Window { public int data; public Recursion() { function(0); } void function(int num) { if (num % 4 == 2) { data = num; Monitor.Pulse(this); Monitor.Wait(SecThread); } function(num + 1); } } } I know I'm doing something laughably-obvious completely wrong, because I don't have any understanding about multi-threading. I hope it's at least a little bit clearer what I'm trying to accomplish.

Jure · Answer

Sorry if I'm wrong, but it seems to me that this just puts the thread to sleep for 5 seconds. I want the thread to actually wait for the main thread to notify it. I think I need to use Monitor.Pulse(object to notify) and Monitor.Wait(object to wait). But I don't know how exactly to implement this. If I understand correctly, it's necessary to put the 2nd thread into its own class. How would I notify one object's thread from another object? Thanks in advance!

Shankey · Answer

Hi, You have to write some thing like below : using System; using System.Threading; // Following is the member of MyThread class public void recursion(String btnText) { Boolean nextclickflag = ( btnText .Equals("Next")?true:false); if(yourcondition == true && (! btn.Text. Equals("Next") )) { btn.Text="Next"; Thread.Sleep(5000); } } class MultiThread { public static void Main() { // First, construct a MyThread object. MyThread myThread = new MyThread(); //Next, construct a thread from that object. Thread newThread = new Thread(new ThreadStart(myThread. recursion(btn.Text)); //Finally, start execution of the thread. newThread.Start(); } }

Multithreading and function recursion

Answers (7)