Thread exception error in C# :Cross-thread operation not valid: Control 'lblp4' accessed from a Cross-thread operation not valid

c#, multithreading

Solution

As the exception suggests, you're accessing controls from a thread other than the one that that created the control (specifically, your ph1..5 threads all attempt to access the UI).

To correct this, you need to use the `Invoke()` method on the controls so that the accesses are performed on the main UI thread.

Perhaps add a function to `Philosopher` as follows:

private void UpdateText(Label label, string text)
{
    // If the current thread is not the UI thread, InvokeRequired will be true
    if (label.InvokeRequired)
    {
        // If so, call Invoke, passing it a lambda expression which calls
        // UpdateText with the same label and text, but on the UI thread instead.
        label.Invoke((Action)(() => UpdateText(label, text)));
        return;
    }
    // If we're running on the UI thread, we'll get here, and can safely update 
    // the label's text.
    label.Text = text;
}

Then, whenever you have something like:

AZsys.Program.frm.lblp1.Text = "Eating...";

Replace it with:

UpdateText(AZsys.Program.frm.lblp1, "Eating...");

Problem

First, sorry for my weak english language, I'm beginner programmer. In my project I use thread class, but when finished it I see this exception : Cross-thread operation not valid: Control 'lblp4' accessed from a thread other than the thread it was created on. In my project, I call 5 functions `Synchronize` from `Philosopher` class in `Form1` class: `Philosopher` class: ``` namespace AZsys { class Philosopher { public Int32 i; public bool e, th; public Philosopher() { } public void main() { lock (AZsys.Program.frm.locker) { while (true) { if (i < 0 || i > 4) { System.Windows.Forms.MessageBox.Show("error"); break; } else { think(); AZsys.Program.frm.chopstick[i].WaitOne(); AZsys.Program.frm.chopstick[(i + 1) % 5].WaitOne(); eat(); AZsys.Program.frm.chopstick[(i + 1) % 5].Release(); AZsys.Program.frm.chopstick[i].Release(); } } } Thread.Sleep(100); } private void eat() { switch (i) { case 1: AZsys.Program.frm.lblp1.Text = "Eating..."; break; case 2: AZsys.Program.frm.lblp2.Text = "Eating..."; break; case 3: AZsys.Program.frm.lblp3.Text = "Eating..."; break; case 4: AZsys.Program.frm.lblp4.Text = "Eating..."; break; case 5: AZsys.Program.frm.lblp5.Text = "Eating..."; break; } e = true; for (int j = 0; j < 992; j++) { if (j % 8 == 0) e = true; } e = false; } private void think() { switch (i) { case 1: AZsys.Program.frm.lblp1.Text = "Thinking..."+Thread.CurrentThread.Name.ToString(); break; case 2: AZsys.Program.frm.lblp2.Text = "Thinking..."+Thread.CurrentThread.Name.ToString(); break; case 3: AZsys.Program.frm.lblp3.Text = "Thinking..."+Thread.CurrentThread.Name.ToString(); break; case 4: AZsys.Program.frm.lblp4.Text = "Thinking..."+Thread.CurrentThread.Name.ToString(); break; case 5: AZsys.Program.frm.lblp5.Text = "Thinking..." + Thread.CurrentThread.Name.ToString(); break; } th = true; for (int j = 0; j < 9924; j++) { if (j % 8 == 0) th = true; } th = false; } } ``` Even in this code, I use lock (`locker`) but don't work!!! `Form1` class: ``` public partial class Form1 : Form { public Semaphore[] chopstick; public object locker; private Philosopher ph1; private Philosopher ph2; private Philosopher ph3; private Philosopher ph4; private Philosopher ph5; public Form1() { InitializeComponent(); chopstick = new Semaphore[5]; } private void Form1_Load(object sender, EventArgs e) { locker = new object(); ph1 = new Philosopher(); ph1.i = 1; ph2 = new Philosopher(); ph2.i = 2; ph3 = new Philosopher(); ph3.i = 3; ph4 = new Philosopher(); ph4.i = 4; ph5 = new Philosopher(); ph5.i = 5; } private void lblp2_Click(object sender, EventArgs e) { } private void btnstart_Click(object sender, EventArgs e) { Thread.CurrentThread.Priority = ThreadPriority.Lowest; Thread t1 = new Thread(ph1.main); Thread t2 = new Thread(ph2.main); Thread t3 = new Thread(ph3.main); Thread t4 = new Thread(ph4.main); Thread t5 = new Thread(ph5.main); t1.Name = "t1"; t2.Name = "t2"; t3.Name = "t3"; t4.Name = "t4"; t5.Name = "t5"; t1.Priority = ThreadPriority.Highest; t2.Priority = ThreadPriority.Highest; t3.Priority = ThreadPriority.Highest; t4.Priority = ThreadPriority.Highest; t5.Priority = ThreadPriority.Highest; // Thread.Sleep(100); t4.Start(); Thread.Sleep(100); t1.Start(); Thread.Sleep(100); t2.Start(); Thread.Sleep(100); t3.Start(); Thread.Sleep(100); t5.Start(); Thread.Sleep(100); } } ``` }

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