C# 中的线程异常错误：跨线程操作无效：从跨线程操作访问的控制“lblp4”无效

Question

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);
    }
}

}

Answer 1

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...");

Answer 2

In my case the intention was to change the behavior of a component "Janus.Windows.Ribbon.CommandBase" (applies to any other type) to enable or disable from another form. And the behavior change was done from an onchange method that listened for the changes of the components.

This implementation was carried out with the steps suggested by the Microsoft documentation ( https://docs.microsoft.com/es-es/dotnet/desktop/winforms/controls/how-to-make-thread-safe-calls-to- windows-forms-controls? view = netframeworkdesktop-4.8)

First the safe call was declared with the parameter that must be of the data type of the property to change.

private delegate void SaveCallDelegate(bool enable);

Variable to capture the properties of the component being modified

private CommandBase cm;

OnChange event queries if the component should be called safely and if so, it calls the "SaveCallChange" method with the value to change.

protected override void OnChanged(Command command){
   base.OnCommandChanged(command);
   
   foreach (KeyValuePair<CommandBase, List<string>> p in Invokers)
   {
      if (p.Key.Ribbon != null && p.Key.Ribbon.InvokeRequired)
      {
         cm = p.Key;
         SaveCallChange((command.Status == CommandStatus.Enabled));
      }
      else {
         p.Key.Enabled = (command.Status == CommandStatus.Enabled);
      }
   }
}

Invokers comes from the EventCommandAdapter class ("Microsoft.Practices.CompositeUI.Commands") which inherits the main class.

Finally there are the two methods that carry out the safe process of change

private void SaveCallChange(bool enable){
    var e = new SaveCallDelegate(setEnable);
    cm.Ribbon.Invoke(e, new object[] { enable });
}

private void setEnable(bool enable) {
    this.cm.Enabled = enable;
}

Answer 3

我可以看到的主要问题是从线程访问用户界面