Why am I Getting This Error:"Cross-Thread Operation Not Valid: Control Lbfolders Accessed from a Thread Other Than the Thread It Was Created On."

Why am I getting this error:Cross-thread operation not valid: Control lbFolders accessed from a thread other than the thread it was created on.?

prgAll.Maximum = lbFolders.SelectedItems.Count;

On that line you perform an assignment (set/add), which by default is not thread-safe.

On the second line it's just a get operation, where thread-safety merely doesn't matter.

EDIT: I don't mean access to the prgAll element.

Accessing the Count property changes the internal state of the ListBox inner collection, that is why it throws the exception.

Cross-thread operation not valid: Control 'label1' accessed from a thread other than the thread it was created on

Update your label in your progress handler instead of inside the worker thread.

// On worker thread so do our thing! 
 void backgroundWorker1_DoWork(object sender, DoWorkEventArgs e) 
 { 
     // Your background task goes here 
     for (int i = 0; i <= 100; i++) 
     { 
         // Report progress to 'UI' thread 
         backgroundWorker1.ReportProgress(i); 
         // Simulate long task 
         System.Threading.Thread.Sleep(100); 
     } 
 } 
 // Back on the 'UI' thread so we can update the progress bar - and our label :)
 void backgroundWorker1_ProgressChanged(object sender, ProgressChangedEventArgs e) 
 { 
     // The progress percentage is a property of e 
     progressBar1.Value = e.ProgressPercentage; 
     label1.Text = String.Format("Trade{0}",e.ProgressPercentage);
 } 

C# Cross-thread operation not valid

You should Invoke backgroundWorker1.ReportProgress(1);, because only UI thread can directly access UI controls. Example of invoking:

private delegate void AddListBoxItemDelegate(object item);

private void AddListBoxItem(object item)
{
   if (this.listBox1.InvokeRequired)
   {
       // This is a worker thread so delegate the task.
       this.listBox1.Invoke(new AddListBoxItemDelegate(this.AddListBoxItem), item);
   }
   else
   {
       // This is the UI thread so perform the task.
       this.listBox1.Items.Add(item);
   }
}

Cross-threading operation not valid

try below code

public void eng_OnGetSpeedDone(OBDIIEngineEventArgs args)
        {
            if (this.InvokeRequired)
            {
                Action action = () => eng_OnGetSpeedDone(args);
                Invoke(action);
                return;
            }
            if (!args.OBDResultNoData)
                brzina_ele.Text = arg.OBDValue.ToString();
            else
                brzina_ele.Text = "0";

        }

Solve a cross-threading Exception in WinForms

When making method calls to a control, if the caller is on a different thread than the one the control was created on, you need to call using Control.Invoke. Here is a code sample:

// you can define a delegate with the signature you want
public delegate void UpdateControlsDelegate();

public void SomeMethod()
{
    //this method is executed by the background worker
    InvokeUpdateControls();
}

public void InvokeUpdateControls()
{
    if (this.InvokeRequired)
    {
        this.Invoke(new UpdateControlsDelegate(UpdateControls));
    }
    else
    {
        UpdateControls();
    }
}

private void UpdateControls()
{
    // update your controls here
}

Hope it helps.

Help needed for 'cross-thread operation error' in C#

Make always sure you update your GUI on the main thread (GUI thread).

foo.MessageReceived += new Agent.MessageReceivedHandler(foo_MessageReceived);

public delegate void MyDelegate(Message msg);
void foo_MessageReceived(Message message)
{ 
   if (InvokeRequired)
   {
      BeginInvoke(new MyDelegate(foo_MessageReceived),new object[]{message});
   }
   else
   {
      label1.Text = message.Body;
   }
}

Solve a cross-threading Exception in WinForms

When making method calls to a control, if the caller is on a different thread than the one the control was created on, you need to call using Control.Invoke. Here is a code sample:

// you can define a delegate with the signature you want
public delegate void UpdateControlsDelegate();

public void SomeMethod()
{
    //this method is executed by the background worker
    InvokeUpdateControls();
}

public void InvokeUpdateControls()
{
    if (this.InvokeRequired)
    {
        this.Invoke(new UpdateControlsDelegate(UpdateControls));
    }
    else
    {
        UpdateControls();
    }
}

private void UpdateControls()
{
    // update your controls here
}

Hope it helps.

Unintentional multithreading in form application

You are using System.Timer. https://msdn.microsoft.com/en-us/library/system.timers.timer(v=vs.110).aspx

For winforms apps you are better off using the one for winforms...

https://msdn.microsoft.com/en-us/library/system.windows.forms.timer(v=vs.110).aspx

It is designed to support single threaded UIs. :-

A Timer is used to raise an event at user-defined intervals. This Windows timer is designed for a single-threaded environment where UI threads are used to perform processing. It requires that the user code have a UI message pump available and always operate from the same thread, or marshal the call onto another thread.

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