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Cross-thread operation not valid: Control 'Form1' accessed from a thread other than the thread it was created on

Im pretty new to C# but I have been playing around with it to learn. So far I have a nice app that does things like starts up the screen saver and controls the windows system volume.
However I'm having some trouble and I'm not sure what is wrong. I have already gone through a bunch of similar questions on the site but I don't think any of them apply to my specific case.
So I wanted to control the app from the web. My plan is to have the app check a webpage on my site for a command every couple seconds. Depending on what the site returns the app will do different things(mute, vol up, etc.) and reset the command on the website to blank. The website command part is all done in PHP and is quite simple, there is no problem with that part of it.
I created a button that calls a function that checks the page and performs the action. It worked fine. But when I tried to make it automatically check the site I'm getting errors. I'm pretty sure it is because I moved the check and perform action to a new thread. So lets move on to the code. These are not the full files but what I think you need to know to help. If you need anything more just let me know.
Form1.cs
public Form1(Boolean init = true)
{
if (init)
{
InitializeComponent(); //initialize UI
startWebMonitor(); //starts web monitor thread for remote web commands
}
}
private void startWebMonitor()
{
Thread t = new Thread(WebMonitor.doWork);
t.Start();
}
public IntPtr getWindowHandle()
{
return this.Handle;
}
WebMonitor.cs
public static void doWork()
{
while(true)
{
checkForUpdate();
Thread.Sleep(1000);
}
}
private static void checkForUpdate()
{
lastCommand = getLastCommand();
if (lastCommand.Equals(""))
{
//No Update
}
else
{
processCommand(lastCommand);
}
}
public static void processCommand(String command)
{
if(command.Equals("mute"))
{
VolumeCtrl.mute(Program.form1.getWindowHandle());
}
HTTPGet req2 = new HTTPGet();
req2.Request("http://mywebsite.com/commands.php?do=clearcommand");
}
VolumeCtrl.cs
private static IntPtr getWindowHandle()
{
Form1 form1 = new Form1(false); //false = do not initialize UI again
return form1.getWindowHandle();
}
public static void mute(IntPtr handle)
{
SendMessageW(getWindowHandle(), WM_APPCOMMAND, getWindowHandle(), (IntPtr)APPCOMMAND_VOLUME_MUTE);
}
Alright so basically the mute function requires the window handle in order to work. But when I try to get the window handle from the new thread it throws the error:
Cross-thread operation not valid: Control 'Form1' accessed from a
thread other than the thread it was created on.
So how do I get around this? I have read other answers on here saying you need to use Invoke or Delegate but I'm not sure how those work or where to use them. I tried to follow one of the answers but I got even more errors.

In this case, write invoke operation inside of method. Then you can access both from control's thread and other threads.
delegate IntPtr GetWindowHandleDelegate();
private IntPtr GetWindowHandle() {
if (this.InvokeRequired) {
return (IntPtr)this.Invoke((GetWindowHandleDelegate)delegate() {
return GetWindowHandle();
});
}
return this.Handle;
}
or, if you want to avoid delegate hell, you could write in more few code with built-in delegate and lambda.
private IntPtr GetWindowHandle() {
if (this.InvokeRequired)
return (IntPtr)this.Invoke((Func<IntPtr>)(GetWindowHandle));
return this.Handle;
}

Try this code it will surely help you
BackgroundWorker bw = new BackgroundWorker();
bw.DoWork += (s, e) => { };
bw.RunWorkerCompleted += (s, e) =>
{
//Call you windowsform method or anything which you want to do
};
bw.RunWorkerAsync();

How to enable form button after process has exited?

I have an windows application developed using C#. In this application, I am creating one process. I want to enable and disable few buttons when Process_Exited() event occures.
In Process_Exited() method, I have written code to enable buttons but at runtime I get error as
"Cross-thread operation not valid:
Control
'tabPage_buttonStartExtraction'
accessed from a thread other than the
thread it was created on."
My code snippet is :
void rinxProcess_Exited(object sender, EventArgs e)
{
tabPage_buttonStartExtraction.Enabled = true;
tabPageExtraction_StopExtractionBtn.Enabled = false;
}
Can anyone suggest how to make this possible?

Move the enable/disable lines in a separate method and call that method from rinxProcess_Exited using Control.Invoke method.

You're attempting to change the UI from a different thread.
Try something like this;
private void SetText(string text)
{
// InvokeRequired required compares the thread ID of the
// calling thread to the thread ID of the creating thread.
// If these threads are different, it returns true.
if (this.textBox1.InvokeRequired)
{
SetTextCallback d = new SetTextCallback(SetText);
this.Invoke(d, new object[] { text });
}
else
{
this.textBox1.Text = text;
}
}
You shouldn't be doing much work on the UI from another thread, as the invocations are quite expensive.
Source: http://msdn.microsoft.com/en-us/library/ms171728.aspx

You must make UI changes on the UI thread. See this question for more details.
Here's the solution applied to your example:
void rinxProcess_Exited(object sender, EventArgs e)
{
if (this.InvokeRequired)
{
this.Invoke((Action)(() => ProcessExited()));
return;
}
ProcessExited();
}
private void ProcessExited()
{
tabPage_buttonStartExtraction.Enabled = true;
tabPageExtraction_StopExtractionBtn.Enabled = false;
}

Call method on the GUI thread from a timers thread

In my application I am using a timer to check for updates in an RSS feed, if new items are found I pop up a custom dialog to inform the user. When I run the check manually everything works great, but when the automatic check runs in the timers Elapsed event the custom dialog is not displayed.
First of all is this a thread issue? (I am assuming it is because both the manual and automatic check use the same code).
When I run the automatic check, do I have to invoke the method that runs the check from the Timers Elapsed event handler?
Is there something I need to do in my custom dialog class?
Edit:
this is a winforms application.
Here is an example of what the code is like. (Please don't point out syntax errors in this code example, this is just a simple example not real code).
public class MainForm : System.Windows.Forms.Form
{
//This is the object that does most of the work.
ObjectThatDoesWork MyObjectThatDoesWork = new ObjectThatDoesWork();
MyObjectThatDoesWork.NewItemsFound += new NewItemsFoundEventHandler(Found_New_Items);
private void Found_New_Items(object sender, System.EventArgs e)
{
//Display custom dialog to alert user.
}
//Method that doesn't really exist in my class,
// but shows that the main form can call Update for a manual check.
private void Button_Click(object sender, System.EventArgs e)
{
MyObjectThatDoesWork.Update();
}
//The rest of MainForm with boring main form stuff
}
public class ObjectThatDoesWork
{
System.Timers.Timer timer;
public ObjectThatDoesWork()
{
timer = new System.Timers.Timer();
timer.Interval = 600000;
timer.AutoReset = true;
timer.Elapsed += new new System.Timers.ElapsedEventHandler(TimeToWork);
timer.Start();
}
private void TimeToWork(object sender, System.Timers.ElapsedEventArgs e)
{
Update();
}
public void Update()
{
//Check for updates and raise an event if new items are found.
//The event is consumed by the main form.
OnNewItemsFound(this);
}
public delgate void NewItemsFoundEventHandler(object sender, System.EventArgs e);
public event NewItemsFoundEventHandler NewItemsFound;
protected void OnNewItemsFound(object sender)
{
if(NewItemsFound != null)
{
NewItemsFound(sender, new System.EventArgs());
}
}
}
After reading some of the comments and answers, I think my problem is that I am using a System.Timers.Timer not a System.Windows.Forms.Timer.
EDIT:
After changing to a Forms.Timer initial testing looks good (but no new items exist yet so have not seen the custom dialog). I added a bit of code to output the thread ID to a file when the update method is called. Using the Timers.Timer the thread ID was not the GUI thread, but using the Forms.Timer the thread ID is the same as the GUI.

Which timer are you using? System.Windows.Forms.Timer automatically fires the event on the UI thread. If you are using other one you will need to use Control.Invoke to call the method on UI thread.

You should use Forms.Timer here, or if you use other kind of timers, serialize calls to UI with .Invoke()

Is your application a WPF-Application? If so, you must delegate the work from your background-thread to the Dispatcher associated with the UI thread.
Post some code, so you can get better help and have a look at the Dispatcher class http://msdn.microsoft.com/en-us/library/system.windows.threading.dispatcher.invoke.aspx

private static System.Threading.SynchronizationContext _UI_Context;
//call this function once from the UI thread
internal static void init_CallOnUIThread()
{
_UI_Context = System.Threading.SynchronizationContext.Current;
}
public static void CallOnUIThread(Action action, bool asynchronous = false)
{
if (!asynchronous)
_UI_Context.Send((o) =>
{
action();
}, null);
else
_UI_Context.Post((o) =>
{
action();
}, null);
}

Understanding Delegates

It's not the first time I come across delegates and I am as confused as I were the last time and the time before that. So once and for all I want to get the delgate-confusion cleared up.
My problem is as followed
Having a Graphical User Interface which only displays a ListView with some boud items, I want to load the data from a Data Connection which takes some time, to increase the comfort of using the application I have instancieted a BackgroundWorker and in the doWork-method I want to fetch the data and display it.
This is how I want it
Create BackgroundWorker and appoint doWork_fetchData() method to the doWork event
Call the Async method of my Worker instance
Have the ListView updated without the User Interface beeing "frozen" during download of data.
Now this is Cross-Thread-Invoking and I wanted to solve this with Delegates which brings me here. Following this tutorial, I got a working Delegate, However it did not solve the problem, inside my delegate I cannot change my ListView, it still says it is on another thread.
I want to find an Easy explenation on delegates and how to use them to solve my problem. Also, should I think or design my software different?

Normally BackgroundWorker communicates with the UI thread using ReportProgress. You would hook up a delegate to receive those progress events before launching the background worker, and then the progress would be reported on the UI thread, where you're safe to change your ListView.
The other alternative in Windows Forms is to call Control.Invoke or Control.BeginInvoke, passing in a delegate which will update the UI. That delegate will be executed on the UI thread. For an example of this, see my threading tutorial or Joe Albahari's.
The equivalent of this in WPF is the Dispatcher - again, Invoke and BeginInvoke. You can access the dispatcher for a control with the Dispatcher property.

You can't change a ui control from a different thread directly, you need to check the Control.InvokeRequired property before you make a change.
See this example on msdn

Checkout this code, it does what you need:
public partial class Form1 : Form
{
public Form1()
{
InitializeComponent();
}
private void btnFill_Click(object sender, EventArgs e)
{
backgroundWorker1.DoWork += new DoWorkEventHandler(backgroundWorker1_DoWork);
backgroundWorker1.RunWorkerAsync();
}
private delegate void AddItemToListViewDelegate(ListView view, ListViewItem item);
private void AddItemToListView(ListView view, ListViewItem item)
{
if (InvokeRequired)
{
Invoke(new AddItemToListViewDelegate(AddItemToListView), new object[] { view, item });
return;
}
view.Items.Add(item);
}
private delegate void ClearListViewItemsDelegate(ListView view);
private void ClearListView(ListView view)
{
if (InvokeRequired)
{
Invoke(new ClearListViewItemsDelegate(ClearListView), new object[] { view });
return;
}
view.Items.Clear();
}
void backgroundWorker1_DoWork(object sender, DoWorkEventArgs e)
{
for (int i = 0; i < 100; i++)
{
if (i == 0)
ClearListView(listView1);
var item = new ListViewItem();
item.Name = i.ToString();
item.Text = item.Name;
AddItemToListView(listView1, item);
}
}
}
And for WPF something similar is required. Note this is not working code. As I don't use WPF I can't vouch that this is solid code, but it should give you an idea. You may need to create an type derived from EventArgs to encapsulate your listview and listviewitems.
If I get time, I'll edit this post so that it works, but that will have to wait until this evening!
using System.Windows.Threading;
...
if (listView1.Dispatcher.Thread != Thread.CurrentThread)
{
listView1.Dispatcher.BeginInvoke(
DispatcherPriority.Normal,
new EventHandler<ListViewAddEventArgs>(AddItemToListView), sender, new object[] { e } );
return;
}
listView1.Items.Add(e.File);

"Cross-thread operation not valid" exception on inner controls

I've been struggling with this for quite a while:
I have a function designed to add control to a panel with cross-thread handling, the problem is that though the panel and the control are in "InvokeRequired=false" - I get an exception telling me that one of the controls inner controls are accessed from a thread other than the thread it was created on, the snippet goes like this:
public delegate void AddControlToPanelDlgt(Panel panel, Control ctrl);
public void AddControlToPanel(Panel panel, Control ctrl)
{
if (panel.InvokeRequired)
{
panel.Invoke(new AddControlToPanelDlgt(AddControlToPanel),panel,ctrl);
return;
}
if (ctrl.InvokeRequired)
{
ctrl.Invoke(new AddControlToPanelDlgt(AddControlToPanel),panel,ctrl);
return;
}
panel.Controls.Add(ctrl); //<-- here is where the exception is raised
}
the exception message goes like this:
"Cross-thread operation not valid: Control 'pnlFoo' accessed from a thread other than the thread it was created on"
('pnlFoo' is under ctrl.Controls)
How can I add ctrl to panel?!
When the code reaches the "panel.Controls.Add(ctrl);" line - both panel and ctrl "InvokeRequired" property is set to false, the problem is the the controls inside ctrl has "InvokeRequired" set to true. To clarify things: "panel" is created on the base thread and "ctrl" on the new thread, therefore, "panel" has to be invoked (causing "ctrl" to need invoke again). Once both of the invokes are done, it reaches the panel.Controls.Add(ctrl) command (both "panel" and "ctrl" doesn't need invocation in this state)
Here is a small snippet of the full program:
public class ucFoo : UserControl
{
private Panel pnlFoo = new Panel();
public ucFoo()
{
this.Controls.Add(pnlFoo);
}
}
public class ucFoo2 : UserControl
{
private Panel pnlFooContainer = new Panel();
public ucFoo2()
{
this.Controls.Add(pnlFooContainer);
Thread t = new Thread(new ThreadStart(AddFooControlToFooConatiner());
t.Start()
}
private AddFooControlToFooConatiner()
{
ucFoo foo = new ucFoo();
this.pnlFooContainer.Controls.Add(ucFoo); //<-- this is where the exception is raised
}
}

As an aside - to save yourself having to create countless delegate types:
if (panel.InvokeRequired)
{
panel.Invoke((MethodInvoker) delegate { AddControlToPanel(panel,ctrl); } );
return;
}
Additionally, this now does regular static checks on the inner call to AddControlToPanel, so you can't get it wrong.

'panel' and 'ctrl' must be created on the same thread, ie. you cannot have panel.InvokeRequired return different value than ctrl.InvokeRequired. That is if both panel and ctrl have the handles created or belong to a container with the handle created. From MSDN:
If the control's handle does not yet
exist, InvokeRequired searches up the
control's parent chain until it finds
a control or form that does have a
window handle. If no appropriate
handle can be found, the
InvokeRequired method returns false.
As it is right now your code is open to race conditions because the panel.InvokeNeeded can return false because the panel is not yet created, then ctrl.InvokeNeeded will certainly return false because most likely ctrl is not yet added to any container and then by the time you reach panel.Controls.Add the panel was created in the main thread, so the call will fail.

Where is pnlFoo being created, and in which thread? Do you know when its handle is being created? If it's being created in the original (non-UI) thread, that's the problem.
All control handles in the same window should be created and accessed on the same thread. At that point, you shouldn't need two checks for whether Invoke is required, because ctrl and panel should be using the same thread.
If this doesn't help, please provide a short but complete program to demonstrate the problem.

Here is a working piece of code :
public delegate void AddControlToPanelDlg(Panel p, Control c);
private void AddControlToPanel(Panel p, Control c)
{
p.Controls.Add(c);
}
private void AddNewContol(object state)
{
object[] param = (object[])state;
Panel p = (Panel)param[0];
Control c = (Control)param[1]
if (p.InvokeRequired)
{
p.Invoke(new AddControlToPanelDlg(AddControlToPanel), p, c);
}
else
{
AddControlToPanel(p, c);
}
}
And here is how I tested it. You need to have a form with 2 buttons and one flowLayoutPanel (I chose this so I didn't have to care about location hwhen dinamically adding controls in the panel)
private void button1_Click(object sender, EventArgs e)
{
AddNewContol(new object[]{flowLayoutPanel1, CreateButton(DateTime.Now.Ticks.ToString())});
}
private void button2_Click(object sender, EventArgs e)
{
ThreadPool.QueueUserWorkItem(new WaitCallback(AddNewContol), new object[] { flowLayoutPanel1, CreateButton(DateTime.Now.Ticks.ToString()) });
}
I that he probem with your exaple is that when you get in the InvokeRequired branch you invoke the same function wou are in, resulting in a strange case of recurssion.

In your own answer you state:
To clarify things: "panel" is created on the base thread and "ctrl" on the new thread
I think this might be the cause of your problem. All UI elements should be created on the same thread (the base one). If you need to create "ctrl" as a consequence of some action in the new thread, then fire an event back to the base thread and do the creation there.

Lots of interesting answers here, but one key item for any multithreading in a Winform app is using the BackgroundWorker to initiate threads, and communicate back to the main Winform thread.

Here is a small snippet of the full program:
public class ucFoo : UserControl
{
private Panel pnlFoo = new Panel();
public ucFoo()
{
this.Controls.Add(pnlFoo);
}
}
public class ucFoo2 : UserControl
{
private Panel pnlFooContainer = new Panel();
public ucFoo2()
{
this.Controls.Add(pnlFooContainer);
Thread t = new Thread(new ThreadStart(AddFooControlToFooConatiner());
t.Start()
}
private AddFooControlToFooConatiner()
{
ucFoo foo = new ucFoo();
this.pnlFooContainer.Controls.Add(ucFoo); //<-- this is where the exception is raised
}
}