I'm trying to hide a form created on the main thread, from a secondary thread but I obviously get a cross-threading issue when I call the hide method. I'm new to this and don't really have a clue as to how to how to correct this. I've always just created a delegate to invoke my method if it's changing stuff created on the main thread, but I don't know how to do that here for the built-in hide method. Let me know if you need more information.
code:
public partial class MainForm : Form
{
ControlPanelForm m_controlPanel = new ControlPanelForm();
// ....
void MeterThread()
{
while (true)
{
// ....
if (EMOdetected)
{
m_controlPanel.Deinitialize();
m_controlPanel.Hide(); // **** //
}
}
}
}
Basically, my MainForm pulls up a control panel form that does some work. In the background I have a thread running and checking for stuff, one of which is an Emergency Off, at which point I want to shut my control panel down and then hide it.
If I try to invoke it right there,
m_controlPanel.Invoke(new EMOHandler(m_controlPanel.Hide)); // **** //
it doesn't look like it executes anything when i debug it. It seems to just pass over the command. Again, I'm new to this so any and all explanations are welcome.
There's no reason to check InvokeRequired or create an anonymous method. Simply write
mainForm.Invoke(new MethodInvoker(mainForm.Hide));
You haven't given any information code-wise but this is a common pattern for manipulating the UI thread from a non-UI thread.
if (mainForm.InvokeRequired)
{
mainForm.Invoke(new Action(() =>
{
mainForm.Hide();
}));
}
else
mainForm.Hide();
As a simple rule, which you already pointed out:
You should not access one window from another thread.
I would suggest you something like this:
public partial class Form1 : Form
{
public Form1()
{
InitializeComponent();
new Thread(SampleFunction).Start();
}
public void AppendTextBox(string value)
{
if (InvokeRequired)
{
this.Invoke(new Action<string>(AppendTextBox), new object[] {value});
return;
}
textBox1.Text += value;
}
void SampleFunction()
{
// Gets executed on a seperate thread and
// doesn't block the UI while sleeping
for(int i = 0; i<5; i++)
{
AppendTextBox("hi. ");
Thead.Sleep(1000);
}
}
}
This is a pretty nice example of how to use MultiThreading, which I got from here.
Though in your case, the method Hide already exists on MainForm and literally waits for you to be invoked, like others already pointed out:
mainForm.Invoke(new MethodInvoker(mainForm.Hide));
Related
This question already has answers here:
How do I update the GUI from another thread?
(47 answers)
Closed 9 years ago.
I have a thread that runs in parallel with the Main Form (UI). All it does (for now) is increment a counter every second. I want to display the value of the counter using a label in Windows Forms. Is that possible?
When I try the following code, I get a compile error in the ShowValue method. I have to declare ShowValue "static" so that I can call it from the background thread. But if I do that, I cannot use the "this." to access the label in ShowValue Form1.
Is this the correct way to do this?
Any tip would be appreciated, thanks!
private void count_secs()
{
while (!stopThread)
{
if (stopThread)
{
break;
}
num2++; // increment counter
Form1.ShowValue(num2); // display the counter value in the main Form
try
{
Thread.Sleep(1000); // wait 1 sec.
}
catch (ThreadInterruptedException)
{
if (stopThread)
{
break;
}
}
}
}
Then in my Form1 class, I have:
public static void ShowValue(int num)
{
this.label7.Text = num.ToString();
// compiler error here: "Keyword 'this' is not valid in a static method.
}
You can't refer a local variable (this.label7) in the static method ShowValue(int num)
your method should look like this:
public void ShowValue(int num)
{
if(label7.InvokeREquired)
{
Action a = () => ShowValue(num);
label7.Invoke(a);
}
else
this.label7.Text = num.ToString();
}
in this code, replace the static call to your form with an instance:
private void count_secs()
{
var frm = new Form1(); //create instance
frm.Show(); // show form
while (!stopThread)
{
if (stopThread)
{
break;
}
num2++; // increment counter
//use form instance
frm.ShowValue(num2); // display the counter value in the main Form
try
{
Thread.Sleep(1000); // wait 1 sec.
}
catch (ThreadInterruptedException)
{
if (stopThread)
{
break;
}
}
}
EDIT
You might want to decalre the form instance outside method count_secs()
You cannot randomly access GUI elements from different threads. The short answer to your problem is: Use existing structures.
If you just want to do things frequently, use a Timer. It will notify your main thread (that "owns" the GUI) when the time is up and you can update the GUI element there.
If you really want to create your own thread, use a Backgroundworker. It will offer thread-safe events from which you can update your GUI elements.
Your first issue is to get the Form Instance, if you don't have the Form instance on your calling form then you cause Application.OpenForms property like:
Form1 frm = Application.OpenForms["Form1"] as Form1;
if(frm != null)
frm.ShowValue(num2);
Your second issue is that you need to modify your method as instance method and to save it from Cross threaded exception modify it like:
public void ShowValue(int num)
{
if (label7.InvokeRequired)
{
label7.BeginInvoke((MethodInvoker)delegate { label7.Text = num.ToString(); });
}
else
{
label7.Text = num.ToString();
}
}
Two issues:
You cannot use the this reference from a static context.
You cannot update your UI from a background thread.
Solutions:
Mark the method ShowValue as an instance method (ie. get rid of the static)
Use a background worker or read this question which explains it very well
It's not mandatory to make the ShowValue function static. Leave it as non static and replace your line of logic Form1.ShowValue(num2) with following code.
if (label1.InvokeRequired)
label1.BeginInvoke(new Action(() => ShowValue(num2)));
else
label1.Invoke(new Action(() => ShowValue(num2)));
On click of a button I have this code with is should show a dialog on top of the current form and display text, wait for one second, change the text and then finally close it:
Form p = new Form();
p.ShowDialog();
p.Text = "Start.";
Thread.Sleep(1000);
p.Text = "Counting.";
Thread.Sleep(1000);
p.Text = "End.";
Thread.Sleep(1000);
p.Close();
However once it executes p.ShowDialog(); it stops the code until the form p is closed and it doesn't work as I intended it to. Can I get some guidance on this? Not necessarily the solution, but at least maybe some keywords I could google on?
UPDATE: due to the difficulties I am facing trying to access business logic, which is irrelevant to the problem, I am delaying providing the working example. Stay tuned and sorry :)
SOLUTION: what I did is in fact used Show() instead of ShowDialog(). Since i was impossible to access form from business logic, BackgroundWorker came in handy and was being used between them. I cannot share any code or the layout of the project structure, but in conclusion, the accepted answer's main statement was the key to the solution :)
That is the point of ShowDialog(). It creates a modal form and does not return control to the calling function until you are done. If it doesn't need to be modal, then use .Show(). If it does need to be modal, then put code in the Form Load method to update the text as needed.
http://msdn.microsoft.com/en-us/library/c7ykbedk.aspx
taken from the link above:
When this method is called, the code following it is not executed until after the dialog box is closed.
if you want to form to display whatever it is you want to display you should write the code inside the the form itself, do that in an eventhandler of the form show event.
As you have found, ShowDialog is a blocking method that does not return until the dialog is closed. Your code to change the text and handle the delay needs to be within the dialogue itself.
However, it's worth noting the next problem that you'll find: if you call Thread.Sleep(1000) from the UI thread, your application will become unresponsive for 1 second at a time. This is probably not what you're aiming for! I'd suggest you look into the Timer or BackgroundWorker classes to handle this more smoothly.
Check this out:
public partial class Form2 : Form
{
delegate void SetTextCallback(string text);
delegate void CloseFormCallback();
public Form2()
{
InitializeComponent();
new Thread(DoMagic).Start();
}
public void DoMagic()
{
this.SetText("Start.");
Thread.Sleep(1000);
this.SetText("Counting.");
Thread.Sleep(1000);
this.SetText("End");
Thread.Sleep(1000);
this.CloseForm();
}
private void CloseForm()
{
if (this.InvokeRequired)
{
CloseFormCallback c = new CloseFormCallback(CloseForm);
this.Invoke(c);
}
else
{
this.Close();
}
}
private void SetText(string text)
{
if (this.label1.InvokeRequired)
{
SetTextCallback d = new SetTextCallback(SetText);
this.Invoke(d, new object[] { text });
}
else
{
this.label1.Text = text;
}
}
}
im new to C# language and i would appreciate any help/feedback on the following issue.
Basically, im trying to develop a program such that it has two classes class1(main thread) and class2. class1 has a button and class2 starts a thread and do some calculations. How can i update any property of the button in class1(main) from class2 within the thread in class2?
here is an example i would like to do
// CLASS 1 is a win form
class1:form
{
public btn_click()
{
// call function startthread from class2
btn.Enabled = false;
startthread()
}
}
// CLASS 2 is not a winform
class2
{
public startthread()
{
Thread worker = new Thread(doCalculation)
}
public doCalculation()
{
// do some calculation then
// blahhh
// blahhh
// HERE I NEED SOMEHOW ENABLED MY BUTTON
}
}
This depends a bit on the UI framework you're using. As an example, say you wanted to update the text written on the button.
If you're using WPF, you could use:
class1Instance.TheButton.Dispatcher
.BeginInvoke( (Action) () => class1Instance.TheButton.Content = "Foo" );
With Windows Forms, you'd use:
class1Instance.TheButton
.BeginInvoke( (Action) () => class1Instance.TheButton.Text = "Foo" );
You have to bring the modifying code back onto the the main thread, regardless whether its WPF or Winforms, cross-thread access to the UI is either explicitly disallowed (usually) or strongly discouraged and buggy.
There are a lot of ways to do this, in Winforms I typically use Control.Invoke like this:
public void UpdateMyTextBox(string NewText)
{
if(InvokeRequired)
Invoke(new Action<string>(UpdateMyTextBox), NewText);
else
myTextBox.Text = NewText;
}
In this example, the InvokeRequired property will check to see if it's being called on the UI thread, and if not, we will call Invoke which will place the request onto the UI thread.
In WPF, you watch to use the Dispatcher, and you do so in a very similar way:
public void UpdateMyWpfTextBox(string NewText)
{
if(!CheckAccess())
Dispatcher.Invoke(new Action<string>(UpdateMyWpfTextBox), NewText);
else
myTextBox.Text = NewText;
}
Not to sure what your trying to do here, but this is best an answer I can come up with, considering the questions. I'm going to assume this is a winforms application
Class2 is going to need a reference to the button in Class 1. So you'll need to pass that to it.
Once Class2 has reference to the button you can attempt to modify the property as
btn.Text = "New Text";
I'm going to assume again that you've tried this route, got an exception and so you posted your question. The reason you got the exception was that your btn and your class2 are running in separate threads. To get around this, you can call invoke on the button. I often handle this like this.
private void SetButtonText(string newText) {
if (btn.InvokeRequired) {
Invoke((MethodInvoker)(() => SetButtonText(newText)));
}
else {
btn.Text = newText;
}
}
I have a windows forms program with a form MainForm. On a button press I start a code that runs (pulses) on every 0.5secs on another thread. I want to modify many things, like labels, progressbars on my MainForm, from the Pulse method. How is this possible?
So I would like to know, how to interract with variables, values, in that thread, and the MainForm. Modify each other, etc..
On foo button click, I tell my pulsator to start.
Pulsator.Initialize();
Here is the Pulsator class:
public static class Pulsator
{
private static Thread _worker;
public static void Initialize()
{
_worker = new Thread(Pulse);
_worker.IsBackground = true;
_worker.Start();
}
public static void Close()
{
if (_worker != null)
{
_worker.Abort();
while (_worker.IsAlive || _worker.ThreadState != ThreadState.Stopped)
{
//closing
}
}
}
public static void Pulse()
{
if (_worker != null)
{
while (true)
{
SomeOtherClass.Pulse();
Thread.Sleep(500);
}
}
else
{
SomeOtherClass.Pulse(); // yeah I know this doesnt needed
}
}
}
SomeOtherClass Pulse method looks like :
public static void Pulse()
{
//here I will have several values, variables, and I want to show results,
// values on my MainForm, like:
Random random = new Random();
MainForm.label1.Text = random.Next(123,321).ToString(); // I hope you know what I mean
}
Of course it's much complicated, it's just a silly example.
Generally, in WinForms it's not safe to modify the state of visual controls outside the thread that owns the control's underlying unmanaged resources (window handle). You have to use the Control.Invoke method to schedule executing the modification on the control's owning thread.
As others already mentioned, you have to use Control.Invoke to change the UI controls from the background thread.
Another option is to use System.ComponentModel.BackgroundWorker (it's available in the form designer toolbox). You could then take a regular forms timer, to call the RunWorkerAsync-Method and do your background work in the DoWork event handler, which is automatically called from another thread.
From there, you can hand data back to the main thread, by calling ReportProgress. This will raise the ProgressChanged event in the main thread, where you are free to update all your UI controls.
Why not use a System.Timers.Timer?
E.g.:
trainPassageTimer = new Timer(500);
trainPassageTimer.AutoReset = true;
trainPassageTimer.Elapsed += TimeElapsed;
...
private void TimeElapsed(object sender, ElapsedEventArgs elapsedEventArgs)
{
// Do stuff
// Remember to use BeginInvoke or Invoke to access Windows.Forms controls
}
C# 2 or higher (VS2005) has anonymous delegates (and C# 3 has lambdas which are a slightly neater version of the same idea).
These allow a thread to be started with a function that can "see" variables in the surrounding scope. So there is no need to explicitly pass it anything. On the downside, there is the danger that the thread will accidentally depend on something that it should not (e.g. a variable that is changing in other threads).
_worker = new Thread(delegate
{
// can refer to variables in enclosing scope(s).
});
I don't know why this is happening, but when I create a new form inside an EventHandler, it disappears as soon as the method is finished.
Here's my code. I've edited it for clarity, but logically, it is exactly the same.
static void Main()
{
myEventHandler = new EventHandler(launchForm);
// Code that creates a thread which calls
// someThreadedFunction() when finished.
}
private void someThreadedFunction()
{
//Do stuff
//Launch eventhandler
EventHandler handler = myEventHandler;
if (handler != null)
{
handler(null, null);
myEventHandler = null;
}
}
private void launchForm(object sender, EventArgs e)
{
mf = new myForm();
mf.Show();
MessageBox.Show("Do you see the form?");
}
private myForm mf;
private EventHandler myEventHandler;
The new form displays as long as the MessageBox "Do you see the form?" is there. As soon as I click OK on it, the form disappears.
What am I missing? I thought that by assigning the new form to a class variable, it would stay alive after the method finished. Apparently, this is not the case.
I believe the problem is that you are executing the code within the handler from your custom thread, and not the UI thread, which is required because it operates the Windows message pump. You want to use the Invoke method here to insure that the form gets and shown on the UI thread.
private void launchForm(object sender, EventArgs e)
{
formThatAlreadyExists.Invoke(new MethodInvoker(() =>
{
mf = new myForm();
mf.Show();
MessageBox.Show("Do you see the form?");
}));
}
Note that this assumes you already have a WinForms object (called formThatAlreadyExists) that you have run using Application.Run. Also, there may be a better place to put the Invoke call in your code, but this is at least an example of it can be used.
I think if you create a form on a thread, the form is owned by that thread. When creating any UI elements, it should always be done on the main (UI) thread.
this looks as if you are not on the form sta thread so once you show the form it is gone and the thread finishes it's job it kills it self since there is nothing referenceing the thread. Its not the best solution out there for this but you ca use a showdialog() rather than a show to accomplish it keeping state if you need a code example i use this exact same process for a "loading...." form
public class Loading
{
public delegate void EmptyDelegate();
private frmLoadingForm _frmLoadingForm;
private readonly Thread _newthread;
public Loading()
{
Console.WriteLine("enteredFrmLoading on thread: " + Thread.CurrentThread.ManagedThreadId);
_newthread = new Thread(new ThreadStart(Load));
_newthread.SetApartmentState(ApartmentState.STA);
_newthread.Start();
}
public void Load()
{
Console.WriteLine("enteredFrmLoading.Load on thread: " + Thread.CurrentThread.ManagedThreadId);
_frmLoadingForm = new frmLoadingForm();
if(_frmLoadingForm.ShowDialog()==DialogResult.OK)
{
}
}
/// <summary>
/// Closes this instance.
/// </summary>
public void Close()
{
Console.WriteLine("enteredFrmLoading.Close on thread: " + Thread.CurrentThread.ManagedThreadId);
if (_frmLoadingForm != null)
{
if (_frmLoadingForm.InvokeRequired)
{
_frmLoadingForm.Invoke(new EmptyDelegate(_frmLoadingForm.Close));
}
else
{
_frmLoadingForm.Close();
}
}
_newthread.Abort();
}
}
public partial class frmLoadingForm : Form
{
public frmLoadingForm()
{
InitializeComponent();
}
}
Is
dbf.Show();
a typo? Is it supposed to be this instead?
mf.Show();
Is it possible that there is another form that you are showing other than the one you intend to show?
You created a window on a non UI thread. When the thread aborts it will take your window along with it. End of story.
Perform invoke on the main form passing a delegate which will execute the method that creates the messagebox on the UI thread.
Since the MessageBox is a modal window, if dont want the launchForm method to block the background thread, create a custom form with the required UI and call show() on it, not ShowDialog().