I want to learn more about threading and created a little test application that change the backcolor of a label.
public partial class Form1 : Form
{
public Form1()
{
InitializeComponent();
}
//lblColor
public Color theLabel
{
get { return this.lblColor.BackColor; }
set { this.lblColor.BackColor = value; }
}
//btnStart
private void btnStart_Click(object sender, EventArgs e)
{
ThreadTest cColor = new ThreadTest();
Thread tColor = new Thread(new ThreadStart(cColor.ChangeColor));
tColor.Start();
}
}
And...
public class ThreadTest
{
public void ChangeColor()
{
Form1 foo = new Form1();
while (true)
{
foo.theLabel = Color.Aqua;
foo.theLabel = Color.Black;
foo.theLabel = Color.DarkKhaki;
foo.theLabel = Color.Green;
}
}
}
The only problem is why can't i make this code work? I can see that the code in ChangeColor runs but the color of the label don't change.
At first glance, you are constructing a new form
Form1 foo = new Form1();
inside ThreadTest and never displaying the form, my guess is you intended to change the form color on the form with btnStart? You have two options, either pass in the form in to a ParameterizedThreadStart or re-write the code to just operate on the existing form.
Also, based on the code written, you will likely need to use Invoke to update the state of the form as you cannot have worker threads update the UI. I will tweak your code and posted a revised example if someone doesn't beat me to it.
Edit
In this case you don't need the invoke... but here is what I think you were intending...
private void btnStart_Click(object sender, EventArgs e)
{
ThreadTest cColor = new ThreadTest();
Thread tColor = new Thread(new ParameterizedThreadStart(cColor.ChangeColor));
tColor.Start(this);
}
public class ThreadTest
{
public void ChangeColor(Object state)
{
Form1 foo = (Form1) state;
while (true)
{
foo.theLabel = Color.Aqua;
foo.theLabel = Color.Black;
foo.theLabel = Color.DarkKhaki;
foo.theLabel = Color.Green;
}
}
}
Also, it is important to set worker threads as background threads, otherwise when you close the form, the thread will keep that appliaction open.
tColor.IsBackground = true;
Additional Example
A slightly different example would be to have multiple threads trying to update the same value and see how they are interleaved... simple snippet to get the ball rolling.
private void btnStart_Click(object sender, EventArgs e)
{
CreateBackgroundColorSetter(Color.Aqua);
CreateBackgroundColorSetter(Color.Black);
CreateBackgroundColorSetter(Color.DarkKhaki);
CreateBackgroundColorSetter(Color.Green);
}
private void CreateBackgroundColorSetter(Color color)
{
var thread = new Thread(() =>
{
while (true)
{
theLabel = color;
Thread.Sleep(100);
}
});
thread.IsBackground = true;
thread.Start();
}
you have 2 issues
a) you are not supposed to update the UI from other threads
b) even if you were allowed to do it you need to tell the UI to repaint
These are non trivial issues
If you are just experimenting with threading then use Debug.WriteLine to see what the background threads are doing
If you actually need to update the UI in the background then lookup BeginInvoke and InvokeNeeded
If you really need to do threading in a GUI app you are going to need BackgroundWorker or something like it. As noted, other threads cannot update the GUI anyway so the sample shown there is a better model for moving long-running work items out of your main thread.
The BackgroundWorker class allows you
to run an operation on a separate,
dedicated thread. Time-consuming
operations like downloads and database
transactions can cause your user
interface (UI) to seem as though it
has stopped responding while they are
running. When you want a responsive UI
and you are faced with long delays
associated with such operations, the
BackgroundWorker class provides a
convenient solution.
If you want do experiments with threading just create a console application and google "threading tutorial c#" to get some example.
Regarding threading and winform you can have a look at the below link just to have an idea about the consideration you should make before access win form property from another thread
In WinForms, why can't you update UI controls from other threads?
Related
I am trying to start a winForm from a thread, but when i do so, the form show but none of the labels are loaded ( the background where they should be is white ) and the form is frozen.
I've tried it with some other winForms that i know that work just fine and it still doesn't seem to work ? Has anyone encountered this problem ?
I know the question is vague but there isn't really any specific code that I could give to help understand the problem.
That is because the Message Loop runs on UI thread only. And when a control or window is created in any other thread, it cannot access that message loop. And thus, cannot process user input.
To solve this, try creating a window from UI thread and create a thread from that window to do whatever you want to do in different thread.
UI thread is supposed to be one.
Then, I suggest you to open your form calling a method of your original form thread, like in the example below:
(To test it just create an empty form called MainForm and paste this code in it)
public delegate void OpenFormDelegate(string txt);
public partial class MainForm : Form
{
public MainForm()
{
InitializeComponent();
var button1 = new Button();
button1.Text = "Run for 5 secs and open new window";
button1.Dock = DockStyle.Top;
this.Controls.Add(button1);
button1.Click += new EventHandler(button1_Click);
}
private void button1_Click(object sender, EventArgs e)
{
Thread t = new Thread(new ThreadStart(Run));
t.Start();
}
public void Run()
{
Thread.Sleep(5000); // sleep for 5 seconds
this.BeginInvoke(new OpenFormDelegate(OpenNewForm), "Hello World !");
}
public void OpenNewForm(string text)
{
Form f = new Form();
f.Text = text;
f.Show();
}
}
It is related to thread access, when the new form is created it will not be able to access the UI thread. Use the main thread to create the form and new thread to process the infomation.
I'm trying to use a background worker to update a listbox used for a status window in my Form in C#. It doesn't appear to work properly when the addToStausLog() method is called from another class outside of the MyForm class even though I pass an instance of the form to the other class that's calling the addToStatusLog update member. Instead the update doesn't happen until the class member finished and returns back to the MyForm class. Maybe there's a better a approach to creating real-time status windows that will run from any class that MyForm is passed into. I'm new to worker threads, so could someone review and let me know what I might be doing wrong or could improve on.
public MyForm()
{
InitializeComponent();
// Setup background task to update listbox status so UI is unaffected
_lListBoxQue = new List<string>();
bw_listBoxBGWorker = new BackgroundWorker();
bw_listBoxBGWorker.DoWork += (o, args) => LstbxThread_doWork();
bw_listBoxBGWorker.RunWorkerCompleted += (o, args) => LstbxThread_completed();
}
private void LstbxThread_doWork()
{
System.Threading.Thread.Sleep(100);
}
private void LstbxThread_completed()
{
// Update listbox
lstStatusBox.BeginUpdate();
lstStatusBox.Items.Clear(); // clear entries
lstStatusBox.Items.AddRange(_lListBoxQue.ToArray());
lstStatusBox.EndUpdate();
}
public String addToStatusLog(String sMsg)
{
_lListBoxQue.Add(sMsg);
if (_lListBoxQue.Count > _iStatusLogMaxLines) // > max?
_lListBoxQue.RemoveAt(0); // remove top element?
if( !bw_listBoxBGWorker.IsBusy ) // background not busy?
bw_listBoxBGWorker.RunWorkerAsync(); // update listbox in back ground task
System.Threading.Thread.Sleep(100);
return sMsg;
}
This is the member that calls another class which attempts to call the addToStatusLog several times during the process, but the updates to the listbox don't happen until the MyClass(this).updateDB() finishes. I need to see real-time updates as the updateDB() function is running. There has to be a way to make this work, I'm hoping...
private void btnUpdateDB_Click(object sender, EventArgs e)
{
if (_bIsUpdateEventRunning == false ) // is event not busy?
{
_bIsUpdateEventRunning = true;
new MyClass(this).updateDB();
_bIsUpdateEventRunning = false;
}
}
Example of class called to update the form listbox.
Public class MyClass{
private MyForm _pForm;
public MyClass(MyForm pForm){ _pForm= pForm; }
public void updateDB(){
_pForm.addToStatusLog("Hello World");
}
}
Updated Fix w/o background worker:
public String addToStatusLog(String sMsg)
{
_lListBoxQue.Add(sMsg);
if (_lListBoxQue.Count > _iStatusLogMaxLines) // > max?
_lListBoxQue.RemoveAt(0); // remove top element?
lstStatusBox.BeginUpdate();
lstStatusBox.Items.Clear(); // clear entries
lstStatusBox.Items.AddRange(_lListBoxQue.ToArray());
lstStatusBox.EndUpdate();
Application.DoEvents();
return sMsg;
}
Thread.Sleep is not the answer here. What you likely need is Application.DoEvents. This processes all messages currently waiting in the Windows message queue.
Thread.Sleep just tells the thread to go to sleep for the number of milliseconds you specify. If your background worker is running on the UI thread, you're putting the UI thread to sleep and it's effectively comatose. (Important: All Windows forms run on the UI thread.)
There are, of course, alternative designs that involve spinning up separate threads of execution. But these have their own issues, and you should be mindful of them before running blindly down that path.
I'm getting some troubles with my Winforms C# app.
I wish to make form named Popup closing after some operations in main thread are done. The problem is an exception caused by cross-thread form closing.
private void loginButton_Click(object sender, EventArgs e)
{
LoginProcess.Start(); // Running Form.show() in new thread
ActiveAcc.IsValid = false;
ActiveAcc.Username = userBox.Text;
try
{
LoginCheck(userBox.Text, passBox.Text);
}
catch (IOException)
{
MessageBox.Show("..");
return;
}
catch (SocketException)
{
MessageBox.Show("..");
return;
}
if (ActiveAcc.IsValid)
{
MessageBox.Show("..");
Close();
}
else
{
Popup.Close(); // Error caused by closing form from different thread
MessageBox.Show("");
}
}
public Login() // 'Main' form constructor
{
InitializeComponent();
ActiveAcc = new Account();
Popup = new LoginWaiter();
LoginProcess = new Thread(Popup.Show); //Popup is an ordinary Form
}
I've been trying to use various tools such as LoginProcess.Abort() or Popup.Dispose() to make it work properly, but even if app is working on runtime environment its still unstable due to Exceptions which are thrown.
I would be grateful for any help, and I am sorry for ambiguities in issue describing.
Why don't you let the UI thread do UI stuff like opening and closing Forms, and spawn the other thread (or background worker, or async task) to do the other stuff?
IMO, having other threads attempt to interact with elements on the UI thread (e.g., have a background thread directly set the text of a label or some such) is asking for heartache.
If you simply must keep your code as is, here is a fairly simple thing you could do. In Popup, add a static bool that defaults to true. Also in Popup, add a timer task that once every X milliseconds checks the status of that boolean. If it finds that the value has been set to false, let Popup tell itself to close within that timer tick.
I'm not crazy about this design, but it could look something like:
public partial class Popup : Form
{
public static bool StayVisible { get; set; }
private System.Windows.Forms.Timer timer1;
public Popup()
{
StayVisible = true;
this.timer1.Interval = 1000;
this.timer1.Tick += new System.EventHandler(this.timer1_Tick);
InitializeComponent();
}
private void timer1_Tick(object sender, EventArgs e)
{
if (!StayVisible) this.Close();
}
}
Then, from another thread, when you want Popup to close, call
Popup.StayVisible = false;
Better yet, you would fire an event that Popup would receive so that it could close itself. Since you intend to use multiple threads, you'll have to deal with raising events cross-thread.
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;
}
I am trying to start a winForm from a thread, but when i do so, the form show but none of the labels are loaded ( the background where they should be is white ) and the form is frozen.
I've tried it with some other winForms that i know that work just fine and it still doesn't seem to work ? Has anyone encountered this problem ?
I know the question is vague but there isn't really any specific code that I could give to help understand the problem.
That is because the Message Loop runs on UI thread only. And when a control or window is created in any other thread, it cannot access that message loop. And thus, cannot process user input.
To solve this, try creating a window from UI thread and create a thread from that window to do whatever you want to do in different thread.
UI thread is supposed to be one.
Then, I suggest you to open your form calling a method of your original form thread, like in the example below:
(To test it just create an empty form called MainForm and paste this code in it)
public delegate void OpenFormDelegate(string txt);
public partial class MainForm : Form
{
public MainForm()
{
InitializeComponent();
var button1 = new Button();
button1.Text = "Run for 5 secs and open new window";
button1.Dock = DockStyle.Top;
this.Controls.Add(button1);
button1.Click += new EventHandler(button1_Click);
}
private void button1_Click(object sender, EventArgs e)
{
Thread t = new Thread(new ThreadStart(Run));
t.Start();
}
public void Run()
{
Thread.Sleep(5000); // sleep for 5 seconds
this.BeginInvoke(new OpenFormDelegate(OpenNewForm), "Hello World !");
}
public void OpenNewForm(string text)
{
Form f = new Form();
f.Text = text;
f.Show();
}
}
It is related to thread access, when the new form is created it will not be able to access the UI thread. Use the main thread to create the form and new thread to process the infomation.