Related
I have a scenario. (Windows Forms, C#, .NET)
There is a main form which hosts some user control.
The user control does some heavy data operation, such that if I directly call the UserControl_Load method the UI become nonresponsive for the duration for load method execution.
To overcome this I load data on different thread (trying to change existing code as little as I can)
I used a background worker thread which will be loading the data and when done will notify the application that it has done its work.
Now came a real problem. All the UI (main form and its child usercontrols) was created on the primary main thread. In the LOAD method of the usercontrol I'm fetching data based on the values of some control (like textbox) on userControl.
The pseudocode would look like this:
CODE 1
UserContrl1_LoadDataMethod()
{
if (textbox1.text == "MyName") // This gives exception
{
//Load data corresponding to "MyName".
//Populate a globale variable List<string> which will be binded to grid at some later stage.
}
}
The Exception it gave was
Cross-thread operation not valid: Control accessed from a thread other than the thread it was created on.
To know more about this I did some googling and a suggestion came up like using the following code
CODE 2
UserContrl1_LoadDataMethod()
{
if (InvokeRequired) // Line #1
{
this.Invoke(new MethodInvoker(UserContrl1_LoadDataMethod));
return;
}
if (textbox1.text == "MyName") // Now it won't give an exception
{
//Load data correspondin to "MyName"
//Populate a globale variable List<string> which will be binded to grid at some later stage
}
}
But it still seems that I've come back to square one. The Application again
becomes unresponsive. It seems to be due to the execution of line #1 if condition. The loading task is again done by the parent thread and not the third that I spawned.
I don't know whether I perceived this right or wrong.
How do I resolve this and also what is the effect of execution of Line#1 if block?
The situation is this: I want to load data into a global variable based on the value of a control. I don't want to change the value of a control from the child thread. I'm not going to do it ever from a child thread.
So only accessing the value so that the corresponding data can be fetched from the database.
As per Prerak K's update comment (since deleted):
I guess I have not presented the question properly.
Situation is this: I want to load data into a global variable based on the value of a control. I don't want to change the value of a control from the child thread. I'm not going to do it ever from a child thread.
So only accessing the value so that corresponding data can be fetched from the database.
The solution you want then should look like:
UserContrl1_LOadDataMethod()
{
string name = "";
if(textbox1.InvokeRequired)
{
textbox1.Invoke(new MethodInvoker(delegate { name = textbox1.text; }));
}
if(name == "MyName")
{
// do whatever
}
}
Do your serious processing in the separate thread before you attempt to switch back to the control's thread. For example:
UserContrl1_LOadDataMethod()
{
if(textbox1.text=="MyName") //<<======Now it wont give exception**
{
//Load data correspondin to "MyName"
//Populate a globale variable List<string> which will be
//bound to grid at some later stage
if(InvokeRequired)
{
// after we've done all the processing,
this.Invoke(new MethodInvoker(delegate {
// load the control with the appropriate data
}));
return;
}
}
}
Threading Model in UI
Please read the Threading Model in UI applications (old VB link is here) in order to understand basic concepts. The link navigates to page that describes the WPF threading model. However, Windows Forms utilizes the same idea.
The UI Thread
There is only one thread (UI thread), that is allowed to access System.Windows.Forms.Control and its subclasses members.
Attempt to access member of System.Windows.Forms.Control from different thread than UI thread will cause cross-thread exception.
Since there is only one thread, all UI operations are queued as work items into that thread:
If there is no work for UI thread, then there are idle gaps that can be used by a not-UI related computing.
In order to use mentioned gaps use System.Windows.Forms.Control.Invoke or System.Windows.Forms.Control.BeginInvoke methods:
BeginInvoke and Invoke methods
The computing overhead of method being invoked should be small as well as computing overhead of event handler methods because the UI thread is used there - the same that is responsible for handling user input. Regardless if this is System.Windows.Forms.Control.Invoke or System.Windows.Forms.Control.BeginInvoke.
To perform computing expensive operation always use separate thread. Since .NET 2.0 BackgroundWorker is dedicated to performing computing expensive operations in Windows Forms. However in new solutions you should use the async-await pattern as described here.
Use System.Windows.Forms.Control.Invoke or System.Windows.Forms.Control.BeginInvoke methods only to update a user interface. If you use them for heavy computations, your application will block:
Invoke
System.Windows.Forms.Control.Invoke causes separate thread to wait till invoked method is completed:
BeginInvoke
System.Windows.Forms.Control.BeginInvoke doesn't cause the separate thread to wait till invoked method is completed:
Code solution
Read answers on question How to update the GUI from another thread in C#?.
For C# 5.0 and .NET 4.5 the recommended solution is here.
You only want to use Invoke or BeginInvoke for the bare minimum piece of work required to change the UI. Your "heavy" method should execute on another thread (e.g. via BackgroundWorker) but then using Control.Invoke/Control.BeginInvoke just to update the UI. That way your UI thread will be free to handle UI events etc.
See my threading article for a WinForms example - although the article was written before BackgroundWorker arrived on the scene, and I'm afraid I haven't updated it in that respect. BackgroundWorker merely simplifies the callback a bit.
I know its too late now. However even today if you are having trouble accessing cross thread controls? This is the shortest answer till date :P
Invoke(new Action(() =>
{
label1.Text = "WooHoo!!!";
}));
This is how i access any form control from a thread.
I have had this problem with the FileSystemWatcher and found that the following code solved the problem:
fsw.SynchronizingObject = this
The control then uses the current form object to deal with the events, and will therefore be on the same thread.
I find the check-and-invoke code which needs to be littered within all methods related to forms to be way too verbose and unneeded. Here's a simple extension method which lets you do away with it completely:
public static class Extensions
{
public static void Invoke<TControlType>(this TControlType control, Action<TControlType> del)
where TControlType : Control
{
if (control.InvokeRequired)
control.Invoke(new Action(() => del(control)));
else
del(control);
}
}
And then you can simply do this:
textbox1.Invoke(t => t.Text = "A");
No more messing around - simple.
Controls in .NET are not generally thread safe. That means you shouldn't access a control from a thread other than the one where it lives. To get around this, you need to invoke the control, which is what your 2nd sample is attempting.
However, in your case all you've done is pass the long-running method back to the main thread. Of course, that's not really what you want to do. You need to rethink this a little so that all you're doing on the main thread is setting a quick property here and there.
The cleanest (and proper) solution for UI cross-threading issues is to use SynchronizationContext, see Synchronizing calls to the UI in a multi-threaded application article, it explains it very nicely.
Follow the simplest (in my opinion) way to modify objects from another thread:
using System.Threading.Tasks;
using System.Threading;
namespace TESTE
{
public partial class Form1 : Form
{
public Form1()
{
InitializeComponent();
}
private void button1_Click(object sender, EventArgs e)
{
Action<string> DelegateTeste_ModifyText = THREAD_MOD;
Invoke(DelegateTeste_ModifyText, "MODIFY BY THREAD");
}
private void THREAD_MOD(string teste)
{
textBox1.Text = teste;
}
}
}
A new look using Async/Await and callbacks. You only need one line of code if you keep the extension method in your project.
/// <summary>
/// A new way to use Tasks for Asynchronous calls
/// </summary>
public class Example
{
/// <summary>
/// No more delegates, background workers etc. just one line of code as shown below
/// Note it is dependent on the XTask class shown next.
/// </summary>
public async void ExampleMethod()
{
//Still on GUI/Original Thread here
//Do your updates before the next line of code
await XTask.RunAsync(() =>
{
//Running an asynchronous task here
//Cannot update GUI Thread here, but can do lots of work
});
//Can update GUI/Original thread on this line
}
}
/// <summary>
/// A class containing extension methods for the Task class
/// Put this file in folder named Extensions
/// Use prefix of X for the class it Extends
/// </summary>
public static class XTask
{
/// <summary>
/// RunAsync is an extension method that encapsulates the Task.Run using a callback
/// </summary>
/// <param name="Code">The caller is called back on the new Task (on a different thread)</param>
/// <returns></returns>
public async static Task RunAsync(Action Code)
{
await Task.Run(() =>
{
Code();
});
return;
}
}
You can add other things to the Extension method such as wrapping it in a Try/Catch statement, allowing caller to tell it what type to return after completion, an exception callback to caller:
Adding Try Catch, Auto Exception Logging and CallBack
/// <summary>
/// Run Async
/// </summary>
/// <typeparam name="T">The type to return</typeparam>
/// <param name="Code">The callback to the code</param>
/// <param name="Error">The handled and logged exception if one occurs</param>
/// <returns>The type expected as a competed task</returns>
public async static Task<T> RunAsync<T>(Func<string,T> Code, Action<Exception> Error)
{
var done = await Task<T>.Run(() =>
{
T result = default(T);
try
{
result = Code("Code Here");
}
catch (Exception ex)
{
Console.WriteLine("Unhandled Exception: " + ex.Message);
Console.WriteLine(ex.StackTrace);
Error(ex);
}
return result;
});
return done;
}
public async void HowToUse()
{
//We now inject the type we want the async routine to return!
var result = await RunAsync<bool>((code) => {
//write code here, all exceptions are logged via the wrapped try catch.
//return what is needed
return someBoolValue;
},
error => {
//exceptions are already handled but are sent back here for further processing
});
if (result)
{
//we can now process the result because the code above awaited for the completion before
//moving to this statement
}
}
This is not the recommended way to solve this error but you can suppress it quickly, it will do the job . I prefer this for prototypes or demos . add
CheckForIllegalCrossThreadCalls = false
in Form1() constructor .
You need to look at the Backgroundworker example:
http://msdn.microsoft.com/en-us/library/system.componentmodel.backgroundworker.aspx
Especially how it interacts with the UI layer. Based on your posting, this seems to answer your issues.
Here is an alternative way if the object you are working with doesn't have
(InvokeRequired)
This is useful if you are working with the main form in a class other than the main form with an object that is in the main form, but doesn't have InvokeRequired
delegate void updateMainFormObject(FormObjectType objectWithoutInvoke, string text);
private void updateFormObjectType(FormObjectType objectWithoutInvoke, string text)
{
MainForm.Invoke(new updateMainFormObject(UpdateObject), objectWithoutInvoke, text);
}
public void UpdateObject(ToolStripStatusLabel objectWithoutInvoke, string text)
{
objectWithoutInvoke.Text = text;
}
It works the same as above, but it is a different approach if you don't have an object with invokerequired, but do have access to the MainForm
I found a need for this while programming an iOS-Phone monotouch app controller in a visual studio winforms prototype project outside of xamarin stuidio. Preferring to program in VS over xamarin studio as much as possible, I wanted the controller to be completely decoupled from the phone framework. This way implementing this for other frameworks like Android and Windows Phone would be much easier for future uses.
I wanted a solution where the GUI could respond to events without the burden of dealing with the cross threading switching code behind every button click. Basically let the class controller handle that to keep the client code simple. You could possibly have many events on the GUI where as if you could handle it in one place in the class would be cleaner. I am not a multi theading expert, let me know if this is flawed.
public partial class Form1 : Form
{
private ExampleController.MyController controller;
public Form1()
{
InitializeComponent();
controller = new ExampleController.MyController((ISynchronizeInvoke) this);
controller.Finished += controller_Finished;
}
void controller_Finished(string returnValue)
{
label1.Text = returnValue;
}
private void button1_Click(object sender, EventArgs e)
{
controller.SubmitTask("Do It");
}
}
The GUI form is unaware the controller is running asynchronous tasks.
public delegate void FinishedTasksHandler(string returnValue);
public class MyController
{
private ISynchronizeInvoke _syn;
public MyController(ISynchronizeInvoke syn) { _syn = syn; }
public event FinishedTasksHandler Finished;
public void SubmitTask(string someValue)
{
System.Threading.ThreadPool.QueueUserWorkItem(state => submitTask(someValue));
}
private void submitTask(string someValue)
{
someValue = someValue + " " + DateTime.Now.ToString();
System.Threading.Thread.Sleep(5000);
//Finished(someValue); This causes cross threading error if called like this.
if (Finished != null)
{
if (_syn.InvokeRequired)
{
_syn.Invoke(Finished, new object[] { someValue });
}
else
{
Finished(someValue);
}
}
}
}
Simple and re-usable way to work around this problem.
Extension Method
public static class FormExts
{
public static void LoadOnUI(this Form frm, Action action)
{
if (frm.InvokeRequired) frm.Invoke(action);
else action.Invoke();
}
}
Sample Usage
private void OnAnyEvent(object sender, EventArgs args)
{
this.LoadOnUI(() =>
{
label1.Text = "";
button1.Text = "";
});
}
Along the same lines as previous answers,
but a very short addition that Allows to use all Control properties without having cross thread invokation exception.
Helper Method
/// <summary>
/// Helper method to determin if invoke required, if so will rerun method on correct thread.
/// if not do nothing.
/// </summary>
/// <param name="c">Control that might require invoking</param>
/// <param name="a">action to preform on control thread if so.</param>
/// <returns>true if invoke required</returns>
public bool ControlInvokeRequired(Control c, Action a)
{
if (c.InvokeRequired) c.Invoke(new MethodInvoker(delegate
{
a();
}));
else return false;
return true;
}
Sample Usage
// usage on textbox
public void UpdateTextBox1(String text)
{
//Check if invoke requied if so return - as i will be recalled in correct thread
if (ControlInvokeRequired(textBox1, () => UpdateTextBox1(text))) return;
textBox1.Text = ellapsed;
}
//Or any control
public void UpdateControl(Color c, String s)
{
//Check if invoke requied if so return - as i will be recalled in correct thread
if (ControlInvokeRequired(myControl, () => UpdateControl(c, s))) return;
myControl.Text = s;
myControl.BackColor = c;
}
this.Invoke(new MethodInvoker(delegate
{
//your code here;
}));
For example to get the text from a Control of the UI thread:
Private Delegate Function GetControlTextInvoker(ByVal ctl As Control) As String
Private Function GetControlText(ByVal ctl As Control) As String
Dim text As String
If ctl.InvokeRequired Then
text = CStr(ctl.Invoke(
New GetControlTextInvoker(AddressOf GetControlText), ctl))
Else
text = ctl.Text
End If
Return text
End Function
Same question : how-to-update-the-gui-from-another-thread-in-c
Two Ways:
Return value in e.result and use it to set yout textbox value in backgroundWorker_RunWorkerCompleted event
Declare some variable to hold these kind of values in a separate class (which will work as data holder) . Create static instance of this class adn you can access it over any thread.
Example:
public class data_holder_for_controls
{
//it will hold value for your label
public string status = string.Empty;
}
class Demo
{
public static data_holder_for_controls d1 = new data_holder_for_controls();
static void Main(string[] args)
{
ThreadStart ts = new ThreadStart(perform_logic);
Thread t1 = new Thread(ts);
t1.Start();
t1.Join();
//your_label.Text=d1.status; --- can access it from any thread
}
public static void perform_logic()
{
//put some code here in this function
for (int i = 0; i < 10; i++)
{
//statements here
}
//set result in status variable
d1.status = "Task done";
}
}
Simply use this:
this.Invoke((MethodInvoker)delegate
{
YourControl.Property= value; // runs thread safe
});
Action y; //declared inside class
label1.Invoke(y=()=>label1.Text="text");
There are two options for cross thread operations.
Control.InvokeRequired Property
and second one is to use
SynchronizationContext Post Method
Control.InvokeRequired is only useful when working controls inherited from Control class while SynchronizationContext can be used anywhere. Some useful information is as following links
Cross Thread Update UI | .Net
Cross Thread Update UI using SynchronizationContext | .Net
I have a scenario. (Windows Forms, C#, .NET)
There is a main form which hosts some user control.
The user control does some heavy data operation, such that if I directly call the UserControl_Load method the UI become nonresponsive for the duration for load method execution.
To overcome this I load data on different thread (trying to change existing code as little as I can)
I used a background worker thread which will be loading the data and when done will notify the application that it has done its work.
Now came a real problem. All the UI (main form and its child usercontrols) was created on the primary main thread. In the LOAD method of the usercontrol I'm fetching data based on the values of some control (like textbox) on userControl.
The pseudocode would look like this:
CODE 1
UserContrl1_LoadDataMethod()
{
if (textbox1.text == "MyName") // This gives exception
{
//Load data corresponding to "MyName".
//Populate a globale variable List<string> which will be binded to grid at some later stage.
}
}
The Exception it gave was
Cross-thread operation not valid: Control accessed from a thread other than the thread it was created on.
To know more about this I did some googling and a suggestion came up like using the following code
CODE 2
UserContrl1_LoadDataMethod()
{
if (InvokeRequired) // Line #1
{
this.Invoke(new MethodInvoker(UserContrl1_LoadDataMethod));
return;
}
if (textbox1.text == "MyName") // Now it won't give an exception
{
//Load data correspondin to "MyName"
//Populate a globale variable List<string> which will be binded to grid at some later stage
}
}
But it still seems that I've come back to square one. The Application again
becomes unresponsive. It seems to be due to the execution of line #1 if condition. The loading task is again done by the parent thread and not the third that I spawned.
I don't know whether I perceived this right or wrong.
How do I resolve this and also what is the effect of execution of Line#1 if block?
The situation is this: I want to load data into a global variable based on the value of a control. I don't want to change the value of a control from the child thread. I'm not going to do it ever from a child thread.
So only accessing the value so that the corresponding data can be fetched from the database.
As per Prerak K's update comment (since deleted):
I guess I have not presented the question properly.
Situation is this: I want to load data into a global variable based on the value of a control. I don't want to change the value of a control from the child thread. I'm not going to do it ever from a child thread.
So only accessing the value so that corresponding data can be fetched from the database.
The solution you want then should look like:
UserContrl1_LOadDataMethod()
{
string name = "";
if(textbox1.InvokeRequired)
{
textbox1.Invoke(new MethodInvoker(delegate { name = textbox1.text; }));
}
if(name == "MyName")
{
// do whatever
}
}
Do your serious processing in the separate thread before you attempt to switch back to the control's thread. For example:
UserContrl1_LOadDataMethod()
{
if(textbox1.text=="MyName") //<<======Now it wont give exception**
{
//Load data correspondin to "MyName"
//Populate a globale variable List<string> which will be
//bound to grid at some later stage
if(InvokeRequired)
{
// after we've done all the processing,
this.Invoke(new MethodInvoker(delegate {
// load the control with the appropriate data
}));
return;
}
}
}
Threading Model in UI
Please read the Threading Model in UI applications (old VB link is here) in order to understand basic concepts. The link navigates to page that describes the WPF threading model. However, Windows Forms utilizes the same idea.
The UI Thread
There is only one thread (UI thread), that is allowed to access System.Windows.Forms.Control and its subclasses members.
Attempt to access member of System.Windows.Forms.Control from different thread than UI thread will cause cross-thread exception.
Since there is only one thread, all UI operations are queued as work items into that thread:
If there is no work for UI thread, then there are idle gaps that can be used by a not-UI related computing.
In order to use mentioned gaps use System.Windows.Forms.Control.Invoke or System.Windows.Forms.Control.BeginInvoke methods:
BeginInvoke and Invoke methods
The computing overhead of method being invoked should be small as well as computing overhead of event handler methods because the UI thread is used there - the same that is responsible for handling user input. Regardless if this is System.Windows.Forms.Control.Invoke or System.Windows.Forms.Control.BeginInvoke.
To perform computing expensive operation always use separate thread. Since .NET 2.0 BackgroundWorker is dedicated to performing computing expensive operations in Windows Forms. However in new solutions you should use the async-await pattern as described here.
Use System.Windows.Forms.Control.Invoke or System.Windows.Forms.Control.BeginInvoke methods only to update a user interface. If you use them for heavy computations, your application will block:
Invoke
System.Windows.Forms.Control.Invoke causes separate thread to wait till invoked method is completed:
BeginInvoke
System.Windows.Forms.Control.BeginInvoke doesn't cause the separate thread to wait till invoked method is completed:
Code solution
Read answers on question How to update the GUI from another thread in C#?.
For C# 5.0 and .NET 4.5 the recommended solution is here.
You only want to use Invoke or BeginInvoke for the bare minimum piece of work required to change the UI. Your "heavy" method should execute on another thread (e.g. via BackgroundWorker) but then using Control.Invoke/Control.BeginInvoke just to update the UI. That way your UI thread will be free to handle UI events etc.
See my threading article for a WinForms example - although the article was written before BackgroundWorker arrived on the scene, and I'm afraid I haven't updated it in that respect. BackgroundWorker merely simplifies the callback a bit.
I know its too late now. However even today if you are having trouble accessing cross thread controls? This is the shortest answer till date :P
Invoke(new Action(() =>
{
label1.Text = "WooHoo!!!";
}));
This is how i access any form control from a thread.
I have had this problem with the FileSystemWatcher and found that the following code solved the problem:
fsw.SynchronizingObject = this
The control then uses the current form object to deal with the events, and will therefore be on the same thread.
I find the check-and-invoke code which needs to be littered within all methods related to forms to be way too verbose and unneeded. Here's a simple extension method which lets you do away with it completely:
public static class Extensions
{
public static void Invoke<TControlType>(this TControlType control, Action<TControlType> del)
where TControlType : Control
{
if (control.InvokeRequired)
control.Invoke(new Action(() => del(control)));
else
del(control);
}
}
And then you can simply do this:
textbox1.Invoke(t => t.Text = "A");
No more messing around - simple.
Controls in .NET are not generally thread safe. That means you shouldn't access a control from a thread other than the one where it lives. To get around this, you need to invoke the control, which is what your 2nd sample is attempting.
However, in your case all you've done is pass the long-running method back to the main thread. Of course, that's not really what you want to do. You need to rethink this a little so that all you're doing on the main thread is setting a quick property here and there.
The cleanest (and proper) solution for UI cross-threading issues is to use SynchronizationContext, see Synchronizing calls to the UI in a multi-threaded application article, it explains it very nicely.
Follow the simplest (in my opinion) way to modify objects from another thread:
using System.Threading.Tasks;
using System.Threading;
namespace TESTE
{
public partial class Form1 : Form
{
public Form1()
{
InitializeComponent();
}
private void button1_Click(object sender, EventArgs e)
{
Action<string> DelegateTeste_ModifyText = THREAD_MOD;
Invoke(DelegateTeste_ModifyText, "MODIFY BY THREAD");
}
private void THREAD_MOD(string teste)
{
textBox1.Text = teste;
}
}
}
A new look using Async/Await and callbacks. You only need one line of code if you keep the extension method in your project.
/// <summary>
/// A new way to use Tasks for Asynchronous calls
/// </summary>
public class Example
{
/// <summary>
/// No more delegates, background workers etc. just one line of code as shown below
/// Note it is dependent on the XTask class shown next.
/// </summary>
public async void ExampleMethod()
{
//Still on GUI/Original Thread here
//Do your updates before the next line of code
await XTask.RunAsync(() =>
{
//Running an asynchronous task here
//Cannot update GUI Thread here, but can do lots of work
});
//Can update GUI/Original thread on this line
}
}
/// <summary>
/// A class containing extension methods for the Task class
/// Put this file in folder named Extensions
/// Use prefix of X for the class it Extends
/// </summary>
public static class XTask
{
/// <summary>
/// RunAsync is an extension method that encapsulates the Task.Run using a callback
/// </summary>
/// <param name="Code">The caller is called back on the new Task (on a different thread)</param>
/// <returns></returns>
public async static Task RunAsync(Action Code)
{
await Task.Run(() =>
{
Code();
});
return;
}
}
You can add other things to the Extension method such as wrapping it in a Try/Catch statement, allowing caller to tell it what type to return after completion, an exception callback to caller:
Adding Try Catch, Auto Exception Logging and CallBack
/// <summary>
/// Run Async
/// </summary>
/// <typeparam name="T">The type to return</typeparam>
/// <param name="Code">The callback to the code</param>
/// <param name="Error">The handled and logged exception if one occurs</param>
/// <returns>The type expected as a competed task</returns>
public async static Task<T> RunAsync<T>(Func<string,T> Code, Action<Exception> Error)
{
var done = await Task<T>.Run(() =>
{
T result = default(T);
try
{
result = Code("Code Here");
}
catch (Exception ex)
{
Console.WriteLine("Unhandled Exception: " + ex.Message);
Console.WriteLine(ex.StackTrace);
Error(ex);
}
return result;
});
return done;
}
public async void HowToUse()
{
//We now inject the type we want the async routine to return!
var result = await RunAsync<bool>((code) => {
//write code here, all exceptions are logged via the wrapped try catch.
//return what is needed
return someBoolValue;
},
error => {
//exceptions are already handled but are sent back here for further processing
});
if (result)
{
//we can now process the result because the code above awaited for the completion before
//moving to this statement
}
}
This is not the recommended way to solve this error but you can suppress it quickly, it will do the job . I prefer this for prototypes or demos . add
CheckForIllegalCrossThreadCalls = false
in Form1() constructor .
You need to look at the Backgroundworker example:
http://msdn.microsoft.com/en-us/library/system.componentmodel.backgroundworker.aspx
Especially how it interacts with the UI layer. Based on your posting, this seems to answer your issues.
Here is an alternative way if the object you are working with doesn't have
(InvokeRequired)
This is useful if you are working with the main form in a class other than the main form with an object that is in the main form, but doesn't have InvokeRequired
delegate void updateMainFormObject(FormObjectType objectWithoutInvoke, string text);
private void updateFormObjectType(FormObjectType objectWithoutInvoke, string text)
{
MainForm.Invoke(new updateMainFormObject(UpdateObject), objectWithoutInvoke, text);
}
public void UpdateObject(ToolStripStatusLabel objectWithoutInvoke, string text)
{
objectWithoutInvoke.Text = text;
}
It works the same as above, but it is a different approach if you don't have an object with invokerequired, but do have access to the MainForm
I found a need for this while programming an iOS-Phone monotouch app controller in a visual studio winforms prototype project outside of xamarin stuidio. Preferring to program in VS over xamarin studio as much as possible, I wanted the controller to be completely decoupled from the phone framework. This way implementing this for other frameworks like Android and Windows Phone would be much easier for future uses.
I wanted a solution where the GUI could respond to events without the burden of dealing with the cross threading switching code behind every button click. Basically let the class controller handle that to keep the client code simple. You could possibly have many events on the GUI where as if you could handle it in one place in the class would be cleaner. I am not a multi theading expert, let me know if this is flawed.
public partial class Form1 : Form
{
private ExampleController.MyController controller;
public Form1()
{
InitializeComponent();
controller = new ExampleController.MyController((ISynchronizeInvoke) this);
controller.Finished += controller_Finished;
}
void controller_Finished(string returnValue)
{
label1.Text = returnValue;
}
private void button1_Click(object sender, EventArgs e)
{
controller.SubmitTask("Do It");
}
}
The GUI form is unaware the controller is running asynchronous tasks.
public delegate void FinishedTasksHandler(string returnValue);
public class MyController
{
private ISynchronizeInvoke _syn;
public MyController(ISynchronizeInvoke syn) { _syn = syn; }
public event FinishedTasksHandler Finished;
public void SubmitTask(string someValue)
{
System.Threading.ThreadPool.QueueUserWorkItem(state => submitTask(someValue));
}
private void submitTask(string someValue)
{
someValue = someValue + " " + DateTime.Now.ToString();
System.Threading.Thread.Sleep(5000);
//Finished(someValue); This causes cross threading error if called like this.
if (Finished != null)
{
if (_syn.InvokeRequired)
{
_syn.Invoke(Finished, new object[] { someValue });
}
else
{
Finished(someValue);
}
}
}
}
Simple and re-usable way to work around this problem.
Extension Method
public static class FormExts
{
public static void LoadOnUI(this Form frm, Action action)
{
if (frm.InvokeRequired) frm.Invoke(action);
else action.Invoke();
}
}
Sample Usage
private void OnAnyEvent(object sender, EventArgs args)
{
this.LoadOnUI(() =>
{
label1.Text = "";
button1.Text = "";
});
}
Along the same lines as previous answers,
but a very short addition that Allows to use all Control properties without having cross thread invokation exception.
Helper Method
/// <summary>
/// Helper method to determin if invoke required, if so will rerun method on correct thread.
/// if not do nothing.
/// </summary>
/// <param name="c">Control that might require invoking</param>
/// <param name="a">action to preform on control thread if so.</param>
/// <returns>true if invoke required</returns>
public bool ControlInvokeRequired(Control c, Action a)
{
if (c.InvokeRequired) c.Invoke(new MethodInvoker(delegate
{
a();
}));
else return false;
return true;
}
Sample Usage
// usage on textbox
public void UpdateTextBox1(String text)
{
//Check if invoke requied if so return - as i will be recalled in correct thread
if (ControlInvokeRequired(textBox1, () => UpdateTextBox1(text))) return;
textBox1.Text = ellapsed;
}
//Or any control
public void UpdateControl(Color c, String s)
{
//Check if invoke requied if so return - as i will be recalled in correct thread
if (ControlInvokeRequired(myControl, () => UpdateControl(c, s))) return;
myControl.Text = s;
myControl.BackColor = c;
}
this.Invoke(new MethodInvoker(delegate
{
//your code here;
}));
For example to get the text from a Control of the UI thread:
Private Delegate Function GetControlTextInvoker(ByVal ctl As Control) As String
Private Function GetControlText(ByVal ctl As Control) As String
Dim text As String
If ctl.InvokeRequired Then
text = CStr(ctl.Invoke(
New GetControlTextInvoker(AddressOf GetControlText), ctl))
Else
text = ctl.Text
End If
Return text
End Function
Same question : how-to-update-the-gui-from-another-thread-in-c
Two Ways:
Return value in e.result and use it to set yout textbox value in backgroundWorker_RunWorkerCompleted event
Declare some variable to hold these kind of values in a separate class (which will work as data holder) . Create static instance of this class adn you can access it over any thread.
Example:
public class data_holder_for_controls
{
//it will hold value for your label
public string status = string.Empty;
}
class Demo
{
public static data_holder_for_controls d1 = new data_holder_for_controls();
static void Main(string[] args)
{
ThreadStart ts = new ThreadStart(perform_logic);
Thread t1 = new Thread(ts);
t1.Start();
t1.Join();
//your_label.Text=d1.status; --- can access it from any thread
}
public static void perform_logic()
{
//put some code here in this function
for (int i = 0; i < 10; i++)
{
//statements here
}
//set result in status variable
d1.status = "Task done";
}
}
Simply use this:
this.Invoke((MethodInvoker)delegate
{
YourControl.Property= value; // runs thread safe
});
Action y; //declared inside class
label1.Invoke(y=()=>label1.Text="text");
There are two options for cross thread operations.
Control.InvokeRequired Property
and second one is to use
SynchronizationContext Post Method
Control.InvokeRequired is only useful when working controls inherited from Control class while SynchronizationContext can be used anywhere. Some useful information is as following links
Cross Thread Update UI | .Net
Cross Thread Update UI using SynchronizationContext | .Net
I have a custom launch configuration. It currently has a JavaArgumentsTab() where I can enter things for VM arguments and Program arguments. But how do I actually get any values entered there?
Ideally I would get them inside my LaunchConfigurationDelegate's launch() method. I expected to find any text entered as arguments inside the LaunchConfiguration or other parameters to that method, and I'm sure this is a newbie question, but I really haven't found anything promising.
TabGroup:
public class LaunchConfigurationTabGroup extends AbstractLaunchConfigurationTabGroup {
#Override
public void createTabs(ILaunchConfigurationDialog dialog, String mode) {
ILaunchConfigurationTab[] tabs = new ILaunchConfigurationTab[] {
new JavaArgumentsTab(),
new CommonTab()
};
setTabs(tabs);
} }
LaunchConfigurationDelegate:
public class LaunchConfigurationDelegate implements ILaunchConfigurationDelegate {
#Override
public void launch(ILaunchConfiguration configuration, String mode,
ILaunch launch, IProgressMonitor monitor) throws CoreException {
// How to get anything entered on my Java tab here...?
} }
Everything from the tabs should already have been set as attribute values in the ILaunchConfiguration when launch is called.
The settings from JavaArgumentsTab are stored in the attributes using constants from IJavaLaunchConfigurationConstants.
The problem: I'm crashing when I want to render my incoming data which was retrieved asynchronously.
The app starts and displays some dialog boxes using XAML. Once the user fills in their data and clicks the login button, the XAML class has in instance of a worker class that does the HTTP stuff for me (asynchronously using IXMLHTTPRequest2). When the app has successfully logged in to the web server, my .then() block fires and I make a callback to my main xaml class to do some rendering of the assets.
I am always getting crashes in the delegate though (the main XAML class), which leads me to believe that I cannot use this approach (pure virtual class and callbacks) to update my UI. I think I am inadvertently trying to do something illegal from an incorrect thread which is a byproduct of the async calls.
Is there a better or different way that I should be notifying the main XAML class that it is time for it to update it's UI? I am coming from an iOS world where I could use NotificationCenter.
Now, I saw that Microsoft has it's own Delegate type of thing here: http://msdn.microsoft.com/en-us/library/windows/apps/hh755798.aspx
Do you think that if I used this approach instead of my own callbacks that it would no longer crash?
Let me know if you need more clarification or what not.
Here is the jist of the code:
public interface class ISmileServiceEvents
{
public: // required methods
virtual void UpdateUI(bool isValid) abstract;
};
// In main XAML.cpp which inherits from an ISmileServiceEvents
void buttonClick(...){
_myUser->LoginAndGetAssets(txtEmail->Text, txtPass->Password);
}
void UpdateUI(String^ data) // implements ISmileServiceEvents
{
// This is where I would render my assets if I could.
// Cannot legally do much here. Always crashes.
// Follow the rest of the code to get here.
}
// In MyUser.cpp
void LoginAndGetAssets(String^ email, String^ password){
Uri^ uri = ref new URI(MY_SERVER + "login.json");
String^ inJSON = "some json input data here"; // serialized email and password with other data
// make the HTTP request to login, then notify XAML that it has data to render.
_myService->HTTPPostAsync(uri, json).then([](String^ outputJson){
String^ assets = MyParser::Parse(outputJSON);
// The Login has returned and we have our json output data
if(_delegate)
{
_delegate->UpdateUI(assets);
}
});
}
// In MyService.cpp
task<String^> MyService::HTTPPostAsync(Uri^ uri, String^ json)
{
return _httpRequest.PostAsync(uri,
json->Data(),
_cancellationTokenSource.get_token()).then([this](task<std::wstring> response)
{
try
{
if(_httpRequest.GetStatusCode() != 200) SM_LOG_WARNING("Status code=", _httpRequest.GetStatusCode());
String^ j = ref new String(response.get().c_str());
return j;
}
catch (Exception^ ex) .......;
return ref new String(L"");
}, task_continuation_context::use_current());
}
Edit: BTW, the error I get when I go to update the UI is:
"An invalid parameter was passed to a function that considers invalid parameters fatal."
In this case I am just trying to execute in my callback is
txtBox->Text = data;
It appears you are updating the UI thread from the wrong context. You can use task_continuation_context::use_arbitrary() to allow you to update the UI. See the "Controlling the Execution Thread" example in this document (the discussion of marshaling is at the bottom).
So, it turns out that when you have a continuation, if you don't specify a context after the lambda function, that it defaults to use_arbitrary(). This is in contradiction to what I learned in an MS video.
However by adding use_currrent() to all of the .then blocks that have anything to do with the GUI, my error goes away and everything is able to render properly.
My GUI calls a service which generates some tasks and then calls to an HTTP class that does asynchronous stuff too. Way back in the HTTP classes I use use_arbitrary() so that it can run on secondary threads. This works fine. Just be sure to use use_current() on anything that has to do with the GUI.
Now that you have my answer, if you look at the original code you will see that it already contains use_current(). This is true, but I left out a wrapping function for simplicity of the example. That is where I needed to add use_current().
I want to make an application in javaFX 2 which opens as a smaller login window, then, when you put in correct data, it takes you to bigger main window. Both are designed in fxml and events are handled within java code.
Yes, I know, it is almost the same as the application in samples and I've tried to do what I want and it worked there.
Now, when I did the same in my project, I hit a problem when I want to change the value of stage.
As you can see in the code below, I have global variable and I set the value of primaryStage in start method to it. Just as a test, I print it out at end of start method and the value is set.
Then, when I try to use it when button is clicked(method buttonClick), the value of stage variable is null, therefore I cannot use it to resize window or anything else.
My question is why is stage variable value reseted despite that I don't use change anything between the two prints?
This code is sample of what I've tried, I've just cut out all code which is not crucial to understand how my application works.
public class App extends Application {
private Stage stage;
#FXML
private AnchorPane pane;
#Override
public void start(Stage primaryStage) {
try {
stage = primaryStage; // Set the value of primaryStage to stage
primaryStage.setScene(new Scene(openScene("Login"))); // Load Login window
primaryStage.show(); // Show the scene
} catch (IOException ex) {
Logger.getLogger(App.class.getName()).log(Level.SEVERE, null, ex);
}
System.out.println(stage);// <-- Here it has the value of primaryStage obviously
}
#FXML
void buttonClick(ActionEvent event) throws IOException {
// Note that even if I try to print here, the value of stage is still
// null, so the code doesn't affect it
// Also, this loads what I want, I just can't change the size.
try{
pane.getChildren().clear(); // Clear currently displayed content
pane.getChildren().add(openScene("MainScene")); // Display new content
System.out.println(stage); // <-- Here, output is null, but I don't know why
stage.setWidth(500); // This line throws error because stage = null
} catch (IOException ex) {
Logger.getLogger(App.class.getName()).log(Level.SEVERE, null, ex);
}
}
public Parent openScene(String name) throws IOException {
//Code from FXML login example
Parent parent = (Parent) FXMLLoader.load(PrijavnoOkno.class.getResource(name
+ ".fxml"), null, new JavaFXBuilderFactory());
return parent;
}
public static void main(String[] args) {
launch(args);
}
}
Although it is not clear by whom and where the buttonClick action method is called, I pressume it is a login button's action in Login.fxml. Also I assume you have defined the App (a.k.a PrijavnoOkno) as a controller of this Login.fxml.
According to these assumptions, there are 2 instances of App.class:
one created when the app starts up and where the stage variable is assigned with primary stage in start() method,
and another instance created by FXMLLoader (while loading Login.fxml) and where the stage variable is not assigned and thus NPE.
One of the right ways can be, create a new Controller class for Login.fxml, call your login action in it. Access the global stage (by making it static in App) from there.