Develop Reference

Raise Events to the UI thread from a Task

I have been trying to determine the proper approach to manipulate the UI from an asynchronous task. My application has become cluttered with threaded Invokes and BeginInvokes. I am trying to alleviate this clutter as well as provide a bit more responsiveness on the UI by taking advantage of C# async and await.
Somewhere on the UI thread I initialize IProgress event handler and pass it to an asynchronous function called DoInfiniteWorkAsync. This function runs infinitely in a background task but often has the need to update portions of the UI.
private void Form1_Load(object sender, EventArgs e)
{
// Create a UIEventHandler on our UI thread.
Progress<string> UIEventHandler = new Progress<string>();
UIEventHandler.ProgressChanged += UIEventHandler_CommandRaised;
// Pass the UIEventHandler to our Asyncronous task.
DoInfiniteWorkAsync(UIEventHandler);
}
void UIEventHandler_EventRaised(object sender, string e)
{
string eventMessage = e;
// Update UI based on the event message.
}
My DoInfiniteWorkAsync function uses the passed in UIEventHandler to report UIEventMessages while running its task.
private async Task DoInfiniteWorkAsync(IProgress<string> UIEventHandler)
{
await Task.Run(() =>
{
// 24/7 running tasks.
// Sets a UIEventMessage to signal UI thread to do something.
UIEventHandler.Report(UIEventMessage);
});
}
Is this the proper way to be updating the UI thread from a long running background thread? The fact that my event handler datatype (IProgress) is specifically directed at progress reporting is making me feel like I'm missing the point.

To bring in data from one asynchronous thread to another you have to invoke it.
Define in your class a field of property:
string _readData = null;
Then fill the string with the eventMessage and call a method to invoke the data.
string eventMessage = e;
_readData = eventMessage;
Msg();
This is the method Msg():
private void Msg()
{
if (this.InvokeRequired)
{
this.Invoke(new MethodInvoker(Msg));
}
else
{
textBox2.Text = textBox2.Text + Environment.NewLine + " >> " + _readData;
}
}

Abort a thread which is running a long query

I have a thread which calls one of the methods, now this method executes a query which can take a very long time possibly 40 minutes or so to complete,
I want to give user a a choice to be able to cancel this operation (meaning stop the thread and stop the query to release database).
I should mention that I am developing WPF Application using .net 4.5, SQL SERVER DB and C#.

You should use backgroundworker, it is exactly what you want.
Eather drag and drop it from the toolbox or create it in code - behind. It supports Cancellation, reports progress, notifies when complete and know if it is running or not.
Here is an example.
void method(){
BackgroundWorker worker = new BackgroundWorker();
worker.RunWorkerCompleted += worker_RunWorkerCompleted;
worker.ProgressChanged += worker_ProgressChanged;
worker.DoWork += worker_DoWork;
worker.WorkerSupportsCancellation = true;
if(!worker.IsBusy)
{
worker.RunWorkerAsync();
}
}
void worker_DoWork(object sender, DoWorkEventArgs e)
{
//do whatever needs to be done on the other thread here.
object argument = e.Argument; //if passed argument in RunWorkerAsync().
object result = new object();
e.Result = result;
//after making worker global, you can report progress like so:
worker.ReportProgress(50); //you can also pass a userState, which can be any object, to show some data already.
}
void worker_ProgressChanged(object sender, ProgressChangedEventArgs e)
{
//you can update a progress bar in here
int progress = e.ProgressPercentage;
}
void worker_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
{
//when done
}
void CancelTheTask()
{
if (worker.IsBusy)
{
//make worker global first, but then
worker.CancelAsync();
}
}
A important things to look at: Never use resources in the DoWork method that are not created inside it. Thus pass things you need in the background worker as Arguments. And things that are created by the backgroundworker should not be set to a global variable ether, pass by result.
When cancelling, RunWorkCompleted will also be fired. Now the query to the database is already being executed, so that is still running, even when your application lost all resources to it.
To cancel that, we would need to know how you execute the query, like #S.Akbari mentioned is one way. Entity Framework 6 also supports cancellation.
For that: check this when using Queryable
here is another example
Or this solution without Entity Framework.

Using Task Parallel Library (TPL) you can use the Task Cancellation pattern.

When you have your Thread blocked on waiting for the query, it's useless for stopping anything.
Make sure the SqlConnection of the query is accessible from your UI and Close it. Abandon the Thread, it will terminate (with an error you've got to suppress).

If the UI thread is doing a Long-time operation it won't be able to process
UI requests. This is also known as Not Responding.
Use ThreadPool like this:
CancellationTokenSource ct;//instantiate it before ThreadPool.QueueUserWorkItem line
private void operation_Click(object sender, RoutedEventArgs e)
{
ct = new CancellationTokenSource();
ThreadPool.QueueUserWorkItem(_ =>
{
var result = LongTimeOperation();//set the operation in another thread so that the UI thread is kept responding
//use the Dispatcher to "return" to the UI thread
Dispatcher.BeginInvoke(new Action(() =>
{
//Use result for example : Label1.Text = result.ToString();
}));
});
}
To give user a choice to be able to cancel the operation use CancellationTokenSource like this:
private void cancel_Click(object sender, RoutedEventArgs e)
{
if (ct != null)
{
ct.Cancel();
ct= null;
}
}
Note: in LongTimeOperation() you must have one more parameter of type CancellationToken
private float LongTimeOperation(CancellationToken ct)
{
if (ct.IsCancellationRequested)
return -1;
....
....
}
This link is useful about Cancellation in Managed Threads.

this is a common problem.But in WPF and WinForm, i'd like to use BackGroundWorker. See Here

C# Asynchronous Task for a Stock Ticker

I've been trying to learn more about asynchronous tasks and threading but not making a ton of headway.
I'm trying to load an "Engine" type of thread that will run in the background upon launch and be able to access the UI Thread to update variables, without hanging the UI Thread.
In the below code, Engine is called, and a Ticker object is created which holds the current value of (Litecoin/USD) called Last, also holds several other values that would be useful. This code successfully assigns the current value to label1.text. I don't necessarily need code but what approach would I take to create a ticker object in the background every second and update the UI thread with each new Ticker objects values.
Is this a good case for a background worker?
private void Form1_Load(object sender, EventArgs e)
{
Engine();
}
private void Engine()
{
Ticker ltcusd = BtceApi.GetTicker(BtcePair.LtcUsd);
label1.Text = "LTC/USD:" + ltcusd.Last;
}
EDIT:
If I do the following, label1 throws an InvalidOperationException due to a Cross-thread operation attempt (label1 in the UI thread).
private void Form1_Load(object sender, EventArgs e)
{
var t = Task.Factory.StartNew(() => Engine());
t.Start();
}
private void Engine()
{
while (true)
{
Thread.Sleep(1000);
Ticker ltcusd = BtceApi.GetTicker(BtcePair.LtcUsd);
label1.Text = "LTC/USD: " + ltcusd.Last;
}
}

Using async/await, the simplest way of getting an "asynchronous" sort of API is to invoke a new task. It's not great, but it'll make things simpler. I would probably create a new class which basically wrapped all the BtceApi methods in tasks:
public class BtceApiAsync
{
public Task<Ticker> GetTickerAsync(BtcePair pair)
{
return Task.Run(() => BtceApi.GetTicker(pair));
}
// etc
}
Then you can use a timer which fires once per second, which will start off a new task and update the UI appropriately:
// Keep a field of type System.Windows.Forms.Timer
timer = new Timer();
timer.Interval = 1000;
timer.Tick += DisplayTicker;
timer.Start();
...
private async void DisplayTicker(object sender, EventArgs e)
{
Ticker ticker = await BtceApiAsync.GetTickerAsync(BtcePair.LtcUsd);
label1.Text = "LTC/USD: " + ltcusd.Last;
}
Note that this doesn't mean the screen will be updated once per second... there will be a new task started once per second, and as soon as each task completes, the UI will be updated.
The use of await here - from an async method started on the UI thread - means you don't need to worry about using the UI; the whole async method will execute on the UI thread, even though the fetch itself happens in a different thread.

You can try ContinueWith to update the Label at the end of the task. If you want to update it event before the task ends then raise an event which is registered by on the UI thread. The event can then update the label.

I suppose this is Windows Forms. You could do it "old school style" and set the label text on the UI thread, and you can do that by passing delegate to the BeginInvoke or Invoke method.
private void Engine()
{
while (true)
{
Thread.Sleep(1000);
Ticker ltcusd = BtceApi.GetTicker(BtcePair.LtcUsd);
UpdateText("LTC/USD: " + ltcusd.Last);
}
}
private void UpdateText(string text)
{
//Inspect if the method is executing on background thread
if (InvokeRequired)
{
//we are on background thread, use BeginInvoke to pass delegate to the UI thread
BeginInvoke(new Action(()=>UpdateText(text)));
}
else
{
//we are on UI thread, it's ok to change UI
label1.Text = text;
}
}

c# thread and a waiting form

I connect to a webserive. While the webservice is connected i want to have a waiting form with an animated gif inside of it. The waiting form is correctly displayed but the animated give is not animated it is fixed.
Can anybody help me. I have already tried : DoEvents but the gif is still not animated.
// Create the new thread object
Thread NewThread = new Thread(new ThreadStart(RunThread));
// Start the new thread.
NewThread.Start();
// Inform everybody that the main thread is waiting
FRM_Wait waitingDialog = new FRM_Wait();
waitingDialog.Show();
waitingDialog.Activate();
Application.DoEvents();
// Wait for NewThread to terminate.
NewThread.Join();
// And it's done.
waitingDialog.Close();
MessageBox.Show("Upload erfolgreich erledigt.", "Upload Erfolgreich",
MessageBoxButtons.OK, MessageBoxIcon.Exclamation);
}
public void RunThread()
{
mfsportservicedev.ServiceSoapClient servicedev = new mfsportservicedev.ServiceSoapClient();
int status = servicedev.addEvent(videosNames, videos);
}

Don't call Join on a thread from within the UI thread. Instead, disable any controls you don't want to act on until the task has completed (e.g. buttons) and then call back into the UI thread when the operation has completed - so move the "And it's done" code into a new method which is invoked at the end of the operation. If you're using .NET 4, I'd suggest using the TPL for this, as it makes it easier to represent "a task which is in progress" and to add a continuation to it. (It's also a good start for what will become the idiomatic way of doing async operations in .NET 4.5.)

The problem is coming from your join. join is synchronous, so basically you are making your UI wait till the thread finishes its work.
You want to use a callback function to come back to your UI.
Edit : ive been skeetified

You problem is here:
NewThread.Join();
This blocks the UI thread until NewThread ends.
Here's one way to do it:
private myDelegate;
// ...
myDelegate = new Action(RunThread);
myDelegate.BeginInvoke(new AsyncCallback(MyCallback),null);
// You RunThread method is now running on a separate thread
// Open your wait form here
// ...
// This callback function will be called when you delegate ends
private void MyCallback(IAsyncResult ar)
{
myDelegate.EndInvoke(ar);
// Note this is still not the UI thread, so if you want to do something with the UI you'll need to do it on the UI thread.
// using either Control.Invoke (for WinForms) or Dispatcher.Invoke (for WPF)
}

Thread.Join is a blocking call that does not pump messages so that is your problem. It is typically advised to avoid calling any kind of synchronization mechanism that causes the UI thread to block.
Here is a solution using the Task class and the Invoke marshaling technique.
private void async InitiateWebService_Click(object sender, EventArgs args)
{
FRM_Wait waitingDialog = new FRM_Wait();
waitingDialog.Show();
Task.Factory.StartNew(
() =>
{
mfsportservicedev.ServiceSoapClient servicedev = new mfsportservicedev.ServiceSoapClient();
int status = servicedev.addEvent(videosNames, videos);
waitingDialog.Invoke(
(Action)(() =>
{
waitingDialog.Close();
}));
});
}
Here is a solution using a raw Thread.
private void async InitiateWebService_Click(object sender, EventArgs args)
{
FRM_Wait waitingDialog = new FRM_Wait();
waitingDialog.Show();
var thread = new Thread(
() =>
{
mfsportservicedev.ServiceSoapClient servicedev = new mfsportservicedev.ServiceSoapClient();
int status = servicedev.addEvent(videosNames, videos);
waitingDialog.Invoke(
(Action)(() =>
{
waitingDialog.Close();
}));
});
thread.Start();
}
C# 5.0 makes this kind of pattern even easier with its new async and await keywords1.
private void async InitiateWebService_Click(object sender, EventArgs args)
{
FRM_Wait waitingDialog = new FRM_Wait();
waitingDialog.Show();
await Task.Run(
() =>
{
mfsportservicedev.ServiceSoapClient servicedev = new mfsportservicedev.ServiceSoapClient();
int status = servicedev.addEvent(videosNames, videos);
});
waitingDialog.Close();
}
1Not yet released.

C# threading issue

To play a bit with threading, delegates and backgroundworkers, I'm putting together a few small applications, I'm having a bit of trouble with one of them.
I've a Windows form, with a textbox, a button and a richttext.
When I press the button, the text in the textbox is used as a paramter to instantiate a class, like this:
public partial class Form1 : Form
{
private BackgroundWorker backgroundWorker;
public Form1()
{
InitializeComponent();
}
private void button1_Click(object sender, EventArgs e)
{
backgroundWorker = new BackgroundWorker();
backgroundWorker.DoWork += new DoWorkEventHandler(worker_DoWork);
backgroundWorker.RunWorkerAsync();
}
void worker_DoWork(object sender, DoWorkEventArgs e)
{
new Thread((ThreadStart)delegate()
{
this.BeginInvoke((ThreadStart)delegate()
{
foreach (string line in textBox1.Lines)
{
Dig digger = new Dig(line, textBox1.Text);
digger.DomainChecked += new Dig.DomainCheckedHandler(OnUpdateTicker);
string response = digger.GetAllInfo();
richTextBox1.AppendText(response);
Application.DoEvents();
}
});
}).Start();
}
void OnUpdateTicker(string msg)
{
new Thread((ThreadStart)delegate()
{
this.BeginInvoke((ThreadStart)delegate()
{
label4.Text = msg;
Application.DoEvents();
});
}).Start();
}
}
When debugging I run into a 'textBox1.Lines' threw an exception of type 'Microsoft.VisualStudio.Debugger.Runtime.CrossThreadMessagingException'
Any tips on how to solve this problem?

First, there is no need to create new threads inside DoWork; the whole idea with the BackgroundWorker is that DoWork is executed on a separate thread. Second, since DoWork is executed on a separate thread and UI controls can be modified only on the UI thread, you need to invoke those updates correctly. So, a rewritten version of worker_DoWork could look like this:
void worker_DoWork(object sender, DoWorkEventArgs e)
{
foreach (string line in textBox1.Lines)
{
Dig digger = new Dig(line, textBox1.Text);
digger.DomainChecked += new Dig.DomainCheckedHandler(OnUpdateTicker);
string response = digger.GetAllInfo();
richTextBox1.Invoke((Action) delegate { richTextBox1.AppendText(response); });
}
}
Note how the code does not explicitly spawn any new threads, and also how the AppendText method call is done through a Control.Invoke call, forcing it to execute on the UI thread.

The main reason is that the textbox is not owned by the background thread.
Your UI thread owns all the UI objects, and you're spinning up a background thread when a button is pressed. That background thread should not have access to any UI objects.
If you want the value of the textbox to be used, you'll need to pass it to your background thread another way.
Have a look here for an explanation (and solution).

You can only update controls on the main thread from the main thread itself, unless you explicitly tell your program that it's ok to do, by using the .Invoke method of the control.
From: http://www.albahari.com/threading/part3.aspx
Control.Invoke
In a multi-threaded Windows Forms application, it's illegal to call a method or property on a control from any thread other than the one that created it. All cross-thread calls must be explicitly marshalled to the thread that created the control (usually the main thread), using the Control.Invoke or Control.BeginInvoke method. One cannot rely on automatic marshalling because it takes place too late – only when execution gets well into unmanaged code, by which time plenty of internal .NET code may already have run on the "wrong" thread – code which is not thread-safe.