How to update UI from other threads?

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When i run my WPF application, i want to do the points given below

> Execute 3 methods(MethodA,MethodB,MethodC) one after another.
> A label control in the Window which shows which method is executing now.(eg: Executing MethodA...)
> Show window without waiting.(UI thread must not wait for other 3 methods to finish).
How to do this?

Posted 4-Aug-15 1:56am

User 8553525

Updated 4-Aug-15 2:29am

v3

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[no name] 4-Aug-15 8:10am

Okay... and? Did you have a question? Do you have some sort of a problem with the code you have written?

[no name] 4-Aug-15 8:26am

How to update label in UI from MethodA?

[no name] 4-Aug-15 8:41am

https://msdn.microsoft.com/en-us/library/system.windows.threading.dispatcher.invoke(v=vs.110).aspx

[no name] 4-Aug-15 8:27am

How to execute 3 methods one by one without make UI wait?

2 solutions

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Sergey Alexandrovich Kryukov · Accepted Answer · 2015-08-04T05:00:00

Solution 2

You cannot call anything related to UI from non-UI thread. Instead, you need to use the method Invoke or BeginInvoke of System.Windows.Threading.Dispatcher (for both Forms or WPF) or System.Windows.Forms.Control (Forms only).

You will find detailed explanation of how it works and code samples in my past answers:
Control.Invoke() vs. Control.BeginInvoke(),
Problem with Treeview Scanner And MD5.

See also more references on threading:
.NET event on main thread[^],
.NET event on main thread,
How to get a keydown event to operate on a different thread in vb.net,
Control events not firing after enable disable + multithreading.

Prathap wrote:

…To display a text in label control, I had written code in ProgressChanged event of BackgroundWorker and it works fine.
When should I use Control.Invoke?

You need to call it each time you need to change something in UI. For example, to change the label text, you could use this method:

C#

static void SetLabelText(Label label, string newText) {
    label.Content = newText;
}

But this is something you could only call from the same UI thread. As I already explained before, you cannot call from a different thread. So, you need to call it from UI thread in all cases, but what to do in another thread? Delegate it to the UI thread. This is how?

```cs
using System.Windows;
using System.Windows.Controls;

// ...

public partial class MyWindow : Window {

public MainWindow() {
InitializeComponent();
} //MainWindow

void SetLabelText(Label label, string newText) {
Dispatcher.Invoke(
new System.Action<label, string="">((aLabel, aText) => {
aLabel.Content = aText;
}), label, newText);
} //SetLabelText

// ...

} //class MyWindow
```

—SA

Posted 4-Aug-15 5:00am

Sergey Alexandrovich Kryukov

Updated 5-Aug-15 5:55am

Chris Maunder772.8K

v10

Comments

[no name] 5-Aug-15 1:44am

Thanks, Sergey Alexandrovich Kryukov.
To display a text in label control, i had written code in ProgressChanged event of BackGroundWorker and it works fine.
When should i use 'Control.invoke'?

Sergey Alexandrovich Kryukov 5-Aug-15 1:46am

You should delegate a simple method which assigns a text value to label.Text.
—SA

Sergey Alexandrovich Kryukov 5-Aug-15 2:29am

Please see to my update to the answer, where I quote your question.
(Sorry, formatting is screwed up a bit; I'll report it as a regression bug.)
—SA

[no name] 5-Aug-15 5:34am

I think i am updating UI control from the main thread itself (assuming ProgressChanged & RunWorkerCompleted events fire in main thread), using the result from worker thread(doWork event).So 'Dispatcher.Invoke' method not needed?

Sergey Alexandrovich Kryukov 5-Aug-15 10:38am

You did not get it! It is needed. Read my words from my answer, not from your head.
—SA

Afzaal Ahmad Zeeshan 5-Aug-15 4:33am

5ed, I personally like the Dispatcher.BeginInvoke method. Allows me to make use of asynchronous programming at its level best.

[no name] 5-Aug-15 5:29am

Dispatcher.BeginInvoke -> To update control in main thread from worker thread?

Afzaal Ahmad Zeeshan 5-Aug-15 5:31am

From any thread, you just use this to the UI-thread and it would perform the actions. Basically what Sergey has said explains it. :)

Sergey Alexandrovich Kryukov 5-Aug-15 10:36am

Probably, you still did not get it. Please see my other update to the question: I added shorter code fragment, to explain what should happen in a UI thread and what invocation does.
—SA

Sergey Alexandrovich Kryukov 5-Aug-15 10:24am

Thank you, Afzaal.
I must say, this is not about personal preference. I depends on what you are doing. In certain cases, BeginInvoke can be a way to screw up presentation. What of the non-UI thread pushes changes much faster then UI can digest it? It's could be better to throttle that thread by UI, to allow showing the state in sync, which would be done by Invoke.
—SA

Afzaal Ahmad Zeeshan 5-Aug-15 10:56am

Good point in that. I would keep it my mind, thank you. :-)

Sergey Alexandrovich Kryukov 5-Aug-15 11:08am

You are very welcome.
—SA

[no name] 5-Aug-15 5:39am

Which is the right way.
Using BackGroundWorker or Creating another Thread and updating UI thread using Dispatcher.Invoke method?

Sergey Alexandrovich Kryukov 5-Aug-15 10:28am

It depends. First Dispatcher.Invoke is not the choice; you always need it, no matter how you start the thread.
Bad thing is to add thread each time on some event; if this is what you mean, my answer would be: neither way is good enough. Most usually, I do different thing: create additional thread once and throttle it, so it would leave the whole life cycle of the whole process in a wait state, waiting for a task. When the user has some job to do, I wake up this thread.
—SA

[no name] 6-Aug-15 3:35am

Thank you sergey.
I tried Dispatcher.Invoke and it and works perfectly.
I have one more question.
> Suppose, I'm using BackGroundWorker to run MethodA on another thread.
> In MethodA, a varibale which contains the message to display in UI label.
message = " you are in MethodA..";
> When i press run, The'ReportProgress' method (written in MethodA) of
BackGroundWorker will get invoked, that results in BackGroundWorker
'ProgressChanged' event get fired.
> The 'ProgressChanged' is called on the UI thread, so in this event i had written
code to display label text. lblMessage.Content = message;
> Please check this

Sergey Alexandrovich Kryukov 6-Aug-15 9:06am

Don't mix it up: an event is invoked, and a method is just called. You don't "write" anything "in event", you write in a method used as an event handler. Why do you need ProgressChanged at all? What happens when you press "run"?
And so on... Not clear what is your concern. Didn't I fully describe how to communicate with the UI thread? This is all that matters.
—SA

[no name] 7-Aug-15 1:31am

Why do you need ProgressChanged at all?
The Window contains a progress bar too and to update it.
What happens when you press "run"?
I meant, run whole application.

Sergey Alexandrovich Kryukov 7-Aug-15 1:53am

Let's say, ReportProgress are very specialized methods which use delegation to UI thread internally, so you don't need Dispatcher to use it. It is not universal, because you can only pass percentage (and other method also some other object of the type System.Object). You cannot pass any action and any other parameters, as with Invoke and BeginInvoke.
—SA

[no name] 7-Aug-15 1:33am

You explained everything very well. Thanks a lot.

Sergey Alexandrovich Kryukov 7-Aug-15 1:53am

You are very welcome.
Good luck, call again.
—SA

Sergey Alexandrovich Kryukov 6-Aug-15 9:10am

You call ReportProgress and handle ProgressChanged. Your handler of this event is called already on the UI thread (otherwise why would you have this mechanism at all?) Please see the code sample here:
https://msdn.microsoft.com/en-us/library/system.componentmodel.backgroundworker%28v=vs.110%29.aspx.

Will you finally accept my answer formally?

—SA

OriginalGriff · Accepted Answer · 2015-08-04T02:35:00

Solution 1

1)

C#

private void MyFunction()
   {
   MethodA();
   MethodB();
   MethodC();
   }

2)

C#

private void MethodA()
   {
   myLabel.Text = "MethodA";
   }
private void MethodB()
   {
   myLabel.Text = "MethodB";
   }
private void MethodC()
   {
   myLabel.Text = "MethodC";
   }

3) Move MyFunction into a BackgroundWorker[^] and use Invoke[^] to set the label text.

Posted 4-Aug-15 2:35am

OriginalGriff

Comments

OriginalGriff 5-Aug-15 2:02am

You only (and always!) need Control.Invoke when your code is running on a thread other than that on which the control was created - the UI thread.

But BackgroundWorker is a very helpful class: the ProgressChanged event is always fired on the original thread, not the background thread, so you don't need to invoke controls in the event handler.
But you can always test Control.InvokeRequired in "general purpose" methods and invoke if necessary.
https://msdn.microsoft.com/en-us/library/system.windows.forms.control.invokerequired.aspx?PHPSESSID=6c7e47uec18v8n3121a6p001h4
It checks the thread, and returns false if it's safe to continue without invoking.

OriginalGriff 5-Aug-15 5:46am

:thumbsup: