Updating a Label content from code behind using dispatcher

Question

hi,
I am trying to update a label's content from code behind.
This part of the code is running in background worker. I wrote the following code to update a label's content:
volumecontrol.Dispatcher.BeginInvoke(new Action(() =>
volumecontrol.Content = volumeupdate;
 i tried using both BeginInvoke and Invoke but the application exits with the error:
System.InvalidOperationException' occurred in WindowsBase.dll
Using Invoke works when updating the UI from another thread but it not working in this case:
Pls help.
Thanks,
Shaleen
TheHexLord

Answer 1

When you do that new action stuff you're capturing variables.
If that means you grab a control's value across from outside the {} then you're trying to capture the variable on the background thread.  If that's some sort of control you're messing with then that will cause a problem as they have thread affinity.
Because you don't want to be blocking the UI thread at all you should use BeginInvoke rather than Invoke.
To explain this clearly - and provide a way you could use to explore what's going on and learn  - we need a separate thread which can be done using Task.Factory.StartNew.
This bit of code allows you to put code onto a background thread:
Task.Factory.StartNew(() =>
// On a separate thread to the UI here
Create a new solution, add a textBlock and Button:
<StackPanel>
<TextBlock Name="tb"/>
<Button Name="btn" Click="btn_Click">Change the text</Button>
</StackPanel>
Then in the button click you can play around with what's going on.
Just to be clear.
That textblock is a control and it is created on the UI thread.
To get from that task thread back to the UI thread you should use Dispatcher.BeginInvoke.  With no control name.
Let's start with a broken piece of code:
private void btn_Click(object sender, RoutedEventArgs e)
Task.Factory.StartNew(() =>
string thingummy = tb.Text + "Banana";
Dispatcher.BeginInvoke(new Action(() => { tb.Text = thingummy; }));
When you click the button it'll error because when you access tb.Text there you do so on a background thread and tb has thread affinity.
This, however, will work OK.
Task.Factory.StartNew(() =>
Dispatcher.BeginInvoke(new Action(() => { tb.Text = tb.Text + "Banana"; }));
That's OK because the Action runs on the UI thread where tb was created and all is good.
Anonymous methods and actions capture variables ( you can google that for more info ).
If you wanted to use a variable which was created on the background thread you can set it here:
Task.Factory.StartNew(() =>
string thingummy = "banana";
Dispatcher.BeginInvoke(new Action(() => { tb.Text = thingummy; }));
or here
string thingummy = "banana";
Task.Factory.StartNew(() =>
Dispatcher.BeginInvoke(new Action(() => { tb.Text = thingummy; }));
They both work.
They are not accessing properties of a UI control because you're just setting the variable to a string.
All of which means you could have a variable in your code which is set to volume or whatever that is from your control  ON THE UI THREAD and then modify that variable on the background thread.  Variables do not have thread affinity.  A
double, string or whatever isn't a control.
And this approach might well be more convenient.
Hope that helps.
Recent Technet articles:
Property List Editing ;  
Dynamic XAML