Call method till it returns value

Question

I want to call a method till it returns a value in my WPF app.

void btnDecode_Click(object sender, RoutedEventArgs e)
{
    var task = new Task(task);
    task.Start();
    task.Wait();
}

async void task()
{
    Task<object> result= DecodedResult((BitmapSource)imageBarcode.Source);
    object i = await result;
    txtBarcodeContent.Text = i.ToString();
}

async Task<object> DecodedResult(BitmapSource renderTargetBitmap)
{
    var reader = new BarcodeReader();
    txtBarcodeContent.Text = "reading";

    return reader.Decode(renderTargetBitmap);
}

But it throws me an error on task.Start();

"Additional information: The calling thread cannot access this object because a different thread owns it."

Why can't I access it and why does another thread own it?

"it throws me error" doesn't say where it's failing - what is "it" here? Please be clearer. (It's also odd to have an `async` method and then start it as a new task explicitly. Why not just make it `async Task mjau` - or ideally with a better name...) — Jon Skeet, Jun 04 '15 at 09:00
Also, please specify more context - is this a web-app? WinForm? — Jon Skeet, Jun 04 '15 at 09:01
possible duplicate of [The calling thread cannot access this object because a different thread owns it.WPF](http://stackoverflow.com/questions/10795948/the-calling-thread-cannot-access-this-object-because-a-different-thread-owns-it) — weston, Jun 04 '15 at 09:12
@MatíasFidemraizer if it asked thousand times... then show me clear answer and solution for this problem... I searched but didn't find any working solution for me. — John, Jun 04 '15 at 09:13
@John Oh yeah, use the search function both in SO or Google. Look for *how to modify a control property from other thread than UI*... — Matías Fidemraizer, Jun 04 '15 at 09:27

score 1 · Accepted Answer · answered Jun 04 '15 at 09:20

1

Whenever you update your UI elements from a thread other than the main thread, you need to use:

this.Dispatcher.Invoke((Action)(() =>
{
       ...// your code here.
}));

You can also use control.Dispatcher.CheckAccess() to check whether the current thread owns the control. If it does own it, your code looks as normal. Otherwise, use above pattern.

answered Jun 04 '15 at 09:20

Noam M

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1

This is the common solution, but is unnecessary with async. – weston Jun 04 '15 at 09:37

score 1 · Answer 2 · edited May 23 '17 at 12:14

This is illegal from anywhere but the UI thread.

txtBarcodeContent.Text = i.ToString();

And task.Wait(); ruins the async effect.

Solution

You can have async on the click method.

This works: Note I only async when I await and I avoid async void (Using async without await).

private async void btnDecode_Click(object sender, RoutedEventArgs e)
{
    string result = await DecodedResult((BitmapSource)imageBarcode.Source);
    txtBarcodeContent.Text = result;
}

private async Task<string> DecodedResult(BitmapSource renderTargetBitmap)
{
    object decoded = await reader.Decode(renderTargetBitmap);
    return decoded.ToString();
}

Apologies should have been `private async void btnDecode_Click` — weston, Jun 04 '15 at 12:56

Call method till it returns value

2 Answers2

Solution