Trackbar vs NumericUpDown

Question

I currently use a NumericUpDown control on a Winform.

I was asked to change it to a Trackbar.

I have a background worker reading the value of the NumericUpDown control and it runs fine.

PlaybackIndex1 = NumericUpDown1.Value

When I replace the NUD with the Trackbar

PlaybackIndex1 = Trackbar1.Value,

I get the error message:

Cross-thread operation not valid: Control 'TrackBar1' accessed from a thread other than the thread it was created on.

Is there something different about the Trackbar?

Why am I not getting the same error message when using the NUD ?

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Answer 1

Hi

Here is one way to work with a BackGroundWorker to get a value from a main thread (UI) control.

In this example, I have a TrackBar1 control on the Form and to get the value inside the BackGroundWorker DoWork, I use

Invoke(Sub() TrackBarValue = TrackBar1.Value)

after which, the variable TrackBarValue holds the value.

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I can't explain why MS have deemed it fitting for a NumericUpDown control on the main thread be readable within another thread with no Delegates or anything. I can confirm that that is the case though.

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Regards Les, Livingston, Scotland

Answer 2

Les,

Thanks!

I thought I was losing my mind, and I am glad you were able to firm that indeed NumericUpDown control is different.

Your solution works.

Is there a difference between

Invoke(Sub() TrackBarValue = TrackBar1.Value)

and

Invoke(Sub() TracBarValue = TrackBar1.Value

End Sub)

?

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Answer 3

Hi

If the Sub() extends into more than a single line then youmwould need the End Sub - otherwise no difference.

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Regards Les, Livingston, Scotland

Answer 4

Great.

I have been reading up on what you did with the Invoke.

It looks like what is contained in the Invoke is a lambda expression?

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Also, the Invoke( ) that is inside the BackgroundWorker's DoWork, 

can you explain is it the background worker that is doing the invoking?

I am usually seeing something like  textbox1.invoke()   

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