"Invalid cross-thread access" when changing a WP8 textBlock - vb.net

Question

I'm coming from VB 6 and am semi-new to VB.NET.
I'm writing a Windows Phone 8 application. There's a grid in which I have several textBlocks that I want to dynamically display data from a file stream (that contains scanned data).
When the WP8 page opens, it automatically loads the data into the textBlocks. This works. Before I load the data from the file, I want to "reset" all textBlocks and hide them. For this I wanted to use a procedure which essentially does the following for every textBlock:
tbl1.Text = ""
tbl1.Visibility = System.Windows.Visibility.Collapsed
This works exactly one time: when the page loads. The procedure does not produce an error.
Now when I call that method again later, when I want to refresh the data shown on the page, I get the following error on the first line of code shown above:
An exception of type 'System.UnauthorizedAccessException' occurred in System.Windows.ni.dll but was not handled in user code
Additional information: Invalid cross-thread access.
I'm a bit lost now. In VB6 I was able to do whatever I wanted with my UI elements. I presume I'm making some kind of newbie mistake here?
I read somewhere about some Dispatcher thing. But that seems overly complicated to simply change a value in a textBlock to me. Is there no simple way?

Answer 1

The solution is indeed to use the Dispatcher.BeginInvoke method.
Dispatcher.BeginInvoke(() =>
{
tbl1.Text = ""
tbl1.Visibility = System.Windows.Visibility.Collapsed
});
The reason why you need this is because you try to access an object that depends on the UI thread from a non-UI thread.

Answer 2

It is probably because you are not calling the method from UI thread. Try using Dispatcher.BeginInvoke:
Dispatcher.BeginInvoke(()=>
{
tbl1.Text = "";
tbl1.Visibility = System.Windows.Visibility.Collapsed;
});