How to obtain data on the size of the element if the dimensions are proportional?

I have a element which is inside a grid. The element is in the cell with proportional (*) size. When you try to display the dimensions using the MessageBox, I receive "not a number". How to be?
April 3rd 20 at 18:54
1 answer
April 3rd 20 at 18:56
In my case, came and ActualWidth properties ActualHeigth

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