I'm using a ScrollViewer to display an Image. The Image has a ScaleTransform set as one of it's LayoutTransforms. I've got it setup to fit the width of the image into the ActualSize of the ScrollViewer. My problem is that if the image height requires the vertical scrollbar to be present (I have it set to Auto) then my image is scaled just a little bit to much. I know how to determine if the scrollbar would be present and how to get the correct scale, but I cannot figure out how to determine what the actual width of the scrollbar is. I guess I could just guess at it, but I'd like something that would work if I later add styles to my application that would result in the scrollbars being a different size. Additionally I'm also doing Fit to Height and would need to get the Height of the horizontal scrollbar when it would be visible (I'm assuming that the answer to getting the width of the vertical scrollbar would make getting the height of the horizontal scrollbar obvious).
You can use SystemParameters.ScrollWidth.
Using ViewableHeight and ViewableWidth instead of ActualHeight and ActualWidth in my scaling calculations along with setting the scroll bars Visibility to Visible instead of Auto works. However I'll accept another answer that allows the scroll bars to be set to Auto instead.
Edit:
OK, I've now got the scroll bars set to Visible. Then I do my calculation with the ViewableHeight and ViewableWidth. Then I set the scroll bars back to Auto. This seems to work even if it's not all that elegant.
来源：https://stackoverflow.com/questions/2777384/determine-the-width-of-the-vertical-scroll-bar-in-a-scrollviewer