wpf / xaml中仅限角的边框

In some xaml, I'd like to have a border that is only out to a few pixels from each corner. So, along the top, the border would be, say, 5 pixels of black at the left edge, then be transparent across the rest of the top until 5 pixels of the right edge, and then the final 5 pixels of the border at the right edge would be black. The intent is to provide some guidance for selection of occasionally transparent things; however, in this application, the presence of a full border would interfere with the content (in the perceptual-visual sense). The corners are not rounded.In some xaml, I'd like to have a border that is