RadiusX Property (RectangleGeometry)
Gets or sets the xradius of the ellipse used to round the corners of the rectangle geometry.
XAML 
<object RadiusX="Double" .../>

Scripting 
value = object.RadiusX object.RadiusX = value 
Property Value
The xradius of the ellipse used to round the corners of the rectangle geometry.
This property is read/write. The default value is 0.
Remarks
For the rectangle geometry to have rounded corners, both the RadiusX and RadiusY properties must be nonzero.
A value greater than or equal to zero and less than or equal to half the rectangle's width that describes the xradius of the ellipse is used to round the corners of the rectangle. Values greater than half the rectangle's width are treated as though equal to half the rectangle's width. Negative values are treated as positive values.
The following illustration shows three rectangles with varying degress of rounded corners.
Rectangles with different RadiusX/RadiusY settings