wpf - {x:Null} vs. Transparent?

Question

What's the difference between the following two?

Background="{x:Null}"

and

Background="Transparent"

Answer 1

Transparent will create a brush that is initialized to a transparent color, null will set the property to null, this means that the destination property has not an brush attached. In WPF it's often important to set a brush to an element. If you for example want to track mouse downs in an element, you must set a background. If you don't want to set a solid color (make it opaque), you can use a transparent brush. This can be done with the string value "Transparent".
The difference lies in the manner, how the property will be set. If you assign null for a brush-property, the property will be set really to null. If you set the string "Transparent", the default value-converter that converts string to brushes converts this to the Brushes.Transparent brush.

Short version: {x:Null} sets the destination property to null. "Transparent" sets the destination property to a transparent brush.