read-only DrawingBrush

I have an object the contains a DP OutBrush of type DrawinfBrush and i set its default value to "new DrawingBrush()"

This brush will be animated

but when I set other brush to this brush I get an exception says its in read only state

button1.Background = myObject.OutBrush.

I worked arround this by doing the below in the Ctor and every thing workss fine

OutBrush = new DrawingBrush();

is this correct or there is any other way?