I have a custom collection defined in my window resources as follows (in a Sketchflow app so the window is actually a UserControl):
<ds:MyCollection x:Key="myKey" x:Name="myName" />
myRef = (MyCollection) this.FindName("myKey");
You should use
Also, a good practice is to create a string constant which maps the name of your key in the resource dictionary (so that you can change it at only one place).