MVC 5 MultiSelectList选择的值不工作

I have a ListBox in a View displaying the possible choices. However, the user's role(s) are not shown as selected. In my UserViewModel the Roles collection contains the complete list of roles, and AspNetRoles collection contains only the roles to which the user belongs. I've tried multiple examples/variations but nothing ever shows as selected. I'm stuck.I have a ListBox in a View displaying the possi