[Allow the user to choose one of them when he creates new content page.]
You want your (User) aka "Editors" the ones that are making [posts/pages] on your website to choose the masterpage for that specific [post or page] that they are creating?
Not "Users" the visitors visiting your website.
So have each [Post/Page] connected to a masterpage instead of all of the content connected to just one.
Is that correct?
This will require some custom coding but possible.
1. Add a field to the admin page/post for the masterpage
2. Then in BE Core have to change the [Page/Post] to enable that field into each [Page/Post]
3. Then have the Code behind file of the site.master.cs select the master page from the current page [masterpage] field.
In theory that should work.
markoh: Do you have a live example of a website thats is currently doing this? Just want to see how it looks.
Also why have so many different masterpages?
Just wanted to know to help out better and to understand what you wanted to do.
What does each masterpage have that different?
If its just the sidebars that can be done another way instead of masterpages.
Have a Great Day!