Can i use Linq to iterate/filter my web.config AppSettings?

Answer 1

Could be because KeysCollection only implements IEnumerable not IEnumerable<T>. Try using the Cast method on the Keys property first, something like:

var query = from q in System.Web.Configuration.WebConfigurationManager.AppSettings.Keys.Cast<string>()
        where q.StartsWith("Foo")
        select q;

Answer 2

Try this if you want the values:

var settings = System.Web.Configuration.WebConfigurationManager.AppSettings;

var query = from string q in settings.Keys
            where q.StartsWith("Foo")
            select settings[q];