gistfile1.cs

namespace System {
    class Exception<T> : Exception {
        public Exception() { }
        public Exception(string message) : base(message) { }
        public Exception(string message, Exception inner)
            : base(message, inner){ }
    }
}

namespace App {
    class UnkownSetting { }
    class Settings {
        // ...
        throw new Exception<UnknownSetting>();
    }
}