Saving a WPF C# App Richtextbox using savefiledialog

Question

How do you go about saving the contents of the richtextbox using the save file dialog? I don't want to point to a specific location (i.e. fStream = new FileStream(@"C:", FileMode.Create); ...as this throws an exception "Access to the path 'C:\Windows\SysWOW64' is denied." Any help on how to implement the save filedialog much appreciated.