C#: To be XML serializable, types which inherit from IEnumerable must have an implementation of Add(System.Object)

The Add() method should have a single argument of the type of the elements in the IEnumerable object. For example, if FindItemsResults implements IEnumerable<T> then you can add method void Add(T value). If you want FindItemsResults to be read-only, you could convert FindItemsResults to a List via the ToList() extension method and serialize the list rather than the FindItemsResults object itself.