public void SomeMethodToTest()
    {
        var collectionPager = new CollectionPager();

        retrieveListService.RetrieveList(collectionPager);
        // in your test collectionPager.Item contains: testItem1, testItem2
    }
 public void RetrieveList(CollectionPager pager)
 {
     pager.List.Add("item1");
     pager.List.Add("item2");
 }