public class Item
  {
      public int Id { get; set; }
      public RelatedItem Related { get; set; }
  }

  public class RelatedItem
  {
      public string Value { get; set; }
  }

var listItem = new List<Item>();

for (int i = 0; i < 100; i++)
{
    listItem.Add(new Item
    {
       Id = i,
       Related = i % 2 == 0 ? new RelatedItem{ Value = i % 4 == 0 ? "Related value " + i : null } : null
    });
}

var result1 = listItem.Select(i => i.Related?.Value);
var result2 = listItem.AsQueryable().Select("Related.Value");