RavenDBIndexDefinition.cs

public interface IBaseItem
{
      int Id { get; set; }
      string Name { get; set; }
}

public class ItemsIndex : AbstractIndexCreationTask<IBaseItem>
{
    public ItemsIndex()
    {
        Map = items => from item in items
                       select new
                       {
                           item.Id,
                           item.Name
                       };
    }
}