public class Dog : RealmObject, IDocument
    {
        [PrimaryKey]
        public Int32 Id { get; set; }
        public string Name { get; set; }
        public int Age { get; set; }
        
    }

public interface IDocument
    {
         Int32 Id { get; set; }
    }
//code
var realm = Realm.GetInstance();

            realm.Write(() =>
            {
                try
                {
                    Int32 newId = NewId<Dog>(realm); 
                  
                    realm.Add(new Dog { Id = newId, Name = "Rex", Age = 1 });
                }
                catch(System.Exception ex)
                {
                    Debug.WriteLine("Error --> " + ex.ToString()); 
                }
            });

private  Int32 NewId<T>( Realm realm) where T : Realms.RealmObject, IDocument
        {
            try
            {
                 //this query gets an exception
                T obj = realm.All<T>().OrderByDescending(x => x.Id).FirstOrDefault();
                if (obj == null)
                {
                    return 1;
                }
                return obj.Id + 1;
            }
            catch (InvalidOperationException)
            {
                return 1;
            }
        }