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Missing objects and the null entity

Last modified on August 15, 2012 17:20

A reference navigation property and some scalar queries return the null entity when there is no entity to return. The null entity (aka, the nullo) is a special version of an entity class that represents the missing object.

In principle, every Order should have a parent Customer. What if a particular noCustOrder doesn't have a parent Customer? What should its Customer navigation property return?

Should it return null (Nothing in VB)? Then the statement noCustOrder.Customer.CompanyName will throw a NullReferenceException because the null value doesn't have a CompanyName property. 

Should we wrap every reference navigation in a giant try/catch block? Or should we follow every entity navigation with a test for null? That might be worse than catching an exception. In some situations - in UI code for example - it is difficult to check whether noCustOrder has a Customer, let alone do something about it.

Fortunately an entity reference navigation neither returns a null value nor throws an exception. Instead, when the entitymanager can't find the Customer, it returns the Customer null entity.

The null entity

The null entity represents the "entity not found". The null entity (known informally as the nullo) is a sentinel object that looks and behaves, for the most part, like a real entity instance.

In most respects it is a real entity

  • it has a specific type (e.g., Customer)
  • it has the properties, methods, and events of its type.
  • it belongs to an EntityManager

The null entity has four important differences:

  • You can tell its the null entity because noCustOrder.Customer.EntityAspect.IsNullEntity returns true
  • You cannot set a nullo property.
  • You cannot create, delete or save a nullo.
  • There is only one nullo per type in a given EntityManager; you can't have two Customer nullos in the same EntityManager.

Actions returning a nullo

Among the ways to get a null entity are

  • a reference navigation that cannot find the parent entity
  • a scalar query that fails to find the requested entity
  • ask the EntityManager for it

Reference navigation

A reference navigation property returns a single entity. The Order.Customer is a reference navigation property returning the order's parent Customer. Because noCustOrder lacks a parent Customer, the statement noCustOrder.Customer returns a Customer nullo.

We don't worry about null entities with collection navigations. A collection navigation property returns a list. The list is never null. It is an empty list if there are no related entities. The statement emptyOrder.OrderDetails returns an empty list of OrderDetail.

Scalar query

A scalar query returns a single object. In the following example, the developer executes a scalar query, requesting that the query return either the first entity found or the null entity if there is no match. There is no Customer with a zero ID.

  cust = manager.Customers
                .Where(c => c.CustomerID == 0) // doesn't exist
  cust = (From c in manager.Customers  _
          Where c = 0 _' doesn't exist
         Select c) _

Note that query would have returned null - the cust value would be null - if the developer executed the query with FirstOrDefault.

Get from manager

Although you can't create a nullo, you can ask an EntityManager to give you the nullo of a particular type.

nullo = manager.GetNullEntity<Customer>(); // the Customer nullo
nullo = manager.GetNullEntity(Of Customer)() ' the Customer nullo

Nullos belong to an EntityManager

A null entity always belongs to a particular EntityManager, the same EntityManager as the entity which produced it. The nullo from noCustOrder.Customer belongs to noCustOrder's manager.

While there can be only one Customer nullo per EntityManager there can be two nullo Customer instances ... if there are two EntityManagers.

The requirement that a null entity belong to cache has an unexpected implication. What if the noCustOrder is not in cache ... if it is detached?

Then the statement noCustOrder.Customer returns the null value. You won't get a nullo from a detached entity. A detached entity can't return a nullo because a null entity always belongs to a particular EntityManager and a detached entity doesn't have an EntityManager. DevForce doesn't know what EntityManager to use. All it can do is return null.

The default nullo

Every entity class defines its own null entity.

You don't have to do anything special. By default, its simple data properties return the default value for the property's type. An int returns 0. A string returns the empty string. A property returning a native .NET object type returns a null object of that type; a nullable int returns null.

Navigation properties are different. 

  • A nullo's reference navigation property returns another nullo. The statement noCustOrder.Customer.Region returns the Region nullo. This makes it safe to chain nullos as in noCustOrder.Customer.Region.Country.CountryName.

  • A collection navigation property returns an empty collection. The statement noCustOrder.Customer.Orders returns an empty list of Orders.

ComplexType properties return a real ComplexType object with property values like a nullo.

Configure the nullo

The CompanyName property of the Customer nullo returns an empty string. Suppose you prefer that it return a canned value such as "N/A" or "[none]". You can change the property values of a nullo if you don't like the defaults by overriding the UpdateNullEntity method in the partial class.

Tags: Navigation
Created by DevForce on December 06, 2010 18:02

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