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An Exploration of the ObjectSpaces Framework

Accessing data through application business domain objects

With the release of the alpha bits of ADO.NET 2.0 and Visual Studio 2005, Microsoft is experimenting with a framework that provides .NET programmers with a new way to access and work with relational data. Traditionally, passing and persisting data to the data storage layer has been very data-centric, as it consisted of thinking of data in terms of tables, rows, and columns. This conflicts with the object-centric paradigm of the business logic layer where data is defined in terms of objects and properties. As a result, a lot of custom code is written and maintained to couple the data-centric database storage layer to the object-centric business layer. ObjectSpaces enables you to work with the data through the business- level objects that make up the application domain. The classes that make up the ObjectSpaces architecture allow you to abstract out the complexity of the data-centric syntax of the storage medium from the object-oriented developer. This article introduces the ObjectSpaces framework and demonstrates the fundamentals of implementing ObjectSpaces in a .NET application.

ObjectSpaces: An Introduction
A valid question you may be asking is, "What is the ObjectSpaces framework and what problem is it trying to solve?" Quite simply, ObjectSpaces is technology built on top of ADO.NET, to provide data-access support. With the addition of ObjectSpaces, ADO.NET supports three main forms of data access technology. The most obvious and utilized data-access technology is the DataSet/DataReader. The strength of DataSets and DataReaders are that they incorporate a relational model common to most relational back-end databases and fully expose the database functionality. They also provide the highest performance compared to the other data-access technologies.

Another data-access technology is SQLXML. The strength of SQLXML is its ability to decouple the data from the database schema. When data needs to be exposed as XML structures, such as Web services and B2B applications, SQLXML is generally the data access of choice. It also provides the ability to map the same data to different schemas for content publishing to various vertical industry standards.

ObjectSpaces is similar to SQLXML in that it decouples the data structure from the database schema. The difference is that ObjectSpaces exposes the schema based on the business-level objects in the application as opposed to XML structures. ObjectSpaces is the way to go if the application has a strong business object layer and the data needs exposure through an object model instead of a relational model.

As you can see, each of these data access technologies exposes a different set of functionalities to solve various problems. It is important that you understand the strengths and weaknesses of each of the data-access technologies. Armed with this knowledge, you will make better decisions on how to design your data-access strategies.

ObjectSpaces is an object-relational mapping (O/RM) framework embedded in ADO.NET. Although O/RM technologies are not new and several third parties offer .NET add-ins, Microsoft is including ObjectSpaces as part of the .NET Framework. O/RM essentially couples the application's object model with the underlying data storage engine. To accomplish this, the O/RM engine uses a mapping file to make the necessary translations.

The ObjectSpaces engine uses an XML mapping file that translates from an object query/update call into the SQL query/update call. By moving the mapping schema into a separate file, objects can be persisted and retrieved from the data storage without writing provider-specific code. Object-oriented programmers can work with the object data using familiar object-oriented syntax.

Mapping Object Relationships
For O/RM to work, the O/RM engine needs a mapping file that defines the schema of the data base, the schema of the objects, and how the schema is mapped between the two. To demonstrate the creation of a mapping file we will create a simple object type – product – and map it to the products table in the Northwind database. Listing 1 (the listings are online at www.sys-con.com/dotnet/sourcec.cfm) defines the product class. Listing 2 shows the XML mapping file used to define the mapping between the product class and the products table.

The database definition schema is defined within the <m:Data Source> </m:DataSource > tags. The object schema is defined within the <m:DataTarget> </m:DataTarget> tags. Notice that object schema tags contain the prefix "osd:" as a namespace alias. (One little caveat of which to be careful: when exposing your data through properties of the class instead of public fields, you need to specify the property as an alias attribute of the field <osd:member> node.)

The mapping schema is defined within the <m:Mappings> </m:Mappings> tags. As you probably realized, creating even the simplest of mapping files is not a trivial exercise and when creating one by hand, using note pad is not practical. Fortunately, Microsoft will provide a mapping utility that will greatly simplify this process. Figure 1 shows the current incarnation of the mapping utility, which allows you to drag and drop data tables and classes onto the design surface and visually create the mappings.

ObjectSpaces supports complex mappings such as one-one, one-many, and many-many. As an example of a one-many mapping, a Supplier class is defined that is related to the Product class. To link the parent Supplier class with the child Product class, a property of a type that implements the IList interface is defined. The child class links back to the parent class through a property of type parent in the child class.

Listing 3 contains the definitions of the Supplier and Product classes. To support this relationship, the mapping file is updated to include an <osd:ObjectRela tionship> node that includes a schema defining the relationship between the two objects. The updated schema also reflects the definition relating the Supplier table and the Products table from the Northwind Database. Listing 4 shows the updated nodes added to the mapping files.

Retrieving Objects
To use ObjectSpaces, the application needs to reference the System.Data.ObjectSpaces and System.Data.SqlXml assemblies. The ObjectSpace class acts as the broker between the database model and the object model. An ObjectSet is then instantiated through the GetObjectSet method of an ObjectSpace. The ObjectSet represents a collection of objects returned from the GetObjectSet method. The GetObjectSet method takes a parameter of type string, which defines the query used to retrieve the object data (more about this later). Listing 5 demonstrates retrieving and populating a collection of Product objects and displaying their ProductName properties in a list. Figure 2 shows the resulting list. The ObjectSet provides functionality similar to the DataSet. For example, it enables data binding to Windows Forms controls and tracking values for optimistic concurrency checking.

Along with the features provided with the ObjectSet, the results of an object query can be returned through an ObjectReader. The ObjectReader is analogous to the DataReader. It provides a forward-only, low-overhead stream of objects. It is useful when you need to read the data out of the Objects but do not need the overhead of persisting the entire object model. Listing 6 demonstrates retrieving the ProductName property of the Product objects and loading them into a list box using an ObjectReader.

Spans and Delay Loading
Spans and delay loading provide more control over the way the object model is loaded. A span identifies the related objects that are returned along with the objects retrieved by the query. For example, when retrieving the Supplier objects, you can use a span to retrieve the associated products. Listing 7 uses a span to retrieve both the Suppliers and their related Products. Each Supplier's CompanyName property is loaded into a TreeView control as a parent node and the associated Products' ProductNames are loaded as the corresponding child nodes. (Remember that the relationship between these objects must also be modeled in the mapping file.) Figure 3 shows the resulting TreeView display.

For performance reasons, there are times when you do not want to load the related objects immediately. ObjectSpaces supports on-demand loading of the related objects using delayed loading. To implement delayed loading, ObjectSpaces provides two classes that provide a wrapper for the classes in the relationship. The ObjectList class provides delayed loading for a one-many relationship and the ObjectHolder class provides delayed loading for a one-one relationship. Listing 8 shows the Supplier class altered to take advantage of delayed loading. Notice the Products field and the Products property are of type ObjectList. Listing 9 shows the loading of a second list box depending on the value selected in the first list box by delayed loading. Figure 4 shows the resulting list boxes.

Querying Objects with OPath
One of the most interesting aspects of the ObjectSpaces framework is the development of an object query language. OPath allows object-oriented programmers the ability to use familiar object-oriented syntax to query the object model. It includes a rich feature set of literals, operators, predicates, and functions. Using OPath, programmers can construct complex queries using the syntax with which they are most comfortable working, while giving the ObjectSpaces engine the arduous task of converting it to the database querying syntax. In the previous examples, an empty string was passed in with the instantiation of the ObjectQuery object. This results in pulling back the entire object set. Listing 10 demonstrates restricting the result set using an OPath query. Listing 11 contains the ensuing SQL generated by the ObjectSpaces engine. Although I can only scratch the surface of the OPath query language in this article, look for a future article devoted to this topic as the language matures.

Conclusion
With the ObjectSpaces framework, Microsoft is providing an intriguing technology to .NET programmers. The classes that make up the ObjectSpaces architecture provide the ability to abstract out the complexity of the data centric syntax of the storage medium. Object-oriented developers can work with the data using familiar OOP syntax and the object centric model of the business layer. ObjectSpaces is evolving to offer a rich feature set including object/relational mapping, delayed loading, data binding, transactional support, and an optimistic concurrency model.

The possibilities of this technology are fascinating and I hope that this article gave you a feel for the direction in which Microsoft is moving with the ObjectSpaces framework. Unfortunately, we will have to wait a little longer for the full release of this framework. Microsoft has recently announced that the release of the ObjectSpaces framework is now aligned with the WinFS/Longhorn release timeframe. On the upside, this indicates that ObjectSpaces is an integral part of Microsoft's future and will only get better and more mature over time.

More Stories By Dan Clark

Dan is a Microsoft Certified Trainer, Microsoft Certified Solution Developer, and a Microsoft Certified Database Administrator. For the past seven years he has been developing applications and training others how to develop applications using Microsoft technologies. Dan has been developing and training Microsoft's .NET technologies since the early betas. He has recently authored the book "An Introduction to Object-Oriented Programming with Visual Basic .NET," published by Apress.

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Most Recent Comments
Zeeshan Shakeel 08/13/07 09:05:27 AM EDT

interesting article, fantastic effort, welldone Dan
take care

Santosh Benjamin 11/22/04 09:51:38 AM EST

Dan,
Sorry i missed that part of the conclusion. I agree that its good for the maturity of the framework, but i think they will also lose some momentum in this space to open source technologies such as NHibernate etc. Anyway, all we can do is wait and see. Perhaps it will turn out to be a killer app!!

Dan Clark 07/07/04 02:34:03 PM EDT

Santosh - You are correct that ObjectSpaces has been stripped from ADO 2.0, as I mentioned in the conclusion. The good news is that it will resurface in the near future.

Santosh Benjamin 07/07/04 10:29:08 AM EDT

Interesting article. As far as i know though, ObjectSpaces is not in ADO.NET 2.0 and has been moved to the Longhorn/Orcas timeframe. See the following articles
http://blogs.msdn.com/aconrad/archive/2004/06/01/146315.aspx
and
http://weblogs.asp.net/aaguiar/archive/2004/05/26/141931.aspx

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