CHAPTER 8    175 Entity Framework Entity Framework (EF) is Microsoft’s Object Relational Mapping (ORM) solution and was first released with .NET 3.5SP1. Entity Framework received much criticism when it was first released and the team at Microsoft has been hard at work to address some of these criticisms in the latest version.  WARNING This chapter is written using a preview version of EF4, so final functionality may differ. It is also worth noting that EF is likely to have releases out of band. For the examples in this chapter you will need to install the EF CTP 2. EF and LINQ to SQL Some developers are understandably confused by the overlap between EF and LINQ to SQL. EF and LINQ to SQL were developed by two different teams—hence the overlap. LINQ to SQL is a great piece of technology and very suitable as a simple, light-weight wrapper to SQL. It is pretty clear, however, that Microsoft is pushing developers to use EF. This is a sensible (although no doubt irritating) move as LINQ to SQL is fundamentally flawed as a generic ORM solution in that • It only works with SQL Server. • Generated classes must have a 1 to 1 relationship with database objects. EF provides an abstraction above the database layer and a number of enhancements that make it superior to LINQ to SQL. Is LINQ to SQL Dead? Er, probably not. In October 2008 Microsoft’s Tim Mallalieu (Program Manager, LINQ to SQL and LINQ to Entities) stated: “We’re making significant investments in the Entity Framework such that as of .NET 4.0 the Entity Framework will be our recommended data access solution for LINQ to relational scenarios.” http://blogs.msdn.com/adonet/archive/2008/10/29/update-on-linq-to-sql-and-linq-to- entities-roadmap.aspx CHAPTER 8  ENTITY FRAMEWORK 176 However, after feedback from a large number of LINQ to SQL customers, Microsoft seemed to back off on this a bit, when Tim said: “We will continue make some investments in LINQ to SQL based on customer feedback. This post was about making our intentions for future innovation clear and to call out the fact that as of .NET 4.0, LINQ to Entities will be the recommended data access solution for LINQ to relational scenarios “ http://blogs.msdn.com/adonet/archive/2008/10/31/clarifying-the-message- on-l2s-futures.aspx LINQ to SQL changes In VS2010/.NET 4.0 LINQ to SQL has a number of welcome performance enhancements and bug fixes. It is slightly troublesome that there is very little information on this at the time of writing, but for a full list please see the blog post by Damien Guard (who works at Microsoft within the data programmability team) at http://damieng.com/blog/2009/06/01/linq-to-sql-changes-in-net-40. Why Use EF? The sections below cover the benefits of using an ORM solution, although please note that some of these advantages are not EF specific—excellent (and more mature) alternatives do exist. Probably the best- known and one of the most mature ORM in the .NET world is NHibernate. You can, however, view an extensive list at http://en.wikipedia.org/wiki/List_of_object-relational_mapping_software. Abstraction Data is generally held in a relational manner that is optimized for storage and quick retrieval (well, in most cases it should be). However, this format tends to not map that easily to how we want to work with objects. ORMs can provide a conceptual mapping that allows you to organize your entities and their properties in a manner that is different than how they are physically stored. For example, I worked on an application for a hospital that stored data about patients in two tables: Person and Patient. These tables had a one-to-one relationship. Person held demographic details and Patient held clinical-specific information. Whether this is the best database design is not important, but you can probably see that when you want to work with a “patient,” it is tedious to have to work with two separate objects. EF allows you to create an object that is spread across two tables, allowing you to write more intuitive code, such as the following: Patient.Firstname Patient.BloodType Code Generation A great advantage of using an ORM solution is that it provides basic CRUD (created, read, update, delete) functionality saving you from having to write this part. As code is automatically generated, the potential for errors is also reduced. Using an ORM can also make it easier to adhere to naming conventions as classes and methods are automatically generated. CHAPTER 8  ENTITY FRAMEWORK 177 It is worth noting that by utilizing an ORM framework for data access, you are introducing additional overhead to data access, and that for high performance applications, you may be better off working with the database directly. Generally, however, the performance implications will probably be pretty minimal and outweighed by the other advantages an ORM solution can bring. Support for Different Databases EF provides support for a number of different databases in addition to SQL Server, such as Oracle, MySQL, SQLAnywhere, and, recently, Synergex’s Synergy and a few more. EF contains its own query language and mechanisms that provide you with a consistent query mechanism that can be used across many different data sources. EF doesn’t care whether your database uses T-SQL or PL/SQL—querying is the same. Design Time Support EF has excellent design-time support that developers will quickly get to grips with. Yes, it was a bit ropey in the original version, but this is much improved in VS2010. I don’t think the majority of ORMs provide this level of GUI functionality, so this is definitely one of EF’s strengths. Utilize LINQ LINQ to Entities allows you to construct some very complex queries utilizing the .NET Framework that would not be possible with standard SQL. N-Tier Application Development EF has a number of features (particularly in the latest release), such as support for POCO (Plain old CLR object e.g.net classes!) and self-tracking of change templates, that make it a great solution for the development of n-tier applications. Where is EF Used? Several areas of .NET 4.0 and VS2010 support or are dependent on EF: • WCF Data Services • Windows Azure • Dynamic data framework • Microsoft Ajax libraries You can, of course, use EF anywhere in your application, but you may find it is particularly suitable to some areas, such as: • ASP.NET MVC • Silverlight/WPF (many controls have inbuilt EF support) • ASP.NET (many controls have built-in support for binding to EF objects) CHAPTER 8  ENTITY FRAMEWORK 178 EF 101 Many developers may be unfamiliar with EF as it was released in between VS2008 and VS2010, or others may have heard bad things about it (not all of which are true). Let’s take a quick look at how to work with EF before looking at the new features in EF4. Entity Data Model All EF applications contain an entity data model (EDM) that describes the objects in the underlying data source. The EDM can be further divided into three layers: • Conceptual model (the view your users see) • Storage model (how data is actually stored) • Mapping model (links the conceptual and storage models) EDMs are stored as XML and are composed of three main sections (which link to the three conceptual layers already described): • CSDL (Conceptual Schema Definition Language) • SSDL (Store Schema Definition Language) • MSL (Mapping Specification Language) It is important to understand the format of the EDM file. EF contains good GUI support, but for more advanced customizations it is necessary to modify the XML file directly. The exact format of your EDM file is also dependent on how it is generated (described next). If you generate your model using the wizard in Visual Studio, the EDM will be held in one file with the extension .edmx with the conceptual, storage, and mapping sections split under the following nodes: • edmx:StorageModels • edmx:ConceptualModels • edmx:Mappings If, however, you generate your model using EDMGen.exe (discussed next) then the model will be split into three separate files: .SSDL, .CSDL, and .MSL. Creating an EDM You can create an EDM in three different ways: • The EDMGen command-line tool • By using the ADO.NET data model wizard in Visual Studio • By creating the model in Visual Studio and then having VS generate your database structure from this model (new to EF4) CHAPTER 8  ENTITY FRAMEWORK 179 EdmGen.exe EdmGen.exe is a command-line tool primarily used for generating an EDM, but it also performs a number of other tasks, such as verification of models and splitting generated files. You may want to utilize EdmGen as part of your application's build process. Creating an Entity Data Model in Visual Studio The easiest way to create an EDM is by using the ADO.NET data model wizard in Visual Studio. 1. Open up Visual Studio. 2. Create a New C# Console application and call it Chapter8.HelloEF. 3. Right-click on the project and select AddNew Item. 4. Select ADO.NET Entity Data Model, and name it Chapter8Model.edmx (Figure 8-1). 5. Click Add. Figure 8-1. Adding an ADO.NET Entity Data Model to our project CHAPTER 8  ENTITY FRAMEWORK 180 6. The Choose Model Contents wizard screen will now appear (Figure 8-2). Select the "Generate from database" option. Figure 8-2. Generate from database 7. Click Next. 8. VS will now ask you for a connection (Figure 8-3). If you haven’t done this already, create a new connection to the example database. Note that my database is called Book, so EF will prefix many settings with the name of the database (you may wish to change this). 9. Click Next. CHAPTER 8  ENTITY FRAMEWORK 181 Figure 8-3. Connection string properties 10. We now need to select the items in the database that we want EF to create entities for. Expand the Tables node and select the following tables (Figure 8-4): • Film • FilmShowing • Order • OrderItem  NOTE The wizard also allows you to create entities for views and stored procedures. CHAPTER 8  ENTITY FRAMEWORK 182 Figure 8-4. Selecting items to generate EF classes for 11. Ensure that you have checked the “Pluralize or singularize generate object names” and "Include foreign key columns in the model" (these are new options in EF—for more information please refer to the following Pluralization section). 12. Set the model namespace as BookModel. 13. Click Finish. EF will then generate entities for the classes we have created and you will be taken to the design view of the generated EF model (Figure 8-5). CHAPTER 8  ENTITY FRAMEWORK 183 Figure 8-5. Design view of EF model Notice how relationships between the entities have been automatically created. This is because our database contained a number of constraints and relationships that were automatically imported into our model. You can also create relationships yourself in the design view. Navigating the EF model When your EF model contains many entities then the designer window can get pretty crowded. You can zoom in and out of the model view by clicking the magnifying icons in the corner of the scroll bars or by right-clicking and selecting the zoom level. Viewing How Entities Are Mapped If you select one of the entities, then view the Mapping Details window, you will see how the individual properties on the entity are mapped to the underlying database fields (Figure 8-6). Now select one of the individual entity properties and Visual Studio will display information in the Properties window, such as the type of field, length, and so on. Notice how you can also change the name of the field and the getter and setter properties' access levels (Figure 8-7). CHAPTER 8  ENTITY FRAMEWORK 184 Figure 8-6. EF Mapping window Figure 8-7. Viewing the properties of Film.Description . the fact that as of .NET 4. 0, LINQ to Entities will be the recommended data access solution for LINQ to relational scenarios “ http://blogs.msdn.com/adonet/archive/ 200 8/ 10/ 31/clarifying-the-message-. Microsoft within the data programmability team) at http://damieng.com/blog/ 200 9 /06 /01 /linq-to-sql-changes-in -net- 40 . Why Use EF? The sections below cover the benefits of using an ORM solution,. e.g .net classes!) and self-tracking of change templates, that make it a great solution for the development of n-tier applications. Where is EF Used? Several areas of .NET 4. 0 and VS 201 0 support