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ADO.NET 3.5 EF allows developers to write less data access code, reduces maintenance, and abstracts the structure of the data into a more business-friendly manner. It can also help to reduce the number of compile-time errors since it generates strongly typed classes from the conceptual model. ADO.NET 3.5 EF generates a conceptual model that developers can write code against using a new data provider called EntityClient, as mentioned previously. EntityClient follows a model similar to familiar ADO.NET objects, using EntityConnection and EntityCommand objects to return an EntityDataReader.
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s Note You can download ADO.NET 3.5 EF and ADO.NET 3.5 Entity Framework Tools from http://
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www.microsoft.com/downloads.
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Understanding the Entity Data Model
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The core of ADO.NET 3.5 EF is in its Entity Data Model. ADO.NET 3.5 EF supports a logical store model that represents the relational schema from a database. A relational database often stores data in a different format from what the application can use. This typically forces developers to retrieve the data in the same structure as that contained in the database. Developers then often feed the data into business entities that are more suited for handling business rules. ADO.NET 3.5 EF bridges this gap between data models using mapping layers. There are three layers active in ADO.NET 3.5 EF s model: Conceptual layer Mapping layer Logical layer These three layers allow data to be mapped from a relational database to a more objectoriented business model. ADO.NET 3.5 EF defines these layers using XML files. These XML files provide a level of abstraction so developers can program against the OO conceptual model instead of the traditional relational data model. The conceptual model is defined in an XML file using Conceptual Schema Definition Language (CSDL). CSDL defines the entities and the relationships as the application s business layer knows them. The logical model, which represents the database schema, is defined in an XML file using Store Schema Definition Language (SSDL). The mapping layer, which is defined using Mapping Schema Language (MSL), maps the other two layers. This mapping is what allows developers to code against the conceptual model and have those instructions mapped into the logical model.
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Most applications running today cannot exist without having a database at the back end. The application and the database are highly dependent on each other that is, they are
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tightly coupled and so it becomes so obvious that any change made either in the application or in the database will have a huge impact on the other end; tight coupling is always two-way, and altering one side will require changes to be in sync with the other side. If changes are not reflected properly, the application will not function in the desired manner, and the system will break down. Let s have look at tight coupling by considering the following code segment, which we used in 11 as part of Listing 11-3: 'create connection Dim conn As SqlConnection = New SqlConnection _ ("Data Source=.\sqlexpress;" & _ "Integrated Security=True;" & _ "database=northwind") 'create command (with both text and connection) Dim sql As String = "select firstname,lastname from employees" Dim cmd As SqlCommand = New SqlCommand(sql, conn) Console.WriteLine("Command created and connected.") Try 'Open connection conn.Open() 'execute query Dim rdr As SqlDataReader = cmd.ExecuteReader Assume you have deployed the preceding code into production along with the database, which has the column names as specified in the select query. Later, the database administrator (DBA) decides to change the column names in all the tables to implement new database policies: he modifies the employees table and changes the firstname column to EmployeeFirstName and the lastname column to EmployeeLastName. After these database changes are made, the only way to prevent the application from breaking is by modifying all the code segments in source code that refers to the firstname and lastname columns, rebuild, retest, and deploy the whole application again. The modified code segment in the preceding code will appear as follows: 'create command Dim sql As String = "select EmployeeFirstName,EmployeeLastName from employees" Though on the surface it doesn t seem difficult to make such changes, if you factor in the possibility that there might be many database-related code segments that require modification of the column names according to the new column naming scheme, this can end up being a tedious approach to upgrade an application so that it can work with the modified database. With ADO.NET 3.5 EF s Entity Data Model, Microsoft has made entity-relationship modeling executable. Microsoft achieved this by a combination of XML schema files and ADO.NET 3.5 EF APIs. The schema files are used to define a conceptual layer to expose the data store s
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