LINQ to SQL and the data access layer in Visual C#

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LINQ to SQL and the data access layer
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To sum up the benefits we ve just listed, we could say that genericity has a cost. What using LINQ to SQL in a RAD way allows is fine-grained customization. We ve just identified using LINQ to SQL directly as an interesting solution for data access, but we said previously that we d consider another option: creating a real data access layer written using LINQ to SQL. Why consider this second option Are there problems if we use LINQ to SQL directly Here are some limitations you should keep in mind if you decide to go the LINQ to SQL RAD route:
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Writing database queries in your presentation layer is not as bad as using SQL code in it (think about ASP.NET s SqlDataSource web control for example), but it s still a questionable practice. If you do it, you re mixing data access code with business logic and presentation code. This means that you ve decided to abandon the benefits of the separation of concerns. There is no single place to look when you want to find all that s related to data access. If all your LINQ to SQL queries are scattered around in your business logic or presentation code, it becomes difficult to review or update all the data access code. Code reuse is not central in such a design. If you want to share LINQ to SQL queries between business objects or screens, where do you put them You can enrich the DataContext class with predefined queries or validation code, but this makes it look like a big bag of tricks without much structure compared to a real data access layer. If you don t create a concrete data access layer, where will you put all the data processing that can t be done with LINQ to SQL This is close to the previous point. Again you can enrich the DataContext class if this is okay with you. Mapping is limited to the table-per-class model. If you want to have entities that span over several tables, as is often the case, it may be better to either use something other than LINQ to SQL or create an application layer that abstracts this limitation and returns richer entities.
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If there were no way to address these concerns then LINQ to SQL would be doomed to be used only in prototypes. If you use LINQ to SQL as is without being careful, it s easy to unknowingly commit the reprehensible architectural sin of mixing the UI, business logic, and data access layers, for example. To avoid this kind of quick-and-dirty use, let s now see how you can create a real data access layer instead of using LINQ to SQL directly in all the layers of your applications.
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LINQ in every layer
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Using LINQ to SQL to create a real DAL If you don t use LINQ to SQL directly in your presentation or business logic layer, you can still use it to create your DAL. This way you can get the benefits of having a true DAL, while still keeping some benefits of LINQ to SQL. Before going further, it may be good to restate the basic goals for a DAL.3
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The DAL should completely hide the underlying data storage and the data access technology used, whether it s an object-relational mapping tool, handgenerated inline SQL, calls to stored procedures, or anything else. This allows the client or upper-level layers to be created with a higher level of abstraction. The DAL should not place any significant constraints on the design of the business object model (also called the domain model). The entire DAL should be replaceable with minimal impact.
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All of the aforementioned goals can be summarized with one word: decoupling. When LINQ to SQL is used in a RAD way, this is not really achieved. This is why you may consider creating a real DAL instead of spreading LINQ to SQL queries all over in your applications. When you create a concrete DAL with LINQ to SQL, there a few points to keep in mind. Here are three of them:
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The ratio of plumbing code to real code can be high compared to direct LINQ to SQL code. If you return LINQ to SQL entities or queries from your DAL, these objects support deferred execution and lazy fetching, weakening your division. Your DAL should return objects that can be passed between components at different tiers.
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We won t address the first point here. It s true that the source code of a DAL that uses LINQ to SQL may look useless because it can be simple and could be used directly in other parts of your applications. However, this is what we want to avoid and one reason for creating a DAL, so this is something you should accept. The fact that the code in your DAL is simpler than equivalent code with literal SQL queries does not mean that it s useless. It s better, in fact! Let s look at the second point. When you write your first DAL method, you may be tempted to write something like listing 13.1.
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Source: Howard Dierking at http://blogs.msdn.com/howard_dierking/archive/2007/04/23/designing-a-domain-driven-data-access-layer.aspx
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