c# qr code reader Updating a disconnected object that has already been changed in Visual C#

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Listing 7.7 Updating a disconnected object that has already been changed
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public static void UpdateSubject(Subject changingSubject) { LinqBooksDataContext context = new LinqBooksDataContext(); Subject existingSubject = context.Subjects .Where(s => s.ID == changingSubject.ID) .FirstOrDefault<Subject>(); existingSubject.Name = changingSubject.Name; existingSubject.Description = changingSubject.Description; context.SubmitChanges(); }
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In the case of objects that have already been updated, simply attaching them to the DataContext will fail. No values would be flagged as needing to be updated, as the change-tracking service will have been unaware of the changes. Here, we need to fetch the record from the database based on the ID of the object that we re trying to update B. Then, we need to update each property as necessary C. If the values in the properties are the same, the change-tracking service will continue to exclude those properties from needing to be updated. When we call SubmitChanges, only the properties and objects that have changed will be submitted. D Be aware that the object we re updating may have been based on values that have been subsequently changed in the database. In order to manage concurrency tracking, our best option is to supply a timestamp that indicates
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Peeking under the covers of LINQ to SQL
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the database version when originally fetching the record. If adding the timestamp column is not an option, we can retain a copy of the original values or a hash of the original values. We can then compare the appropriate values and manage the concurrency ourselves. The DataContext s object identity and change-tracking services play a crucial role in the object s life cycle. If we simply need to read the values, we can set the DataContext to a read-only mode by setting the ObjectTrackingEnabled to false, thus bypassing these services. However, if we need to be able to change and persist the values, tracking the objects and changes is critical.
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Summary
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On the surface, LINQ to SQL allows for easy access to querying and updating abilities without the need to manually define the database access. Under the covers, it offers a powerful set of mapping structures, expression parsing, and entity management. You can use the technology without fully understanding how everything works under the covers. The more you understand how it works, the less likely you are to find yourself experiencing unexpected results. If nothing else, it is important to understand how the DataContext manages the object identity and change management so that you make sure you update the correct information. At this point, we ve covered the core concepts behind LINQ to SQL and pulled back the covers to get an understanding of how it works. In the next chapter, we ll continue to examine LINQ to SQL by exploring some more advanced functionality. By the time we re done, you should have a full tool belt for using LINQ with the SQL Server family of databases.
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Advanced LINQ to SQL features
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This chapter covers:
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Handling concurrency Working directly with the database using passthrough queries, stored procedures, and user defined functions Improving the business tier with compiled queries, partial classes, partial methods, and object inheritance Comparison of LINQ to SQL with LINQ to Entities
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Advanced LINQ to SQL features
In the last couple of chapters, we discussed the core components of working with relational data using LINQ to SQL. We saw how the mapping metadata combined with the IQueryable interface and expression trees to enable us to apply the same LINQ to Objects query expressions to relational data stores. By leveraging common APIs, we can eliminate vast amounts of data plumbing code and focus more directly on the business needs. In this chapter, we re going to extend the basic concepts and see some of LINQ to SQL s more advanced features. We ll begin by expanding on our discussion of the object life cycle, focusing on concurrency and transaction issues. We ll continue by exploring how we can work more directly with the database and take advantage of some of the more specific functionality offered by SQL Server. Moving beyond the data tier, we ll look at options LINQ to SQL gives us to customize the business tier, including precompiling query expressions, using partial classes, and polymorphism via inheritance. We ll conclude by briefly exploring the upcoming Entity Framework as an alternative to LINQ to SQL for accessing relational data.
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