c# qr code library The entity life cycle in C#

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The entity life cycle
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Tracking changes
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As we make changes to our objects, the DataContext maintains both the original value of that property and the newly changed value by a change tracking service. By retaining both the original and new values, we can optimize the submission back to the database and only update the changed records. In listing 7.4, we establish two different DataContext objects. Each of them manages its own set of object identities and change tracking.
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Listing 7.4 Identity management and change tracking
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LinqBooksDataContext context1 = new LinqBooksDataContext(); LinqBooksDataContext context2 = new LinqBooksDataContext(); context1.Log = Console.Out; context2.Log = Console.Out;
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Guid Id = new Guid("92f10ca6-7970-473d-9a25-1ff6cab8f682"); Subject editingSubject = context1.Subjects.Where(s => s.ID == Id).SingleOrDefault(); ObjectDumper.Write(editingSubject); ObjectDumper.Write(context2.Subjects.Where(s => s.ID == Id)); editingSubject.Description = @"Testing update";
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ObjectDumper.Write(context1.Subjects.Where(s => s.ID == Id)); ObjectDumper.Write(context2.Subjects.Where(s => s.ID == Id));
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As we did in listing 7.3, we begin by setting up our two data context objects B. We use two contexts for the purpose of this example to simulate two separate users. Each context manages the identity and change tracking services separately. In this example, we retrieve the subject that corresponds to a given Guid. In order to demonstrate the identity management, we fetch the same record from our database in two separate contexts. We also include logging C for each context to output the results to the console (or output) window to prove that we re actually requesting information from the database. We fetch the editingSubject from the first context and display the values in both the editingSubject and the database in the second context D. As shown in table 7.4, the output for both values should be identical. We then change the description on the editing subject, but we don t commit the change to the database E. At this point, the change is only retained in memory via the change tracking service of the context1 instance. Context2 has no knowledge of the change.
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Peeking under the covers of LINQ to SQL
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Table 7.4 State of values returned by queries before and after changes Action Value returned from original query Make change and requery Context1 Original Changed Context2 Original Original Database Original Original
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When we output the results by reissuing the original query, we now see that the description returned by the query on context1 returns our new description value, but the one using context2 still returns the original value. Remember, since each context mimics a different user, this would mean that two users would see different representations of the data. If we were to check the value in the database, we would see that it still retains the original values as well. It is important to realize the difference between the value stored in memory for each context object and the value stored in the database. The results of the query on the second context object are not surprising. However, what is not expected is that requerying the first context returns the object from the identity tracking service instead of a new object from the database. Actually, the second context is returning the object it has retained as well, but since we haven t made any change to the object tracked by the second context, it appears to be identical to the values stored in the database for that row. In addition to tracking changes in column-mapped properties, the changetracking service also monitors changes in object associations. Thus if we were to move a comment from one book to another, the change tracking would maintain that change in memory until we actually submit the change back to the database.
Submitting changes
So far, all of the changes we ve made have been retained in memory and not persisted to the database. It only appears that the changes have been applied on subsequent queries due to the identity tracking on the DataContext. As we demonstrated in chapter 6, submitting changes to the database is done with a single call of SubmitChanges on the context. When SubmitChanges is called, the context compares the original values of the objects that it is tracking with the current values. If these differ, the context packages up the changes and creates the necessary query string to be executed by the database. Assuming no conflicts occurred in the update and the appropriate records are updated, the context flushes its list of changes. If there are problems, the changes are rolled back in the database based on the concurrency management
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