9: Modifying Obje cts with LINQ to SQL in Visual C#.NET

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9: Modifying Obje cts with LINQ to SQL
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corresponding values To illustrate the difference, let s consider a slightly modi ed version of the entity class that contains three nonkey values , , and Figure 93 shows the effect of each object that was concurrently updated, causing of the options for an a con ict between the original and database values
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Client wins, or the option: The original values is are reset to the database values so that when called again, the new original values are the same as those that caused the con ict Hence, no con ict occurs during the second call, and the current, in-memory values overwrite the concurrent update Hence, this option circumvents optimistic concurrency by pushing through the updates anyway, despite the optimistic concurrency check speci ed in the mapping So this is most appropriate after some check about the values or a user overmethod when ride This is the default value used by the the option is not explicitly speci ed option: The current Database wins, or the are overwritten and effectively lost This values in the allows changes to other entities that were not in con ict to succeed call The con icting update is simply with the next omitted
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Merge, or the option: The concurrent changes are merged such that the values of properties updated in the current are retained As a result, if con icting version in the changes are made to a property, the client values win over the database values Note that in the particular example in Figure 93, this option does not make much sense based on the semantics of the three properties method also supports an additional Boolean parameter The to ignore con icts in case of delete operations Con ict resolution requires two key decisions: selecting the resolution option, and deciding on the number of retries before abandoning the current set of changes and starting over Each resolution option involves overwriting someone s change or potentially merging inconsistent changes Hence, it is worth seeking human input from a user if the application has a user interface Otherwise, automatic con ict resolution based on predetermined choices may be the only option The number of retries typically is decided in advance and is de ned in the program Optimistic concurrency is intended for cases in which con icting concurrent changes are rare Hence, a small number of retries, often just one, is appropriate and suf cient In practice, repeated con icts in succession may indicate that optimistic concurrency is an unsuitable option Perhaps a more pessimistic approach using transactions is appropriate
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Refreshing Entities Simultaneous changes also can affect entities that have been retrieved but not modi ed Modi ed entities can be refreshed when a con ict is detected method For unmodi ed entities, an explicit by using the call is required to retrieve the latest values Refresh capability is particularly handy when a unit of work runs for a long time An example is when the results are bound to a user interface and the user comes back after a lunch break Another example is when certain entities need to be selectively refreshed for a second unit of work The following code is a call added modi ed version of a previous code sample with a changes only the scalar values and does not affect relaNote that tionship properties such as
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9: Modifying Obje cts with LINQ to SQL
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LINQ to SQL computes the set of changes before generating SQL commands to make the changes persistent You can ask for the set of changes method This gives you a way to inspect the using the changes and possibly do some validations before deciding to call It also provides a handy tool for debugging your applications without affecting the rows in the database The changes are returned as three read-only collections containing entities to be inserted, also has a convenient updated, and deleted, respectively implementation that lists the number of inserts, updates, and class from the Visual deletes Alternatively, you can use the Studio sample project mentioned in the preceding chapter to see the results The following listing shows a usage pattern:
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