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context.SaveChanges(); } catch (OptimisticConcurrencyException ex) { Console.WriteLine("Concurrency Exception! {0}", ex.Message); } catch (Exception ex) { Console.WriteLine("Exception! {0}", ex.Message); } } The following is the output of the code in Listing 14-1: High Country Backpacking Tent Unit Price: $199.95 Changing High Country Backpacking Tent's Unit Price to: $239.95 Concurrency Exception! Store update, insert, or delete statement affected an unexpected number of rows (0). Entities may have been modified or deleted since entities were loaded. Refresh ObjectStateManager entries.
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Optimistic concurrency is not enabled by default when tables are imported into a model. To enable optimistic concurrency, change the Concurrency Mode property of one of the entity s properties to Fixed. You do not have to use a TimeStamp property as we did in this recipe. You do need to choose a property that you know will be changed in every update to the underlying table. Typically, you would use a column whose value is generated by the database on each update. The TimeStamp column is a good candidate. If you choose another column, be sure to set the StoreGeneratedPattern property to Computed for the corresponding entity property. This will tell Entity Framework that the value is generated by the database. Entity Framework recognizes the TimeStamp data type as a Computed property. In Listing 14-1, we inserted a new product into the database. We queried the model for the one product we inserted. Once we had the product, we updated the row out-of-band using the ExecuteStoreCommand() method to send a SQL update statement to the database changing the row. On the database side, this update caused the UnitPrice to be changed to $229.95 and the TimeStamp column to be updated automatically by the database. After the out-of-band update, we changed the UnitPrice on the product in the object context to $239.95. At this point, the object context believes (incorrectly) that it has the most recent values for the product, including an update to the UnitPrice now set at $239.95. When we call SaveChanges(), Entity Framework generates an update statement with a where clause that includes both the ProductId and the TimeStamp values we have for the product. The value for this TimeStamp is one retrieved when we read the product from the database before the outof-band update. Because the out-of-band update caused the TimeStamp to change, the value for the TimeStamp column in the database is different than the value of the TimeStamp property on the product entity in the object context. The update statement will fail because no row is found in the table matching both the ProductId and the TimeStamp values. Entity Framework will respond by rolling back the entire transaction and throwing an OptimisticConcurrencyException.
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In responding to the exception, the code in Listing 14-1 printed a message and continued. This is probably not how you would handle a concurrency violation in a real application. One way to handle this exception is to refresh the entity with the current value of the concurrency column from the database. With the correct value for the concurrency column, a subsequent SaveChanges() will likely succeed. Of course, it might not for the same reason that it failed the first time, and you need to be prepared for this as well. The Refresh() method on the object context is used to refresh an entity including the current value of the concurrency column. It takes two parameters: a RefreshMode and an entity. With RefreshMode.ClientWins, property changes made to the entity in the object context are not updated from the database. Only the values, such as the concurrency column value, that are different in the database but not changed in the object context are updated on the entity. In short, changes made in the object context (client) are kept. With RefreshMode.StoreWins, changes made to the entity in the object context are overwritten by values from the database. The database wins. After calling Refresh(), the entity in the object context has the values reflecting either the client winning or the database winning. Calling SaveChanges() at this point will generate a new update statement with the new values. This update will either succeed or fail with possibly another OptimisticConcurrencyException if an intervening update statement once again changed the values in the database.
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