Tài liệu Dealing with Update Failures pptx

Dealing with Update Failures

So far, the examples you've seen have assumed that the updates pushed to the database by

the Update() have succeeded. In this section, you see what happens when updates fail-and

what you can do about it.

Note You'll find all the code examples shown in this section in the

HandlingUpdateFailures.cs file located in the ch11 directory. This program listing

is omitted from this book for brevity.

In the examples in this section, assume that the CommandText property of a

SqlDataAdapter object's UpdateCommand is set as follows:

UPDATE Customers

SET

CompanyName = @NewCompanyName,

Address = @NewAddress

WHERE CustomerID = @OldCustomerID

AND CompanyName = @OldCompanyName

AND Address = @OldAddress

This UPDATE statement uses optimistic concurrency because the updated columns are

included in the WHERE clause.

An Update Failure Scenario

Consider the following scenario that shows an update failure:

1. User 1 retrieves the rows from the Customers table into a DataTable named

customersDataTable.

2. User 2 retrieves the same rows.

3. User 1 updates the CustomerName DataColumn of the DataRow with the

CustomerID DataColumn of J5COM and pushes the change to the database. Let's

say User 1 changes the CustomerName from J5 Company to Updated Company.

4. User 2 updates the same DataRow and changes the CompanyName from J5

Company to Widgets Inc. and attempts to push the change to the database. User 2

then causes a DBConcurrecy-Exception object to be thrown and their update fails.

(The same exception occurs if User 2 tries to update or delete a row that has

already been deleted by User 1.)

Why does the update fail in step 4? The reason is that with optimistic concurrency, the

CompanyName column is used in the WHERE clause of the UPDATE statement.