CS 112D Lecture Notes - Concurrency Conflicts, DataViews, and Typed DataSets (Chapter 7)

Concurrency Conflicts

• Pessimistic Concurrency Control (Locking) is not available in ADO.NET
• Optimistic Concurrency Control is available, however. In this scheme, no records are locked, but a DBConcurrencyException is thrown if a record we are about to change has already been changed by another user / process.
• The way changes are detected is by having a very detailed WHERE clause in UPDATE and DELETE queries. For example, here is an update query generated by the DataAdapter configuration wizard:

    UPDATE tblEmployee
    SET FirstName = @FirstName, LastName = @LastName,
        DepartmentID = @DepartmentID, Salary = @Salary
    WHERE (ID = @Original_ID) AND (DepartmentID = @Original_DepartmentID
             OR @Original_DepartmentID IS NULL AND DepartmentID IS NULL)
           AND (FirstName = @Original_FirstName) AND (LastName = @Original_LastName)
           AND (Salary = @Original_Salary OR @Original_Salary IS NULL AND Salary IS NULL)
    

• If no record is found matching the above WHERE clause when the query is executed via a Command object, a DBConcurrencyException is thrown by ADO.NET.

DataViews

• A DataView object allows you to select and/or sort records in a DataTable.
• It is more powerful than using the DataTable's Select method, because it is a real-time view of the DataTable's content, reflecting changes as they are made.
• Every DataTable has a DefaultView() method which returns a DataView we may use, setting its RowFilter and/or Sort properties as needed.
• There is a DataView control you may add to your form, which allows you to set filter and sort criteria as well.

Strongly Typed DataSets

• Typed DataSet - a subclass of DataSet that can be generated for us by Visual Studio. It defines properties that are specific to the Schema of our DataSet.
• Typed DataSets are preferable to using the generic DataSet class because compile-time type checking becomes available, speeding up development and execution time, plus improving reliability.
• For example, The following code for a typed DataSet:

          Dim row As Integer
          Dim newValue As String
          DsCustomerInstance.tblCustomer.Item(row).FirstName = newValue

  is equivalent to the following code for an untyped DataSet (using the declarations from above):

          Dim dsCust As DataSet
          dsCust.Tables("Customers").Rows.Item(row).Item("FirstName") = newValue

• To produce a Typed Dataset, first add a DataAdapter object to your form, then use the Generate DataSet dialog box, which is available from the Data Menu, or by right-clicking on your form.

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