Workshop

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This workshop will help reinforce the concepts covered in today's lesson.

Quiz

1:	Why can't you pass a data reader by value between application domains?
A1:	Data reader classes are derived from MarshalByRefObject , which can be referenced from remote application domains but not serialized and copied between them. As a result, you can pass a data reader by reference but not by value. However, passing it by reference means that each time a method or property of the data reader is accessed from the remote domain, a call to the hosting domain must be made and the results returned. This results in unnecessary overhead.
2:	When would you use GetValues instead of methods such as GetString , GetInt32 , and GetByte ?
A2:	The GetValues method retrieves all the columns in the row and places them in an array of type System.Object . This is more efficient than reading each column individually, although to use the data with a strong type, you then need to access the element of the array and cast it to the appropriate type.
3:	How can I retrieve a large binary value with a data reader?
A3:	For large binary values, you would typically want to call the GetBytes method on the column repeatedly, each time retrieving a specific amount of data and placing it in a buffer (an array of bytes).
4:	Why would you declare a parameter as IDataReader rather than as SqlDataReader or OleDbDataReader ?
A4:	Creating methods that accept parameters or variables declared as the IDataReader interface rather than a derived type allows your code to work with any .NET Data Provider. Two specific examples include writing code in an ASP.NET page that casts the data reader returned from a data access class into IDataReader and then binds the data reader to a DataGrid , and writing methods that manipulate any object that implements the IDataReader interface.

Exercise

Q1:

Write a method that uses a SqlDataReader to save the cover images of all titles for which images exist in the database. (Note that the images are JPEG and are all the same cover.)

A1:

One possible solution might be Private Sub SavePhotos(ByVal connect As String) Dim con As New SqlConnection(connect) Dim com As New SqlCommand( _ "SELECT ISBN, cover FROM Titles WHERE cover IS NOT NULL", con) Dim dr As SqlDataReader Dim isbn As String Dim cover() As Byte Try con.Open() dr = com.ExecuteReader(CommandBehavior.CloseConnection) Do While dr.Read isbn = Trim(dr.GetString(0)) cover = CType(dr.GetValue(1), Byte()) ' Now write out the file Dim fs As New FileStream(isbn & ".jpg", FileMode.OpenOrCreate) Dim br As New BinaryWriter(fs) br.Write(cover) br.Close() fs.Close() Loop Catch e As Exception ' Handle the error Console.WriteLine(e.Message) Finally dr.Close() End Try End Sub

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