Assembly Language: Step-by-Step - part 9 pps

The following code shows an example of reading an XML file into the DataSet and then displaying the data in a DataGrid: Private Sub Button13_Click _ ByVal sender As System.Object, _ ByVa

Reading an XML Document into the DataSet

Reading XML data into a DataSet can be done by simply using the ReadXmlmethod of the DataSet This method has several overloads, but one of the over-loads allows a filename to be passed into the method The filename must be aphysical path, which means that when the XML document is on the Webserver, the Server.MapPath method can be used with a relative virtual address

to obtain the physical path The following code shows an example of reading

an XML file into the DataSet and then displaying the data in a DataGrid:

Private Sub Button13_Click( _ ByVal sender As System.Object, _ ByVal e As System.EventArgs) _ Handles Button13.Click

Dim ds As New DataSet(“MyCompany”) ds.ReadXml(“C:\EmployeeList.XML”) DataGrid1.DataSource = ds.Tables(“Employee”) DataBind()

End Sub

The browser output is shown in Figure 9.12 This code reads the eeList.XML file into the DataSet The DataSet parses the repeating rows intotables The end result is that two tables are created: the Employee table and theAddress table

Employ-The DataSet does well at identifying the XML data, but all of the data typesare strings and many of the data types, such as dates and numbers, producethe desired results This can be corrected by supplying an XML schema AnXSL schema can be supplied as a separate file, or it can be embedded into theXML file For the EmployeeList.XML file, an XML schema might look like thefollowing:

<xs:attribute name=”LastName” type=”xs:string” />

<xs:attribute name=”FirstName” type=”xs:string” />

<xs:attribute name=”Salary” type=”xs:decimal” />

Figure 9.12 The EmployeeList is read into memory and bound to the DataGrid.

Writing an XML Document from the DataSet

A DataSet can be saved to an XML file using the WriteXml method, regardless

of its original source One option that is available is the ability to change theoutput type of each column when writing the data For example, the HireDatecan be an element or an attribute The following code changes all columns ofall tables to attributes and then writes the XML with an embedded schema to

a file called EList.XML:

Private Sub Button14_Click( _ ByVal sender As System.Object, _ ByVal e As System.EventArgs) _ Handles Button14.Click

Dim ds As New DataSet(“MyCompany”) ds.ReadXmlSchema(“c:\el.xsd”) ds.ReadXml(“C:\EmployeeList.XML”) Dim t As DataTable

For Each t In ds.Tables Dim c As DataColumn For Each c In t.Columns c.ColumnMapping = MappingType.Attribute Next

Next ds.WriteXml(“c:\EList.XML”, XmlWriteMode.WriteSchema) End Sub

The code changes all columns by changing the ColumnMapping properties

of all Columns of all Tables to MappingType.Attribute The options for theMappingType are Attribute, Element, Hidden, or SimpleContent

Another change that can be made to the XML output is nesting of parent andchild tables The relation has a Nested property that can be set to control thenesting The following code sets the Nested property to false for all relation-ships in the DataSet:

Dim r As DataRelation For Each r In ds.Relations r.Nested = False Next

When the columns are all changed to attributes and the nesting is set tofalse, the XML output looks like the following:

<?XML version=”1.0” standalone=”yes”?>

<EmployeeList>

<Employee HireDate=”2003-01-01T00:00:00.0000000-05:00” EmpID=”1”

LastName=”GaryLast” FirstName=”Gary” Salary=”50000”

Employee_Id=”0” />

<Employee HireDate=”2003-01-02T00:00:00.0000000-05:00” EmpID=”2”

LastName=”RandyLast” FirstName=”Randy” Salary=”40000”

Employee_Id=”1” />

<Address Street1=”123 MyStreet” Street2=”” City=”MyCity”

State=”My” ZipCode=”12345” Employee_Id=”0” />

<Address Street1=”234 MyStreet” Street2=”” City=”MyCity”

State=”My” ZipCode=”23456” Employee_Id=”1” />

Using the XmlDataDocument with a DataSet

There may be times when is it more desirable to work with data in an XMLfashion instead of table rows and columns This can be done by creating anXmlDataDocument and passing a DataSet into the class constructor In the fol-lowing example, the Suppliers table is read from the Northwind database, andthen an XmlDataDocument is created from the DataSet Finally, a resultanttable is created, containing the SupplierID, CompanyName, and Contact-Name, as shown in Figure 9.13

Figure 9.13 Creating an HTML table by navigating the XmlDataDocument.

Private Sub Button15_Click( _ ByVal sender As System.Object, _ ByVal e As System.EventArgs) _ Handles Button15.Click

‘Connection Dim cn As New SqlConnection() Dim cnstr As String

cnstr = “server=.;integrated security=yes;database=northwind” cn.ConnectionString = cnstr

‘Command Dim cmd As New SqlCommand() cmd.CommandText = _

“Select SupplierID, CompanyName, ContactName from Suppliers” cmd.Connection = cn

Dim da As New SqlDataAdapter(cmd) Dim ds As New DataSet(“NW”) da.Fill(ds, “Suppliers”) Dim x As New XmlDataDocument(ds) Dim nav As XPathNavigator = x.CreateNavigator() Dim node As XPathNodeIterator

node = nav.Select(“//Suppliers”) Response.Write(“<table border=’1’>”)

Do While node.MoveNext() Response.Write(“<tr>”) Dim nav2 As XPathNavigator nav2 = node.Current Response.Write(“<td>”) nav2.MoveToFirstChild() ‘ID Response.Write(nav2.Value & “ “)

Response.Write(“</td>”) Response.Write(“<td>”) nav2.MoveToNext() Response.Write(nav2.Value & “ “) Response.Write(“</td>”)

Response.Write(“<td>”) nav2.MoveToNext() Response.Write(nav2.Value & “<br>”) Response.Write(“</td>”)

Response.Write(“</tr>”) Loop

Response.Write(“</table>”) End Sub

This code builds a simple table containing the SupplierID, CompanyName,and ContactName, using an XPathNavigator

The XmlValidatingReader class performs forward-only validation of a stream

of XML The XmlValidatingReader constructor can be passed to an XmlReader,

a string, or a stream This class has a ValidationType property that can be set toAuto, DTD, None, Schema, or XDR If the setting is set to None, this classbecomes an XmlTextReader

In the next example, the file in Listing 9.1 is validated using the following code:

Private Sub Button16_Click( _ ByVal sender As System.Object, _ ByVal e As System.EventArgs) _ Handles Button16.Click

Dim vr As New XmlValidatingReader( _ New XmlTextReader(“C:\xmltest.XML”)) vr.ValidationType = ValidationType.DTD Dim xd As New XmlDocument()

xd.Load(vr) Response.Write(“Valid Document!<br>”) vr.Close()

End Sub

This code simply opens the XML file with an XmlTextReader, and the reader

is used as the input to the XmlValidatingReader Since this code has an ded DTD, the document is validated

embed-In the next test, the input file has been modified

<?XML version=”1.0” encoding=”utf-8”?>

<!DOCTYPE myRoot [

<!ELEMENT myRoot ANY>

<!ELEMENT myChild ANY>

<!ELEMENT myGrandChild EMPTY>

<!ATTLIST myChild ChildID ID #REQUIRED

>

Figure 9.14 The error that is generated when an invalid document is validated using the XmlValidatingReader.

Lab 9.1: Working with XML Data

You have a requirement to be able to save a customer’s orders, along withthe order details, to an XML file The XML file must have only three levels

of elements The first level is the root element, which is called Customers,and has attributes for the CustomerID, CompanyName, and Contact-Name The second level is the Orders element, which contains an Orderselement for each order that the customer has The third level containsOrder_Details elements, which contain an element for each item in theorder The XML document essentially contains an element for each row ofeach table, and all column data must be presented as XML attributes

In this lab, you modify the DataGrid from the previous lab, to add aSave Orders button to the DataGrid; this button writes the current customer’s orders to an XML file

Retrieving the Data

In this section, you modify the Bindtable method to retrieve the tomers, orders and order details for all customers, and store the results in

cus-a Session vcus-aricus-able Dcus-atcus-aRelcus-ations cus-also is crecus-ated to join these tcus-ablestogether, and the ColumnMapping must be set to be an attribute forevery column in the DataSet

1. Start this lab by opening the OrderEntrySolution from Lab 8.1

2. Right-click the OrderEntrySolution in the Solution Explorer, andclick Check Out This checks out the complete solution

3. Open the CustomerList.aspx.vb code-behind page

4. In the Bindtable method, modify the code to check for the existence

of a Session variable named Customers If it exists, assign the sion variable to a DataSet

Ses-5. If the Session variable does not exist, populate a new DataSet withCustomers, Orders, and Order Details from the Northwind SQLdatabase Add relations between the Customers and Orders tables,and between the Orders and Order Details tables

6. Add a loop, which enumerates all tables and all columns of theDataSet, setting the ColumnMapping to Attribute

7. Store the DataSet in the Customers Session variable Your codeshould look like the following:

Public Sub Bindtable() Dim ds As DataSet

If Session(“Customers”) Is Nothing Then

cnstr = “server=.;integrated security=yes;” _

& “database=northwind”

Dim cn As New SqlConnection(cnstr) Dim sql As String = _

sql = “Select CustomerID, CompanyName, ContactName “ _

& “ from customers”

Dim da As New SqlDataAdapter(sql, cn)

ds = New DataSet(“NW”)

‘Fill Customers da.Fill(ds, “Customers”) ds.Tables(“Customers”).PrimaryKey = _ New DataColumn() _

{ds.Tables(“Customers”).Columns(“CustomerID”)}

‘Fill Orders sql = “Select * from Orders”

da.SelectCommand.CommandText = sql da.Fill(ds, “Orders”)

ds.Tables(“Orders”).PrimaryKey = _ New DataColumn() _

{ds.Tables(“Orders”).Columns(“OrderID”)}

‘Fill Order Details sql = “Select * from [Order Details]”

da.SelectCommand.CommandText = sql da.Fill(ds, “Order_Details”) ds.Tables(“Order_Details”).PrimaryKey = _ New DataColumn() _

{ds.Tables(“Order_Details”).Columns(“OrderID”), _ ds.Tables(“Order_Details”).Columns(“ProductID”)}

‘Create Customers to Orders Relation ds.Relations.Add( _

“CustomersOrders”, _ ds.Tables(“Customers”).Columns(“CustomerID”), _ ds.Tables(“Orders”).Columns(“CustomerID”), _ True)

ds.Relations(“CustomersOrders”).Nested = True

‘Create Orders to Order Details Relation ds.Relations.Add( _

“OrdersOrderDetails”, _ ds.Tables(“Orders”).Columns(“OrderID”), _ ds.Tables(“Order_Details”).Columns(“OrderID”), _ True)

Next Session(“Customers”) = ds

ds = CType(Session(“Customers”), DataSet) End If

dgCustomers.DataSource = ds.Tables(“Customers”) dgCustomers.DataKeyField = “CustomerID”

DataBind() End Sub

Preparing the Data Grid

The DataGrid needs to be updated to have a Save button beside each tomer The Save button is used to initiate the storing of customer data in

cus-an XML file

1. In the Init event method of the DataGrid, add code to create a buttoncolumn

2. Set the properties of the button Be sure that the CommandName is

called Save This is used in the ItemCommand method, in order to

find out which button was pressed

3. Your code should look like the following:

Private Sub dgCustomers_Init( _ ByVal sender As Object, _ ByVal e As System.EventArgs) _ Handles dgCustomers.Init Dim colButton As New ButtonColumn() With colButton

.ButtonType = ButtonColumnType.PushButton CommandName = “Save”

.ItemStyle.Width = New Unit(100, UnitType.Pixel) ItemStyle.HorizontalAlign = HorizontalAlign.Center HeaderStyle.HorizontalAlign = HorizontalAlign.Center HeaderText = “Save Orders<br>as XML”

.Text = “Save”

End With dgCustomers.Columns.Add(colButton) End Sub

Save Customer’s Orders to XML File

In this section, you add code to the ItemCommand method of the Grid This code retrieves the customer primary key of the selected cus-tomer The code then uses an XmlDataDocument to get data from theDataSet and write the data to the XML file

Data-1. Add an if statement to the ItemCommand, which checks to see if theCommand is Save All additional code is placed inside the if statement

2. Add code to retrieve the DataSet from the Session variable

3. Declare a variable called XML as a XmlDataDocument Check to see

if a Session variable called CustomersXml exists If so, assign the

Session variable to the XML variable If not, create a new Document, based on the DataSet, and assign it to the XML variable.

XmlData-4. The XML file is stored in the current Web site folder Add code to getthe current path

5. Add a variable called CustomerKey Retrieve the CustomerKey fromthe DataKeys collection of the DataGrid

6. Declare a variable called xmlWriter, and assign a new instance of theXmlTextWriter to it The filename is the CustomerKey name, with a.XML extension This file is stored in the current folder

7. Write an XML declaration to the file

8. Write code to locate the customer within the XmlDataDocument,and write the customer details to the file

9. Add code to close the XmlTextWriter

10. Save your work Your code should look the following:

Private Sub dgCustomers_ItemCommand( _ ByVal source As Object, _

ByVal e As System.Web.UI.WebControls.DataGridCommandEventArgs) _ Handles dgCustomers.ItemCommand

If e.CommandName = “Save” Then Dim ds As DataSet = _ CType(Session(“Customers”), DataSet) Dim XML As XmlDataDocument

If Session(“CustomersXml”) Is Nothing Then XML = New XmlDataDocument(Session(“Customers”)) Session(“CustomersXml”) = XML

Else XML = CType( _ Session(“CustomersXml”), XmlDataDocument) End If

Dim path As String = Server.MapPath(“.”) & “\”

‘Get Customer Key Dim CustomerKey As String CustomerKey = dgCustomers.DataKeys(e.Item.ItemIndex) path &= CustomerKey & “.XML”

‘Open the XmlWriter.

Dim xmlWriter As New XmlTextWriter(path, _ System.Text.Encoding.UTF8)

xmlWriter.WriteStartDocument() Dim CustomerXml As XmlNode Dim xPathQuery As String xPathQuery = String.Format( _

“//Customers[@CustomerID=’{0}’]”, CustomerKey) CustomerXml = XML.SelectSingleNode(xPathQuery) CustomerXml.WriteTo(xmlWriter)

xmlWriter.Close() End If

Test the DataGrid

The DataGrid can be tested by setting the CustomerList.aspx as the startpage and running the application

1. Right-click the Customer project in the Solution Explorer Click Set

As StartUp Project

2. Right-click the CustomerList.aspx page Click Set As Start Page

3. Run the application The result should look like that shown in Figure 9.15

Figure 9.15 The CustomerList page filled with Customers.

4. Click the Save button for CustomerID ANTON The browser output

is shown in Figure 9.16 Notice that there is only one root element,which represents ANTON, followed by the orders and order items

Figure 9.16 The browser output of CustomerID = ANTON.

5. Check you work back into Visual SourceSafe

Review Questions

1. What class can be used to create an XML document from scratch?

2. What class can be used to perform data type conversion between NET data types and XML types?

3. What class can be used to perform XSL transformations?

4. What is the simplest method of storing a DataSet in an XML file?

5. How are large XML files quickly searched without loading the complete file into memory?

Answers to Review Questions

1. The XmlDocument class

2. The XmlConvert class

3. The XslTransform class or the ASP.NET XML Web control

4. Using the WriteXml method of the DataSet

5. Using the XPathDocument class with the XpathNavigator

When data must be transferred from one location to another, a method of moving data across the media is required This method typically involves thesending of bytes in a sequential fashion and the ability to read and processthese bytes in chunks, while the information is still being received Streams arethe answer to this problem

The previous chapters have looked at data access using ADO.NET and XMLtechnologies Although those technologies should be the primary technologiesfor storing and retrieving data, there are many instances where the need forfile and folder access is necessary

It’s also a common requirement to persist, or store, objects with their state,and to retrieve these persisted objects This is sometimes referred to as objectdehydration and rehydration, but is more commonly called serialization

Many of the types covered in this chapter are located in the System.IOnamespace This chapter starts by exploring streams in detail After that, fileand folder classes are covered Finally, this chapter covers serialization

Streams, File Access,

and Serialization

C H A P T E R

10

Q: Is it possible to allow users to upload files to the Web server?

A: Yes The HTML file field control can be used for this We will look

at this control in this chapter

Q: Can serialization be used to make copies of objects?

A: Absolutely Serialization can be used to perform a deep copy of anobject by serializing to a memory stream then deserializing to anew object

Stream Classes

In the NET Framework, many classes require the ability to move data Thisdata movement may be to and from a file, a TCP socket, memory, or somethingelse If a class were written to simply write to a file, there could be a problemlater when the requirement for writing to a file changed to writing content to abrowser window This is where streams can help

The stream provides a method for moving data to and from somewhere,depending on the stream class that is implemented Instead of writing to a file,

a class should write to a stream This allows the programmer to decide whatthe destination of the stream will be

In the NET Framework, some NET streams have endpoints, or data sinks,such as a file stream The NET Framework also provides intermediate streamsthat provide processing and are spliced into other streams, such as the bufferedstream and the Crypto stream

All streams typically have the same pattern for reading and writing data, asshown here This section examines each of these streams in detail

‘Writing data Open the stream While more data exists Write the data Close the stream

‘Reading data

Open the stream While more data exists Read the data Process the data Close the stream

The NET Framework provides stream classes, which are classes that derivefrom System.IO.Stream, and helper classes, which are wrapper classes that use

a stream and provide additional methods to simplify stream access The helperclasses are typically called reader and writer classes Figure 10.1 shows therelationship between the stream and reader/writer classes

Stream

The Stream class is an abstract base class for all stream classes The constructorfor this class is protected, which means that it is not possible to create a newinstance of this class The Stream class members are shown in Table 10.1

The Stream class has a Close method, which releases all resourses, such asfile handles and windows sockets The opening of the stream is accomplished

in the constructor of the Stream class

Using one of the available streams helps to isolate the programmer from thelow-level operating system and device details

All stream classes handle the movement of binary data using bytes or bytearrays The System.Text.Encoding class provides routines for converting bytesand byte arrays to and from Unicode text

Figure 10.1 Stream class children and stream helper classes.

BufferedStream FileStream MemoryStream CryptoStream NetworkStream

Table 10.1 Stream Properties and Methods

STREAM MEMBER DESCRIPTION

Null A static property that can be used to send data to the bit

bucket Use this when a stream is required, but there is no desire to actually move data

CanRead Returns a Boolean, indicating whether the stream can be

read This is an abstract method that must be overridden CanSeek Returns a Boolean, indicating whether this stream

supports seeking This is an abstract method that must be overridden.

CanWrite Returns a Boolean, indicating whether this stream can

be written to This is an abstract method that must be overridden.

Length Returns a Long, indicating the length of the stream This is

an abstract method that must be overridden.

Position This changeable property can be used to get or set the

position within the stream The stream must support seeking to use this property This is an abstract method that must be overridden.

BeginRead Starts an asynchronous read from the stream.

BeginWrite Starts an asynchronous write from the stream.

Close Closes the stream This method will also flush all data that

is buffered All resources, including file and socket handles will be released.

EndRead Called to wait for a pending asynchronous read operation

to complete.

EndWrite Called to wait for a pending asynchronous write operation

to complete.

Flush Forces the movement of any data that is in memory to

its destination This is an abstract method that must be overridden.

Read If the stream supports reading, this method is used to

retrieve a sequence of bytes from the stream and update the position within the stream This is an abstract method that must be overridden.

ReadByte If the stream supports reading, this method is used to

read a single byte from a stream and update the position within the stream.

Table 10.1 (continued)

STREAM MEMBER DESCRIPTION

Seek This method is used to set the position within the stream.

The stream must support seeking to execute this method.

This method requires an offset and a relative origin The relative origin can be Begin, Current, or End This is an abstract method that must be overridden.

SetLength If the stream supports writing and seeking, this method

can be used to expand or truncate the current stream This

is an abstract method that must be overridden.

Write If the stream supports writing, this writes a sequence of

bytes to the current stream and advances the position.

This is an abstract method that must be overridden.

WriteByte If the stream supports writing, this method writes a byte

to the current stream and advances the position

FileStream

The FileStream class provides the ability to move data to and from a disk file.This class inherits all the methods in the Stream class and has additional file-centric properties and methods

Append. Using the Append mode opens the file and sets the position

to the end of the file If the file does not exist, a new file is created Thisoption can only be used with the FileAccess property set to Write Anattempt to read from the file will throw an ArgumentException

Create. Using the Create mode opens a new file if the file does not exist

If the file does exist, the file will be truncated

CreateNew. The CreateNew mode creates a new file if the file does notexist If the file exists, an IOException will be thrown

Open. The Open mode opens an existing file If the file does not exist, aFileNotFound exception will be thrown

OpenOrCreate. The OpenOrCreate mode opens the file if it exists

If the file does not exist, a new file is created This mode differs from theCreateNew mode in that this mode does not truncate an existing file

Truncate. The Truncate mode opens and truncates an existing file forwriting If the file does not exist, a FileNotFoundException is thrown If

an attempt is made to read from the file, an exception will be thrown

FileAccess

The FileAccess parameter specifies whether the file is being opened for read orwrite access This is a bit enumeration method, which means that settings can

be combined with the Or operator The following settings are available.

Read. Read access is used to specify that the file will be opened for only use

read-Write. Write access is used to specify that the file will be opened forwrite-only use

ReadWrite. ReadWrite access is used to specify that the file will beopened for read or write access

FileShare

The FileShare parameter is used to specify how other streams can access thisfile Available options are listed here Generally, the best setting is None (thedefault), unless all users need read-only access to the file, in which the settingcould be set to Read

Inheritable. The Inheritable share specifies that the file handle is table by child processes This option is not available with Win32

inheri-None. The None share does not allow any sharing of this file until the file

is closed This is the default when the FileShare parameter is not specified

An additional attempt to open the file will result in an IOExceptionbeing thrown

Read. The Read share allows other processes to also open the same filefor read access The file cannot be opened for write access until the filehas been closed

ReadWrite. The ReadWrite share allows other processes to open this filefor reading and writing

Write. The Write share allows other processes to open this file for writing.This file cannot be opened for read access until the file has been closed

BufferSize

The BufferSize specifies the size of the buffer to be used when accessing thisfile If the number is between zero and eight, the buffer size will be set to eight.Generally, performance gains can be realized by increasing this number

UseAsync

The UseAsync setting can be used to allow asynchronous access to the file.When set to true, file access is done by using the BeginRead and BeginWritemethods

FileStream Examples

The section examines several ways of creating a FileStream object Theseexamples explore several options that are available when opening and work-ing with the FileStream

Opening and Writing to a File

The following code examples show how a file can be opened, written to, andclosed:

Private Sub Button1_Click( _ ByVal sender As System.Object, _ ByVal e As System.EventArgs) _ Handles Button1.Click

Dim s1 As New FileStream( _

“c:\test.txt”, FileMode.Create) Dim s1options As String

s1options = String.Format( _

“s1 - CanRead:{0} CanSeek:{1} CanWrite:{2}<br>”, _ s1.CanRead, _

s1.CanSeek, _ s1.CanWrite) Response.Write(s1options) Dim b As Byte()

b = System.Text.Encoding.Unicode.GetBytes(“Hello World”) s1.Write(b, 0, b.Length)

s1.Close() End Sub

The browser displays the following information about the stream:

s1 - CanRead:True CanSeek:True CanWrite:True

Figure 10.2 shows the file contents when viewed in the Visual Studio NETbinary editor Viewing the output in the binary editor reveals that the messagewas saved using two bytes per character (Unicode) Writing to the streamrequired either a byte or an array of bytes Therefore, this code converts theUnicode string to an array of bytes and writes the byte array, starting at offsetzero of the byte array, and writing all bytes by setting the count to the length

of the byte array

Writing and Reading from the FileStream

Since the CanRead and CanSeek properties were set to true, the code can bemodified to reset the position to the beginning of the file and read its contents.The following code shows the writing and reading of the file:

Private Sub Button1_Click( _ ByVal sender As System.Object, _ ByVal e As System.EventArgs) _ Handles Button1.Click

Dim s1 As New FileStream( _

“c:\test.txt”, FileMode.Create) Dim s1options As String

s1options = String.Format( _

“s1 - CanRead:{0} CanSeek:{1} CanWrite:{2}<br>”, _ s1.CanRead, _

s1.CanSeek, _ s1.CanWrite) Response.Write(s1options) Dim b As Byte()

b = System.Text.Encoding.Unicode.GetBytes(“Hello World”) s1.Write(b, 0, b.Length)

Dim strOutput As String = “”

Dim bInput(9) As Byte Dim count As Integer = bInput.Length

Do While (count > 0) count = s1.Read(bInput, 0, bInput.Length) strOutput &= _

System.Text.Encoding.UTF8.GetString(bInput, 0, count) Loop

s1.Close() Response.Write(strOutput & “<br>”) End Sub

Figure 10.2 Displaying the file in the binary editor reveals that hello world was stored using two bytes per character (Unicode)

To read the file, a byte array must be supplied to act as a buffer The size ofthe buffer could be set much higher to achieve better performance Each timethe loop is executed, count will hold the quantity of bytes read from thestream This loop will run until the Read method returns zero, then the file isclosed and the string is output to the browser The browser output is shown inFigure 10.3

The stream is not obliged to fill the buffer each time the Read method is executed The stream is only obliged to return one or more bytes If no bytes have been received, the call will block until a single byte has been received This operation works especially well in situations where a slow stream is involved The loop can process bytes while the slow stream is sending data.

Opening the Same File with Multiple Streams

In this example, two streams can be opened Both streams are opening thesame file, and each stream has its own position

Figure 10.3 Browser output when writing and reading a file.

Private Sub Button2_Click( _

ByVal sender As System.Object, _ ByVal e As System.EventArgs) Handles Button2.Click

Dim s1 As New FileStream( _

“c:\test.txt”, _ FileMode.OpenOrCreate, _ FileAccess.Read, FileShare.Read) Dim s1options As String

s1options = String.Format( _

“s1 - CanRead:{0} CanSeek:{1} CanWrite:{2}<br>”, _ s1.CanRead, _

s1.CanSeek, _ s1.CanWrite) Dim s2 As New FileStream( _

“c:\test.txt”, _ FileMode.OpenOrCreate, _ FileAccess.Read, FileShare.Read) Dim s2options As String

s2options = String.Format( _

“s2 - CanRead:{0} CanSeek:{1} CanWrite:{2}<br>”, _ s2.CanRead, _

s2.CanSeek, _ s2.CanWrite) Response.Write(s1options) Response.Write(s2options) s1.Seek(0, SeekOrigin.Begin) s2.Seek(0, SeekOrigin.Begin) Dim strOutput As String = “”

Dim bInput(10) As Byte Dim count As Integer count = s1.Read(bInput, 0, bInput.Length)

Do While (count > 0) strOutput &= _ System.Text.Encoding.UTF8.GetString(bInput, 0, count) count = s1.Read(bInput, 0, bInput.Length)

strOutput &= “<br>”

Loop count = s2.Read(bInput, 0, bInput.Length)

Do While (count > 0) strOutput &= _ System.Text.Encoding.UTF8.GetString(bInput, 0, count) count = s2.Read(bInput, 0, bInput.Length)

strOutput &= “<br>”

Loop s1.Close() s2.Close() Response.Write(strOutput & “<br>”) End Sub

Figure 10.4 Browser output with two streams open for reading.

This code would normally throw an IOException, but does not because theFileAccess is set to Read on both streams Also, an HTML line break has beenadded to the output each time through the loops This gives an indication of thenumber of times that the loop has run Figure 10.4 shows the browser output

Null Stream

The Null Stream is a bit bucket, meaning that it is a dummy stream This can

be useful in situations where a stream is required to execute a process, butthere is no desired endpoint

The following code example shows how a Null stream can be written to andread from Note that a new Stream instance is not, and cannot be, created TheStream class is abstract, which means that it must be inherited Instead, theassignment is made to System.Null

Private Sub Button4_Click( _

ByVal sender As System.Object, _ ByVal e As System.EventArgs) _ Handles Button4.Click

Dim s1 As Stream = Stream.Null Dim s1options As String s1options = String.Format( _

“s1 - CanRead:{0} CanSeek:{1} CanWrite:{2}<br>”, _ s1.CanRead, _

s1.CanSeek, _ s1.CanWrite) Response.Write(s1options) Dim b As Byte()

b = System.Text.Encoding.Unicode.GetBytes(“Hello World”) s1.Write(b, 0, b.Length)

s1.Seek(0, SeekOrigin.Begin) Dim strOutput As String = “”

Dim bInput(10) As Byte Dim count As Integer = bInput.Length count = s1.Read(bInput, 0, bInput.Length)

Do While (count > 0) strOutput &= _ System.Text.Encoding.UTF8.GetString(bInput, 0, count) count = s1.Read(bInput, 0, bInput.Length)

Loop s1.Close() Response.Write(strOutput & “<br>”) End Sub

This code is the same as the code in the FileStream example for opening andwriting to a file, except that the FileStream was replaced with the Stream classand initialized to Stream.Null The browser output is shown in Figure 10.5.When writing to the Null stream, calling the Write method results in it sim-ply returning without writing anything, and executing the Read methodreturns zero, indicating that the end of the stream has been reached CanReadreturns true, CanWrite returns true, and CanSeek returns true

MemoryStream

The MemoryStream class provides the ability to move data to and from amemory buffer This class inherits all methods in the Stream class and hasadditional memory-centric properties and methods MemoryStreams are use-ful in helping to eliminate the need for temporary files when processing data.The memory buffer that is created by the memory stream is directly accessible

Capacity. An optional integer that sets the initial size of the internalbuffer Writing past the end of the buffer will cause the buffer to increaseits size Using the SetLength method will also update the length of thebuffer

Figure 10.5 Browser output when using the Null stream.

Writeable. An optional setting that can be used with the Buffer parameter

to indicate whether the buffer can be written to

Index. An optional parameter that can be used with the Buffer parameter;

it indicates the starting location in the buffer that will be used

Count. An optional parameter that can be used with the Buffer parameter;

it indicates the quantity of bytes that may be used in the buffer

PubliclyVisible. An optional parameter that enables the GetBuffermethod The GetBuffer method returns the buffer as an unsigned byte array

MemoryStream Examples

The section examines several ways of working with the MemoryStream object.These examples explore several of the options that are available when openingand working with the MemoryStream

Opening and Writing to a MemoryStream

The following code example shows how a Memory stream can be written toand read from

Private Sub Button3_Click( _

ByVal sender As System.Object, _ ByVal e As System.EventArgs) _ Handles Button3.Click

Dim s1 As New MemoryStream() Dim s1options As String s1options = String.Format( _

“s1 - CanRead:{0} CanSeek:{1} CanWrite:{2}<br>”, _ s1.CanRead, _

s1.CanSeek, _ s1.CanWrite) Response.Write(s1options) Dim b As Byte()

b = System.Text.Encoding.Unicode.GetBytes(“Hello World”) s1.Write(b, 0, b.Length)

s1.Seek(0, SeekOrigin.Begin) Dim strOutput As String = “”

Dim bInput(10) As Byte Dim count As Integer count = s1.Read(bInput, 0, bInput.Length)

Do While (count > 0) strOutput &= _ System.Text.Encoding.UTF8.GetString(bInput, 0, count) count = s1.Read(bInput, 0, bInput.Length)

Loop s1.Close() Response.Write(strOutput & “<br>”) End Sub

This code is the same as the code in the FileStream example for opening andwriting to a file, except that the FileStream was replaced with the Memory-Stream class and initialized with the empty constructor The browser output isshown in Figure 10.6 CanRead returns true, CanWrite returns true, andCanSeek returns true

Accessing the MemoryStream’s Buffer

The following code example shows how a Memory stream’s internal buffer can

be accessed directly This simplifies retrieving the data, and doesn’t requirechanging the stream’s position

Private Sub Button5_Click( _

ByVal sender As System.Object, _ ByVal e As System.EventArgs) _ Handles Button5.Click

Dim s1 As New MemoryStream() Dim s1options As String s1options = String.Format( _

“s1 - CanRead:{0} CanSeek:{1} CanWrite:{2}<br>”, _ s1.CanRead, _

s1.CanSeek, _ s1.CanWrite) Response.Write(s1options) Dim b As Byte()

b = System.Text.Encoding.Unicode.GetBytes(“Hello World”) s1.Write(b, 0, b.Length)

Dim strOutput As String = “”

Dim bInput() As Byte = s1.GetBuffer() strOutput &= _

System.Text.Encoding.UTF8.GetString(bInput, 0, bInput.Length) s1.Close()

Response.Write(strOutput & “<br>”)

Figure 10.6 Browser output when using the MemoryStream.

NetworkStream

The NetworkStream class provides the ability to move data to and from a work endpoint This class inherits all the methods in the Stream class and hasadditional network-centric properties and methods NetworkStreams can beused to access a Web site, as well as to communicate between local computers

net-It takes a little bit more code to set up the NetworkStream Figure 10.7 shows

a high-level view of the program flow when using the NetworkStream toretrieve the default Web page from a Web server

The following helper objects are required in order to communicate using theNetworkStream class:

IPAddress. The IPAddress class is used to encapsulate an IP address thatrepresents the URL of the final endpoint This uses the static Resolvemethod of the Dns class to perform a Domain Name Service (DNS)search of the Internet for the IP address that corresponds to the domainname that is supplied by the user Since the endpoint may be part of aWeb farm, an array of addresses may be returned, of which the firstaddress is customarily used, unless there is a communication failure

IPEndPoint. The IPEndPoint class consists of the IPAddress and the Portnumber of the endpoint The IPEndPoint is required to create a Socket

Encoder.ASCII. The Encoder.ASCII class is used to create an encoderthat will be used to convert ASCII strings to byte arrays and back

Figure 10.7 A high-level view of the progam flow when working with the NetworkStream

to retrieve the default Web page from a Web server.

OwnsSocket

Setting this to true causes the Close method of the NetworkStream to also callthe Close method of the Socket The default is false, so this should always beset to true unless the Socket is being used for other purposes

NetworkStream Example

The section examines the creation and use of a NetworkStream object Thisexample looks up the IP address of www.wiley.com, then creates an IPAddressobject based on the URL Next, the IPEndPoint will be created, and finally theSocket is created and connected The code is as follows:

Private Sub Button6_Click( _

ByVal sender As System.Object, _ ByVal e As System.EventArgs) _ Handles Button6.Click

Dim strServer As String = “www.wiley.com”

Dim getCmd As String Dim getBytes As Byte() Dim recvBytes(1024) As Byte Dim strRespose As String = “”

Dim enc As Encoding = Encoding.ASCII

Create IPEndPoint with IP and Port

Send Get Request

Receive Response from Stream

‘Resolve the DNS name to an IP Address.

‘by taking the first address in the resolved list Dim host As IPAddress = Dns.Resolve(strServer).AddressList(0) Dim EPhost As New IPEndPoint(host, 80)

‘Creates the Socket for sending data over TCP Dim sokt As New Socket(

AddressFamily.InterNetwork, _ SocketType.Stream, _

ProtocolType.Tcp) getCmd = String.Format( _

“GET / HTTP/1.1{0}Host: {1}{0}Connection: Close{0}{0}”, _ ControlChars.CrLf, strServer)

End If Dim s1 As New NetworkStream(sokt, True) Dim s1options As String

s1options = String.Format( _

“s1 - CanRead:{0} CanSeek:{1} CanWrite:{2}<br>”, _ s1.CanRead, _

s1.CanSeek, _ s1.CanWrite) Response.Write(s1options) s1.Write(getBytes, 0, getBytes.Length) Dim strOutput As String = “”

Dim bInput(10) As Byte Dim count As Integer count = s1.Read(bInput, 0, bInput.Length)

Do While (count > 0) strOutput &= _ System.Text.Encoding.UTF8.GetString(bInput, 0, count) count = s1.Read(bInput, 0, bInput.Length)

Loop s1.Close() Response.Write(strOutput) End Sub

The browser output, shown in Figure 10.8, demonstrates that the stream’sCanRead and CanWrite properties are true, while the CanSeek property is set

to false An attempt to seek will cause an exception to be thrown

This stream operates much like the previous streams that have been examined, except that more setup is required in order to communicate over thenetwork

Figure 10.8 An example of using the NetworkStream to read a Web page.

CryptoStream

The CryptoStream class provides the ability to move and process data fromone stream to another This stream does not have an endpoint This class inher-its all the methods in the Stream class and has additional cryptographic-centricproperties and methods CryptoStream can be used to encrypt any data.The CryptoStream provides the ability to perform symmetrical encryptionwith little extra work Symmetrical encryption is done by sharing a secret Inthis case, the secret will be the initialization vector (IV) and the key Depend-ing on the usage, it may be desirable to regenerate the IV and key each timethat a session has started, or it may be desirable to store the IV and key forrepeated use

TripleDESCryptoServiceProvider, it may be better to use one of the explicitCryptoServiceProvider classes The following CryptoServiceProvider classesare available:

CryptoStream Encryption Example

The following example shows how encryption may be done by encrypting the

words Encrypted Hello World and placing them in a file called c:\test.txt.

Private Sub Button7_Click( _

ByVal sender As System.Object, _ ByVal e As System.EventArgs) _ Handles Button7.Click

Dim s1 As New FileStream( _

“c:\test.txt”, FileMode.Create) Dim s1options As String

s1options = String.Format( _

“s1 - CanRead:{0} CanSeek:{1} CanWrite:{2}<br>”, _ s1.CanRead, _

s1.CanSeek, _ s1.CanWrite) Response.Write(s1options) Dim cryptoProvider As TripleDESCryptoServiceProvider cryptoProvider = TripleDESCryptoServiceProvider.Create()

If Session(“iv”) Is Nothing Then Session(“iv”) = cryptoProvider.IV Session(“key”) = cryptoProvider.Key txtIV.Text = Convert.ToBase64String(cryptoProvider.IV) txtKey.Text = Convert.ToBase64String(cryptoProvider.Key) End If

Dim xfrm As ICryptoTransform xfrm = cryptoProvider.CreateEncryptor( _ Session(“key”), Session(“iv”)) Dim c1 As New CryptoStream(s1, xfrm, CryptoStreamMode.Write) Dim c1options As String

s1options = String.Format( _

c1.CanRead, _ c1.CanSeek, _ c1.CanWrite) Response.Write(c1options) Dim b As Byte()

b = System.Text.Encoding.Unicode.GetBytes( _

“Encrypted Hello World”) c1.Write(b, 0, b.Length) c1.Close()

s1.Close() End Sub

This code creates a FileStream, then creates the Provider The TripleDESCryptoServiceProvider will generate an IV and key,which must be recorded to decrypt the message They are saved into Sessionvariables Next, the CreateEncryptor method is executed to produce an ICrypto-Transform, and finally the CryptoStream is opened with the appropriate para-meters The browser output, shown in Figure 10.9, demonstrates that althoughthe FileStream supports reading, writing, and seeking, the CryptoStream onlysupports writing

TripleDESCryptoService-CryptoStream Decryption Example

The following example shows how decryption may be done by decrypting the

words Encrypted Hello World from the file called c:\test.txt and sending them to

the browser

Private Sub Button8_Click( _

ByVal sender As System.Object, _ ByVal e As System.EventArgs) _ Handles Button8.Click

Dim s1 As New FileStream( _

“c:\test.txt”, FileMode.Open) Dim s1options As String

s1options = String.Format( _

“s1 - CanRead:{0} CanSeek:{1} CanWrite:{2}<br>”, _ s1.CanRead, _

s1.CanSeek, _ s1.CanWrite) Response.Write(s1options) Dim cryptoProvider As TripleDESCryptoServiceProvider cryptoProvider = TripleDESCryptoServiceProvider.Create() Dim xfrm As ICryptoTransform

xfrm = cryptoProvider.CreateDecryptor(Session(“key”), Session(“iv”))

Dim c1 As New CryptoStream(s1, xfrm, CryptoStreamMode.Read) Dim c1options As String

c1options = String.Format( _

“c1 - CanRead:{0} CanSeek:{1} CanWrite:{2}<br>”, _ c1.CanRead, _

c1.CanSeek, _ c1.CanWrite) Response.Write(c1options) Dim strOutput As String = “”

Dim bInput(10) As Byte Dim count As Integer count = c1.Read(bInput, 0, bInput.Length)

Do While (count > 0) strOutput &= _ System.Text.Encoding.UTF8.GetString(bInput, 0, count) count = c1.Read(bInput, 0, bInput.Length)

Loop Response.Write(strOutput & “<br>”) c1.Close()

s1.Close() End Sub

The browser output, Figure 10.10, shows the decrypted file contents TheFileStream supports reading, writing, and seeking, but the CryptoStream onlysupports reading Also note that the IV and key were initialized from thestoredSession variables

BufferedStream

The BufferedStream class provides the ability to take an existing stream andgive it buffering capabilities This stream does not have an endpoint This classinherits all the methods in the Stream class and has additional buffer-centricproperties and methods

Figure 10.9 Encryption IV and key This also reveals that the CryptoStream only supports writing.

Figure 10.10 The file is decrypted and displayed in the browser.

BufferedStream Example

The following example shows how a BufferedStream can be used with theexisting FileStream:

Private Sub Button9_Click( _

ByVal sender As System.Object, _ ByVal e As System.EventArgs) _ Handles Button9.Click

Dim s1 As New FileStream( _

“c:\test.txt”, FileMode.Create) Dim s1options As String

s1options = String.Format( _

“s1 - CanRead:{0} CanSeek:{1} CanWrite:{2}<br>”, _ s1.CanRead, _

s1.CanSeek, _ s1.CanWrite) Response.Write(s1options) Dim b1 As New BufferedStream(s1) Dim b1options As String

b1options = String.Format( _

“b1 - CanRead:{0} CanSeek:{1} CanWrite:{2}<br>”, _ b1.CanRead, _

b1.CanSeek, _ b1.CanWrite) Response.Write(b1options) Dim b As Byte()

b = System.Text.Encoding.Unicode.GetBytes(“Hello World”) b1.Write(b, 0, b.Length)

b1.Seek(0, SeekOrigin.Begin) Dim strOutput As String = “”

Dim bInput(10) As Byte Dim count As Integer count = b1.Read(bInput, 0, bInput.Length)

Do While (count > 0) strOutput &= _ System.Text.Encoding.UTF8.GetString(bInput, 0, count) count = b1.Read(bInput, 0, bInput.Length)

Loop b1.Close() s1.Close() Response.Write(strOutput & “<br>”) End Sub

The browser output is shown in Figure 10.11 Notice that the buffered streamsupports reading, writing, and seeking

Response.OutputStream

The Response.OutputStream is an instance of the Stream object The Stream can be used when outputting binary information to the browser Thefollowing example (see Figure 10.12) opens a FileStream containing a picture.The FileStream is read and then written to the OutputStream

Output-Figure 10.11 In this browser output, notice that the BufferedStream supports reading, writing, and seeking.

Figure 10.12 An image file is read with a FileStream object and written to the Response object’s output stream.

Private Sub Button10_Click( _

ByVal sender As System.Object, _ ByVal e As System.EventArgs) _ Handles Button10.Click

Response.Clear() Response.ContentType = “image/gif”

Dim s1 As New FileStream( _

“c:\whe_logo.gif”, FileMode.Open) Dim s2 As Stream = Response.OutputStream() Dim b(512) As Byte

Dim count As Integer count = s1.Read(b, 0, b.Length)

Do While (count > 0) s2.Write(b, 0, count) count = s1.Read(b, 0, b.Length) Loop

Response.End() End Sub

To ensure that the image is output properly without being corrupted, theresponse buffer is cleared first Next, the content type is set to image/gif,which tells the browser what kind of file is being sent Finally, a loop is created,which reads a block of bytes from the FileStream, and outputs the block to theOutputStream

This same concept can be applied to delivering other document types, such

as Word documents and Excel spreadsheets

Stream Helper Classes

Many stream helper classes can be used to simplify the coding of the streamsthat have been covered in this chapter This section explores some of theseclasses and provide examples where appropriate

The BinaryReader class provides many Write overloads to simplify writing to

a file The following example shows how this can be used:

Private Sub Button14_Click( _ ByVal sender As System.Object, _ ByVal e As System.EventArgs) _ Handles Button14.Click

Dim s1 As New FileStream( _

“c:\test.txt”, FileMode.Create) Dim b1 As New BinaryWriter(s1, Encoding.Unicode) b1.Write(“Hello BinaryWriter World”)

b1.Close() s1.Close() End Sub

This code can reduce the complexity of working with the stream directly,but the BinaryWriter requires a stream to be passed to its constructor

BinaryReader

The BinaryWriter class provides many Read overloads to simplify the reading

of data from files The following is an example of its use:

Private Sub Button15_Click( _

ByVal sender As System.Object, _ ByVal e As System.EventArgs) _ Handles Button15.Click

Dim s1 As New FileStream( _

“c:\test.txt”, FileMode.Open) Dim b1 As New BinaryReader(s1, Encoding.Unicode) Response.Write(b1.ReadString())

b1.Close() s1.Close() End Sub

This code simplifies the retrieval of data, but requires that two streams beopened

TextWriter and TextReader

These classes are abstract and provide many methods that simplify readingand writing textual information