vb .net tutorial - 10

Performing File I/O Operations and Implementing Multithreading in Visual Basic .NET

©NIIT Performing File I/O Operations and Implementing Multithreading in VB .NET/Lesson 10/ Slide 1 of 38

Objectives

In this lesson, you will learn to:

Identify the benefits of the .NET System.IO model

Identify the benefits of the Visual Basic .NET run-time functions

Identify the benefits of multithreading

Implement threads in Visual Basic .NET



File

Is a collection of bytes that has a persistent storage.

I/O operations in Visual Basic .NET are handled in two different ways:

Using the .NET System.IO model

Using the Visual Basic .NET run-time functions



System.IO Model

Specifies a set of classes that are available to all the .NET languages.

Contains classes that are used for creating, copying, moving, and deleting files.

Contains the following frequently used classes:

FileStream

BinaryReader and BinaryWriter

StreamReader and StreamWriter

Directory



The File Hierarchy in the System.IO Model



The FileStream Class

Provides access to files and file-related information.

Is used with the File class, which is contained in the System.IO namespace, to create, copy, delete, move, and

open files.

Can also be used with another class called Path, to manipulate the strings that represent folder or file paths.

Opens a file either in synchronous or asynchronous mode.

Opens files in synchronous mode by default.

Has constructors that use enumerations, such as FileMode, FileAccess, and FileShare, to specify how a file is

created, opened, and shared.



FileMode Enumerations

Members Description

Append It is used to open a file if it exists and move the file pointer to the end of the file, or create a new file. Append can only be used with the FileAccess.Write.

Create It is used to specify that the operating system should create a new file. However, if the file already exists, it will be overwritten.

CreateNew It is used to specify that the operating system should create a new file. However, if the file already exists, it will throw an exception.



FileMode Enumerations (Contd.)

Open It is used to specify that the operating system should open an existing file.

OpenOrCreate It is used to specify that the operating system should open an existing file. However, if the file does not exist, a new file should be created.

Truncate It is used to specify that the operating system should open an existing file. After the file is opened, the file should be truncated so that its size is zero bytes.

Members Description



FileAccess Enumerations Members Description

Read It is used to specify Read access to a file so that data can be read from the file.

ReadWrite It is used to specify Read and Write access to a file so that data can be written to and read from the file.

Write It is used to specify Write access to a file so that data can be written to the file.



FileShare Enumerations

Members Description

None It is used to decline sharing of the current file. When this constant is used with the FileShare enumeration, a request to open the file will fail until the file is closed.

Read It is used to allow subsequent opening of a file for reading. If this flag is not used, a request to open the file for reading will fail until the file is closed.

ReadWrite It is used to allow subsequent opening of a file for reading or writing. If this flag is not used, any request to open the file for writing or reading will fail until the file is closed.

Write It is used to allow subsequent opening of a file for writing. If this flag is not used, any request to open the file for writing will fail until the file is closed.



BinaryReader and BinaryWriter Classes

Are used to read from and write to a binary file.

StreamReader and StreamWriter Classes

Are used to read and write data as streams of characters.

Use specific encoding to convert characters to and from bytes.



The Directory Class

Enables you to work with drives and folders.

Method Usage

CreateDirectory() It is used to create the directories specified by a path.

Delete() It is used to delete a directory and all its contents.

Exists() It is used to determine whether the specified path refers to an existing directory on the disk.



Directory Class (Contd.)

Method Usage

GetDirectoryRoot() It is used to return the root portion of a specified path.

GetLogicalDrives() It is used to retrieve the names of the logical drives on the current computer.

Move() It is used to move a directory and its contents to a new path.



Just a Minute…

1. The code snippet generates a build error. Identify the error.

2. What is the difference between the CreateNew and Create constants of the FileMode enumeration?

3. Complete the code to ensure that the file Product.txt is opened with no share specification.



Visual Basic .NET Run-time Functions

Provide compatibility with earlier versions of Visual Basic.

Allow three types of file access:

Sequential

Random

Binary

Are defined in the System.IO.File namespace.



Functions Used to Access Files

Dir – It is used to retrieve the name of a file or directory that matches a specific pattern based on the file attribute or volume label of a file.

EOF – It is used to check whether the end of the file is reached. When a file is opened for random or sequential access, if the end of the file is reached, the function returns a Boolean value True.

FileCopy – It is used to create a copy of an existing file.

FileDateTime – It is used to retrieve the date and time when a file was created or last modified.

FileLen – It is used to retrieve the length of a file in bytes.



Functions Used to Access Files (Contd.)FreeFile – It is used to retrieve the next file number available for use by the FileOpen() function.

GetAttr – It is used to retrieve a FileAttribute value representing the attributes of a file or directory.

Loc – It is used to retrieve a value specifying the current read/write position within an open file.

LOF – It is used to retrieve a value representing the size, in bytes, of a file opened using the FileOpen()function.

Seek – It is used to retrieve a value specifying the current read/write position within a file opened using the

FileOpen()function or sets the position for the next read/write operation within a file opened using the FileOpen() function.

SetAttr – It sets the attribute information for a file.



Functions Used to Access Data from a File

With sequential access, a file can be opened in the Input, Output, or Append mode. In the Input mode, a file is

opened for reading data from the file. In the Output or Append mode, a file is opened for writing data to the file. The syntax for using the FileOpen() function with sequential access is as follows:

FileOpen(FileNumber, FileName, OpenMode.mode type)



Functions Used to Access Data from a File (Contd.)

With random access, the FileOpen() function is used to read and write data to a file without closing the file.

Random-access files store data in records, which makes it easy to locate relevant information quickly. The syntax for using the FileOpen() function with random access is as follows:

FileOpen(FileNumber, FileName, OpenMode.Random,OpenAccess=OpenAccess.Default, OpenShare=OpenShare.Default,RecordLength= length of the record)



Functions Used to Access Data from a File (Contd.)

With binary access, the FileOpen() function is used to read or write to any byte position in a file. The syntax for using the FileOpen()function with binary access is as follows:

FileOpen(FileNumber, FileName, OpenMode.Binary)



Functions Used to Write Data to and Read Data From Files

Access type Functions for writing data

Functions for reading data

Sequential Print, PrintLine Input, InputString

Random FilePut FileGet

Binary FilePut FileGet



Just a Minute…

Complete the code to display the contents of the file named Product.txt in the output window.



Problem Statement 10.D.1

The users at the call centers of Diaz Telecommunications should be able to create a text file for storing customer feedback on the services provided by Diaz Telecommunications. The users should also be able to open the file for future assessment.



Task List

Identify the mechanism to implement the user requirement.

Identify the controls needed to provide the user interface.

Perform the appropriate steps to design the form.

Write the code to implement the identified mechanism.

Save the application.

Run the application to validate the conditions applied on the form.



Task 1: Identify the mechanism to implement the user requirement.

Result:

You can fulfill the user requirement by using either the classes in the System.IO model or the Visual Basic .NET run-time functions. However, since the System.IO model is a part of the .NET Framework class library, you should implement the relevant classes of the System.IO model to

make full use of the .NET Framework features of Visual Basic .NET.



Task 2: Identify the controls needed to provide the user interface.

Result:

As per the plan, you need to add a RichTextBox control, a label control, and two buttons, Read Data and Write

Data, to the form named frmCustomerFeedback.

In addition, a Reset button should be used to clear the contents of the RichTextBox control.

To ensure that data is first written and then read from the file, initially the Read Data button will be disabled. To

clear the contents of the RichTextBox, the Reset button will be used.



Task 3: Perform the appropriate steps to design the form.

Task 4: Write the code to implement the identified mechanism.

Task 5: Save the application.

Task 6: Run the application to validate the conditions applied on the form.



Problem Statement 10.P.1

The users at the call centers of Diaz Telecommunications should be able to create a text file for storing product details that are available for sale. The users should be able to read from or write data to the file as a sequence of characters. The users should also be able to open the file for future assessment.



ThreadsHelp in performing multiple activities simultaneously.

MultiThreading

Is a process in which individual activities are executed on separate threads.

Is also called free threading in Visual Basic .NET.

Enables you to create scalable applications since you can add threads to an application as the workload increases.



Creating a Thread

Requires declaring a variable of the System.Threading. Thread class and calling the constructor of the thread

class with the AddressOf operator and the name of the procedure you want to execute on the new thread.

Example

Dim MyThread As New System.Threading.Thread(AddressOf MySub)

Starting Execution of a Thread

You can start a thread by using the Start()method.

Stopping Execution of a Thread

You can stop the execution of a thread by calling the Abort()method.



Thread Priorities

Are defined by using the Priority property.

By default set to Normal.

Can be changed by using the ThreadPriority enumeration.

Values Description

AboveNormal Specifies that the thread has a higher priority than Normal

BelowNormal Specifies that the thread has the lower priority

Highest Specifies that the thread has the highest priority

Lowest Specifies that the thread has the lowest priority

Normal Specifies that the thread has an average priority



Thread State

Is accessed by using the ThreadState property.

Can be a combination of the values in the System.Threading.Threadstate enumeration, that is,

a thread can be in more than one state at a given time.

Can be changed by the following methods:

Thread.Sleep()

Thread.Suspend()

Thread.Interrupt()

Thread.Resume()

Thread.Abort()



Thread Lifecycle

Starts when an object of the System.Threading.Thread class is created and stops when any of the following

occurs:

The method the thread is running completes

The process containing the thread terminates

The Abort() method is called for the thread object



Thread Lifecycle (Contd.)

The following diagram illustrates the thread states and the methods that would cause the thread to leave each

state.



Multithreaded Procedures

Can accept arguments.

Can return values.

Thread Synchronization

Is done by using events in an application.

The SyncLock Statement

Ensures that multiple threads do not access shared data at the same time.



Just a Minute…

There is a class called CalcClass, which has a Sub procedure called CalculateArea(). You need to execute this procedure in a separate thread. To execute the CalculateArea() procedure, you need to declare a variable of the thread type. Complete the declaration of the variable in the code snippet.



Summary

In this lesson, you learned that:

The file I/O operations in Visual Basic .NET can be done in two different ways:

Using the .NET System.IO model

Using the Visual Basic .NET run-time functions

The System.IO model specifies the use of the common classes available to all the .NET languages.

The classes in the System.IO namespace that are mostly used are:

FileStream

BinaryReader



Summary (Contd.)

BinaryWriter

StreamReader

StreamWriter

Directory

Visual Basic .NET run-time functions allow three types of file access:

Sequential

Random

Binary

Threads can be implemented in Visual Basic by declaring a variable of the System.Threading.Thread type.



Summary (Contd.)

The methods that are mostly used to manipulate a thread are:

Start()

Sleep()

Suspend()

Resume()

Abort()

vb .net tutorial - 10

Documents

file io operations

new file

file class

file paths

file pointer

existing file

netthe file hierarchy

filerelated information