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21.1
Introduction
To be able to open a file and read the data from a storage unit of a
computer, such as a hard drive and able to save the data into the
storage unit are important functions of a computer program. In fact,
the ability to store, retrieve and modify data makes a computer a powerful tool
in database management.
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In this lesson, we
will learn how to manage data that is stored as a text file. Using text
file is an easy way to manage data, although it is not as sophisticated
as full fledged database management software such as SQL Server,
Microsoft Access and Oracle. Visual Basic 2008 allows the user to create
a text file, save the text file as well as read the text file. It is
relatively easy to write code for the above purposes in VB2008 compared
to VB6.
Reading and writing to
a text file in VB2008 required the use of the StreamReader class
and the StreamWriter class respectively. StreamReader is a tool
that enables the streaming of data by moving it from one location to
another so that it can be read by the user. For example, it allows the
user to read a text file that is stored in a hard drive. On the other
hand, the StreamWriter class is a tool that can write data input by the
use to a storage device such as the hard drive. |
21.2 Reading a
Text File
In order to read a file from the hard disk or any storage
device, we need to use the
StreamReader class.
To achieve that, first of all we need to include the following statement
in the program code:
Imports System.IO
This line has to precede the whole program code as it is
higher in hierarchy than the StreamReader Class. In Fact, this is the concept of
object oriented programming where StreamReader is part of the namespace
System.IO . It has to be put on top of the whole program(i.e. above the
Public Class Form 1 statement). The word import means we import the namesapce
System.IO into the program. Once we have done that , we can declare a variable
of the streamReader data type with the following statement:
Dim
FileReader As
StreamReader
If we don't include the
Imports System.IO, we have
to use the statement
Dim
FileReader As
IO.StreamReader
each time we
want to use the StreamReader class.
Now, start a new project and
name it in whatever name you wish. Now, insert the OpenFileDialog control
into the form because we will use it to read the file from the storage device.
The default name of the OpenFileDialog control is OpenFileDialog1,
you can use this
name or you can rename it with a more meaningful name. The OpenFileDialog control will return
a DialogResult value which can determine whether the user clicks the OK
button or Cancel button . We will also insert a command button and change its
displayed text to 'Open'. It will be used by the user to open and read a certain
text file. The following statement will accomplish the task above.
Dim
results
As
DialogResult
results =
OpenFileDialog1.ShowDialog
If
results = DialogResult.OK
Then
'Code to be
executed if OK button was clicked
Else
'Code to be
executed if Cancel button was clicked
End If
End Sub
Next, we insert a textbox and
set its Multiline property to true. It is used for displaying the text from a
text file. In order to read the text file, we need to create a new instant
of the streamReader and connect it to a text file with the following statement:
FileReader = New
StreamReader(OpenFileDialog1.FileName)
In addition, we need to
use the ReadToEnd method to read the entire text of a text file.
The syntax is:
TextBox1.Text = FileReader.ReadToEnd()
Lastly, we need to close the
file by using the Close() method. The entire code is shown in the box below:
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The Code
Imports System.IO
Public
Class Form1
Private
Sub
BtnOpen_Click(ByVal
sender As
System.Object, ByVal
e As
System.EventArgs) Handles
BtnOpen.Click
Dim
FileReader As
StreamReader
Dim
results As
DialogResult
results = OpenFileDialog1.ShowDialog
If
results = DialogResult.OK
Then
FileReader = New
StreamReader(OpenFileDialog1.FileName)
TextBox1.Text = FileReader.ReadToEnd()
FileReader.Close()
End
If
End
Sub
| The Design Interface
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| The Open
Dialog box
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| The Output
Interface
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21.3 Writing to a
Text File
Writing a text file
means storing the text entered by the user via a textbox into a storage device
such as a hard drive. It also means saving the file. To accomplish this task, we
need to deploy the StreamWriter Class. You also need to insert the
SaveFileDialog control into the form as it is used to save the data into the
storage unit like a hard drive. The default bame for the SaveFileDialog control
is
SaveFileDialog1. The Code is basically the same as the code for reading
the file, you just change the StreamReader to StreamWriter, and
the method from ReadToEnd to Write. The code is shown in the
following table:
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The code
Imports System.IO
Public
Class Form1
Private
Sub
BtnSave_Click(ByVal
sender As
System.Object, ByVal
e As
System.EventArgs)
Dim
FileWriter As
StreamWriter
Dim
results As
DialogResult
results = SaveFileDialog1.ShowDialog
If
results = DialogResult.OK Then
FileWriter = New
StreamWriter(SaveFileDialog1.FileName,
False)
FileWriter.Write(TextBox1.Text)
FileWriter.Close()
End
If
End
Sub
The Output Interface
When you
click the save button, the program will prompt you to key in a file name
and the text will be save as a text file. Finally, you can combine the
two programs together and create a text editor that can read and write
text file, as shown in the diagram below.
he |
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