File Handling in Visual Basic: Operations, Methods, and Examples

Introduction to Programming Lecture 6 Msury Mahunnah, Department of Informatics, Tallinn University of Technology

Operations With Text Files

Text Files Text files have an extension that ends with .txt It is called a Sequential File

Working with files File handling in Visual Basic is based on System.IO namespace with a class library that supports string, character and file manipulation. These classes contain properties, methods and events for creating, copying, moving, and deleting files. The BinaryReader, StreamWriter. most commonly used classes StreamReader are FileStream, BinaryWriter, and

You can open a Text File for ... 1. Reading 2. Writing 3. Appending

Opening a Text File for Reading Const fname$ = C:\Users\Owner\Documents\test.txt Dim objReader As New System.IO.StreamReader(fname) After this sentence the file is not opened yet As New This means Create a New Object . The type of the object is a StreamReader: System.IO.StreamReader

Opening a Text File for Reading One of the StreamReader methods available for reading is ReadToEnd (reading whole file at once) This method read the whole text, right to the end. i.e. TextBox1.Text = objReader.ReadToEnd (After this sentence the file is opened and the text is read) It is imporant to close the StreamReader after reading (otherwise you ll get error messages) objReader.Close()

ReadToEnd example Const fname$ = C:\Users\Owner\Documents\test.txt" If System.IO.File.Exists(fname) = True Then Dim objReader As New System.IO.StreamReader(fname) richtextbox1.AppendText(objReader.ReadToEnd) objReader.Close() Else MsgBox( File Does Not Exist ) End If

Reading Line by Line Method ReadToEnd is used for reading the whole file at once Method ReadLine for reading one line at a time To go through the whole file, use Do ... Loop statement with Peek method Peek signifies the end of the text file. Returns the next available character but does not consume it. If Peek returns -1 the reading sequence (process) is at the end of the file.

ReadLine example Const fname$ = C:\Users\Owner\Documents\test.txt If System.IO.File.Exists(fname) = True Then Dim objReader As New System.IO.StreamReader(fname) Dim TextLine As String = Do While objReader.Peek() <> -1 TextLine = TextLine & objReader.ReadLine() & vbNewLine Loop richtextbox1.AppendText(TextLine) objReader.Close() Else MsgBox( File Does Not Exist ) End If

Opening a Text File for Writing Const fname$ = C:\Users\Owner\Documents\test2.txt" Dim objWriter As New System.IO.StreamWriter(fname) After this sentence the file is not opened yet The type of the object is a StreamWriter: System.IO.StreamWriter

Writing into Text File Method Write add text into Text File. VB insists that the file must exist before it can actually do something with it! objWriter.Write( Hello world ) or objWriter.WriteLine( Hello world ) - This sentence writes Hello word into file and goes to new line

Write example Const fname$ = C:\Users\Owner\Documents\test.txt If System.IO.File.Exists(fname) = True Then Dim objWriter As New System.IO.StreamWriter(fname) objWriter.Write( Hello world! ) objWriter.Close() MsgBox( Text written to file! ) Else MsgBox( File Does Not Exist ) End If

Creating a file if it doesnt exist Const fname$ = C:\Users\Owner\Documents\test.txt Dim objWriter As New System.IO.StreamWriter(fname, False) objWriter.Write( Hello world! ) objWriter.Close() MsgBox( Text written to file! ) (Add a comma and False)

Opening a Text File for Appending Const fname$ = C:\Users\Owner\Documents\test.txt" If System.IO.File.Exists(fname) = True Then Dim objWriter As New System.IO.StreamWriter(fname, True) objWriter.Write( Hello world! ) objWriter.Close() MsgBox( Text written to file! ) Else (Add a comma and True) MsgBox( File Does Not Exist ) End If

Reading from a file and writing into an array Dim TextLine$ Dim seq As Object Dim t&=-1 Dim i&, V$() .... TextLine = objReader.ReadLine() seq = Strings.Split(TextLine, ) For i = Lbound(seq) To Ubound(seq) t+=1 Redim Preserve V(t) V(t) = seq(i) Next ...

All txt-files in certain folder Dim files() As String files = System.IO.Directory.GetFiles(path, pattern) Dim files() As String files = System.IO.Directory.GetFiles( C:\Users\John\", "*.txt") MsgBox("Found " & files.Length & " txt-files") Dim file As String For Each file In files RTB.AppendText(file & vbNewLine) Next RTB - RichTextBox

Create and delete a directory System.IO.Directory.CreateDirectory(path) System.IO.Directory.CreateDirectory( C:\Users\John\folder1\folder2\ ) Possibility to add so many subfolder as needed! System.IO.Directory.Delete(path) System.IO.Directory.Delete( C:\Users\John\folder1\folder2\ )

Delete all txt-files in folder Dim files() As String files = System.IO.Directory.GetFiles( C:\Users\John", "*.txt") MsgBox("Found " & files.Length & " txt-files") Dim file As String ForEach file In files System.IO.File.Delete(file) Next

Retrieving the Names of All Drives in the Computer Dim drives() As String drives = System.IO.Directory.GetLogicalDrives Dim drive As String ForEach drive In drives RTB.AppendText(drive & vbNewLine) Next RTB - RichTextBox