File IO

 File Input/Output  StreamWriter  StreamReader  Text Files  Binary Files

 Significant programs will require you to read/write data from data files  Configuration files  Data Files  For complex applications you will use Databases  Microsoft SQL Server/Access  We will only concentrate on simple text files

 Simple text files are nothing more than a series of characters Hello there. This is text in a text file This is a second line in the same text file.  At the end of the file there is a character called EOF (End Of File). You can’t see it  But you can test for it!!

 To use a file, you must:  Include a special Imports statement at the top of your program: Imports System.IO  Open the file  If the file doesn’t exist, you can’t open it  Then you must create it  Data can be written to the file (always at the end of the file!!)  Data can be read from the end of the file  Files can have Random Access to write/read from any point of the file – beyond the scope of this class  These are called Binary files.  Can’t view them in an editor!!  When finished with the file, close it!!!

 A Visual Basic Object that you declare (Dim) so that you can write to a file Dim filMyFile As StreamWriter  To create a new text file, use File.CreateText filMyFile = File.CreateText(“myfile.txt”)  To add to an existing file, use AppendText filMyFile = File.AppendText(“myfile.txt”)

 Once the file is open, you write to it with the Write / WriteLine statements filMyFile.WriteLine(“This is a line of text!!”)  The Write statement does NOT add a newline after the text (does not press the return key)  The WriteLine statement adds a newline after the text  To put a blank line in the file, just add a WRiteLine without text filMyFile.WriteLine(“”)

 You must always close files when finished with it  Use the Close statement filMyFile.Close()  Only close files if they’re open  Once you close it, you can’t close it again until it’s open

 You can add text to an existing file  Open it with the AppendText command  Then just write to it as you would any other text file  When finished, just close it as you would any other text file

Dim filTestFile As StreamWriter filTestFile = File.CreateText(“mytest.txt”) filTestFile.WriteLine(“This is the first line of text!”) filTestFile.WriteLine(“Here is the second!”) filTestFile.WriteLine(“”) filTestFile.WriteLine(“The previous line was blank!!”) filTestFile.Close() filTestFile = File.AppendText(“mytest.txt”) filTestFile.WriteLine(“”) filTestFile.WriteLine(“Oops. We forgot to add this line the first time!”) filTestFile.Close()

 Object in Visual Basic used to read text files  Declare the variable with a Dim statement Dim filTestFile As StreamReader  To open the file, use the OpenText method: filTestFile = File.OpenText(filTestFile)  To read data from the file, use the ReadLine method Dim txtALine As String txtALine = filTestFile.ReadLine()  Data can ONLY be read in a forward direction

 Check to see if a file exists or not (FileExists) If File.Exists(strFileName) Then ‘ Do something here End If  You can line things up nicely with vbTab  It is a string, which adds one tab to the text lstMyListBox.items.Add(“Name: “ & vbTab & vbTab & “Bill Clinton”) lstMyListBox.items.Add(“Name: “ & vbTab & vbTab & “George Bush”)

 You can find if you’re at the end of the file with Peek Dim myFile As StreamReader Dim strMyStr As String myFile = File.OpenText(“grades.txt”) Do Until myFile.Peek = -1 strMyStr = myFile.ReadLine() lstMyListBox.Items.Add(strMyStr) Loop myFile.Close()