File dialog boxes
Function args and overloading
Examples in this show
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List directory contents
Open a file for viewing
Read/Write to a file
Add file I/O to an old project
reading/writing array contents
Streamreader
Overloaded functions
Listing text files in a directory
File load
Private Sub Form1_Load(ByVal sender As
System.Object, ByVal e As
System.EventArgs) Handles MyBase.Load
txtfile.Text = CurDir()
End Sub
the open button –check for legal path
Dim NewPath As String
' NewPath holds the path the user has entered.
NewPath = txtfile.Text
' Change the location to NewPath.
Dim errormessage As String
Try
ChDir(NewPath)
' This checks to make sure the path is not blank.
Catch ex As Exception When NewPath = ""
ErrorMessage = "You must enter a path."
' This catches errors caused by a path that is not valid.
Catch
ErrorMessage = "You must enter a valid path. If trying" & _
"to access a different drive, remember to include the drive" & _
"letter."
Finally
' Display the error message only if one exists.
If ErrorMessage <> "" Then
MsgBox(ErrorMessage)
End If
End Try
Display txt contents of directory
Dim contentitem As String
lstData.Items.Clear()
' Only look at the .txt files.
ContentItem = Dir("*.txt")
If ContentItem = "" Then
errormessage = "No text files found."
MsgBox(errormessage)
End If
' Iterate through the directory contents.
Do Until ContentItem = ""
' Add it to the list.
lstData.Items.Add(contentitem)
' Move to the next textfile.
ContentItem = Dir()
Loop
Using filedialogs: this is a dialog
box
“Open” and “save” file dialog boxes are in
your toolbox and are added to the
component tray
Button action
• Open and save buttons display the file dialog,
get the results of the dialog.
• If the result is cancel the sub is exited via a
return
• If the filename doesn’t end in “.txt” (not a
guarantee of anything) the sub is exited. We do
this by using the endswith() string function to
check the end of the string.
• I added a try… catch in the hopes of eliminating
file io exceptions which might shut down the
application.
Button action: open
• The open button gets a filename, opens a
filereader for a file with this name.
• As long as there is more text in the file we
read it and append it to our multiline
textbox.
Button action: Save
• The save button gets a filename from the dialog
(or a cancel), opens a filewriter for a file with this
name.
• As long as there is more text (ie. Line by line) in
the multi-line textbox we read it and append it to
our multi-line textbox. (Multi-line textbox is just a
textbox with multi-line property set to true).
• We do this by using indexof string function to
find a crlf in the contents string and the substring
function to cut this part off.
Open button
Private Sub btnopen_Click(ByVal sender As System.Object, ByVal e As
System.EventArgs) Handles btnopen.Click
Dim objinput As StreamReader
Dim tmp, filename As String
Dim result As DialogResult = windows.forms.objOpenFileDialog.ShowDialog
If result = DialogResult.Cancel Then
Return
End If
filename = objOpenFileDialog.FileName
If filename.EndsWith(".txt") = False Then
Return
End If
Try
objinput = New StreamReader(filename)
Dim contents As String
contents = objinput.ReadLine
Do While contents <> ""
txtContents.AppendText(contents & ControlChars.CrLf)
contents = objinput.ReadLine
Loop
objinput.Close()
Catch ex As Exception
End Try
End Sub
Save button
Private Sub btnSave_Click(ByVal sender As System.Object, ByVal e As
System.EventArgs) Handles btnSave.Click
Dim objoutput As StreamWriter
Dim tmp As String
Dim result As DialogResult =
windows.forms.objSaveFileDialog.ShowDialog
If result = DialogResult.Cancel Then
Return
End If
Dim filename As String = objSaveFileDialog.FileName
If filename.EndsWith(".txt") = False Then
Return
End If
Try
objoutput = New StreamWriter(filename)
Dim contents As String = txtContents.Text
objoutput.WriteLine(contents)
objoutput.Close()
Catch ex As Exception
End Try
End Sub
Open button
Adding file output to a previous
project
Recommended way to do file i/o
• Use a streamwriter and streamreader
objects (for writing or reading
respectively).
• Use a filedialog box for opening or saving
a file.
• This example should not be used as a
basis for file io
The save button
Private Sub btnsave_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles
btnsave.Click
' FileOpen will create the file if it doesn't exist.
' value 1 is the file handle
Dim errormessage, displaystring As String
Dim i, j As Integer
Try
FileOpen(1, "records.txt", OpenMode.Append)’ this should be redone with stream writer
For i = 0 To classsize - 1
displaystring = students(i)
For j = 0 To 2
If exams(i, j) = -99 Then
displaystring = displaystring & ControlChars.Tab & "no grade"
Else
displaystring = displaystring & ControlChars.Tab & exams(i, j)
End If
Next
WriteLine(1, displaystring)
Next
Catch ex As Exception
errormessage = ex.ToString()
MsgBox(ErrorMessage)
Exit Sub
Finally
FileClose(1)
End Try
End Sub
Open/View and Save
Open button provides dialog box
Open button code
Private Sub btnopen_Click(ByVal sender As System.Object, ByVal e As System.EventArgs)
Handles btnopen.Click
Dim objinput As StreamReader
Dim tmp, filename As String
Dim result As DialogResult = objOpenFileDialog.ShowDialog
If result = DialogResult.Cancel Then
Return
End If
filename = objOpenFileDialog.FileName
If filename.EndsWith(".txt") = False Then
Return
End If
Try
objinput = New StreamReader(filename)
Dim contents As String
contents = objinput.ReadLine
Do While contents <> ""
txtContents.AppendText(contents & ControlChars.CrLf)
contents = objinput.ReadLine
Loop
objinput.Close()
Catch ex As Exception
End Try
End Sub
Create a new file (type contents)
Dialog box for save button
Save button code
Private Sub btnSave_Click(ByVal sender As System.Object, ByVal e As
System.EventArgs) Handles btnSave.Click
Dim objoutput As StreamWriter
Dim tmp As String
Dim result As DialogResult = windows.forms.objSaveFileDialog.ShowDialog
If result = DialogResult.Cancel Then
Return
End If
Dim filename As String = objSaveFileDialog.FileName
If filename.EndsWith(".txt") = False Then
Return
End If
Try
objoutput = New StreamWriter(filename)
Dim contents As String = txtContents.Text
objoutput.WriteLine(contents)
objoutput.Close()
Catch ex As Exception
End Try
End Sub
Filling/saving array contents
Saved file contents
Save button: open file and write to it
Private Sub btnsave_Click(ByVal sender As System.Object, ByVal e As
System.EventArgs) Handles btnsave.Click
' FileOpen will create the file if it doesn't exist.
' value 1 is the file handle
Dim errormessage As String
Dim i As Integer
Dim outpt As StreamWriter
Try
outpt = File.CreateText("values2.txt") 'could use dialog box
For i = 0 To array.Length - 1
outpt.WriteLine(array(i))
Next
Catch ex As Exception
errormessage = ex.ToString()
MsgBox(errormessage)
Exit Sub
Finally
outpt.Close()
End Try
End Sub
Open button: open file and read from it
Private Sub btnopen_Click(ByVal sender As System.Object, ByVal e As
System.EventArgs) Handles btnopen.Click
Dim input As streamreader
Dim errormessage As String
Dim i As Integer
lstData.Items.Clear()
Try
input = File.OpenText("values.txt") 'could use dialog box or textbox for name
For i = 0 To array.Length - 1
array(i) = CInt(input.ReadLine)
lstData.Items.Add(array(i))
Next
Catch ex As Exception
errormessage = ex.ToString()
MsgBox(ErrorMessage)
Exit Sub
Finally
input.Close()
End Try
End Sub
notes
• Could have used a file dialog, or got
filename(s) from the textbox.
• Global array declared
• Need to import system.io for streamreader
and streamwriter
Another streamreader example
About this example
• Streamreader can be opened from a string
• Need imports System.io at the top of the
application.
• I first checked if the file existed, then
displayed all the file contents
Just the code for the open button
Private Sub btnopen_Click(ByVal sender As System.Object, ByVal e As
System.EventArgs) Handles btnopen.Click
Dim fname, line As String
line = "“ ‘initialize the line to empty
Dim strmreader As System.IO.StreamReader
fname = txtfile.Text ‘get filename…a string
If File.Exists(fname) Then ‘check if legal
strmreader = System.IO.File.OpenText(fname) ‘note code here
Do Until strmreader.Peek = -1’ and here…read to EOF
line = strmreader.ReadLine
’streamreader can read a line
lstdisplay.Items.Add(line)
Loop
Else
MessageBox.Show("illegal file name")
End If
End Sub
Reading individual characters: file
contents
123 456 789
12
34
56
7
Display ASCII codes for chars
Using read (instead of readline)
Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As
System.EventArgs) Handles Button1.Click
Dim data As String
Lstdisplay.Items.Clear()
Dim sr As IO.StreamReader = IO.File.OpenText("C:\data.txt")
Do While sr.Peek <> -1
data = sr.Read() 'read char by char
Lstdisplay.Items.Add(data)
Loop
sr.Close()
End Sub
The stream methods
Functions or subs may have
optional arguments
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You can specify that a procedure argument is optional and does not have to be
supplied when the procedure is called. Optional arguments are indicated by the
Optional keyword in the procedure definition. The following rules apply:
Every optional argument in the procedure definition must specify a default value.
The default value for an optional argument must be a constant expression.
Every argument following an optional argument in the procedure definition must also
be optional.
The following syntax shows a procedure declaration with an optional argument:
Sub subname(ByVal arg1 As type1, Optional ByVal arg2 As type2 = default)
Calling Procedures with Optional Arguments
When you call a procedure with an optional argument, you can choose whether to
supply the argument. If you do not, the procedure uses the default value declared for
that argument.
When you omit one or more optional arguments in the argument list, you use
successive commas to mark their positions. The following example call supplies the
first and fourth arguments but not the second or third:
Call subname(arg1, , , arg4) ' Leaves out arg2 and arg3.
compute degrees F or C based on which is passed
Private Sub calc(Optional ByVal F As String = Nothing, Optional ByVal C As String = Nothing)
Dim convert As Double
Try
If F Is Nothing Then ‘convert celsius temp to F
Dim celsius As Double = CDbl(C)
convert = 9 / 5 * celsius + 32
txtf.Text = convert.ToString("n2")
Else ‘ must use F since C must be nothing
Dim fahr As Double = CDbl(F)
convert = 5 / 9 * (fahr - 32)
txtc.Text = convert.ToString("n2")
End If
Catch ex As FormatException
MessageBox.Show("at least on field must be a double")
End Try
End Sub
Private Sub btnconvert_Click(ByVal sender As System.Object, ByVal e As System.EventArgs)
Handles btnconvert.Click
If txtf.Text = "" Then
calc(, txtc.Text)
Else
calc(txtf.Text, )
End If
End Sub
Running optional args example
Overloaded functions
• A function “signature consists of the name,
and parameters.
• If function signatures differ (even though
the names are the same), then VB, C++,
VC++, java, etc, can distinguish which
function you are trying to use.
• So we could also code two versions of a
given function.
Function overloading: add ints,
doubles and strings
Function overload
Private Sub btndostuff_Click(ByVal sender As System.Object, ByVal e As
System.EventArgs) Handles btndostuff.Click
Dim s, t As String
s = txtone.Text
t = txttwo.Text
txtint.Text = ADD(CInt(s), CInt(t))
txtdouble.Text = ADD(CDbl(s), CDbl(t))
txtstring.Text = ADD(s, t)
End Sub
Function ADD(ByVal s As String, ByVal t As String) As String
ADD = s & t
End Function
Function ADD(ByVal s As Integer, ByVal t As Integer) As String
ADD = (s + t).ToString
End Function
Function ADD(ByVal s As Double, ByVal t As Double) As String
ADD = (s + t).ToString("n3")
End Function