File dialog boxes Function args and overloading

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File dialog boxesFunction args and overloading
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Examples in this show

• 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

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Listing text files in a directory
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File load

• Private Sub Form1_Load(ByVal sender As
  System.Object, ByVal e As System.EventArgs)
  Handles MyBase.Load
• txtfile.Text CurDir()
• End Sub

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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 ltgt "" Then
• MsgBox(ErrorMessage)

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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

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Using filedialogs this is a dialog box
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Open and save file dialog boxes are in your
toolbox and are added to the component tray
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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 doesnt 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.

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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.

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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.

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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 ltgt ""
• txtContents.AppendText(contents
  ControlChars.CrLf)
• contents objinput.ReadLine
• Loop

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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

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Open button
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Adding file output to a previous project
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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

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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

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Open/View and Save
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Open button provides dialog box
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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 ltgt ""
• txtContents.AppendText(contents
  ControlChars.CrLf)
• contents objinput.ReadLine

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Create a new file (type contents)
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Dialog box for save button
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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

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Filling/saving array contents
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Saved file contents
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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()

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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

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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

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Another streamreader example
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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

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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 filenamea
  string
• If File.Exists(fname) Then check if
  legal
• strmreader System.IO.File.OpenText(f
  name) note code here
• Do Until strmreader.Peek -1 and
  hereread 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

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Reading individual characters file contents

• 123 456 789
• 12
• 34
• 56
• 7

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Display ASCII codes for chars
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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 ltgt -1
• data sr.Read() 'read char by char
• Lstdisplay.Items.Add(data)
• Loop
• sr.Close()
• End Sub

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The stream methods
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Functions or subs may have optional arguments

• 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.

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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

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Running optional args example
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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.

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Function overloading add ints, doubles and
strings

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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