Learning the .NET Framework: Project A Currency Changer - Accepting Data Entry

Users enter the amount of currency they wish to convert using the buttons 0 to 9 on the first tab page, which invokes the cmdButtons( ) event. Note that this event is fired whenever one of the 11 buttons (0 to 9 and the decimal point button) is pushed. It must also perform error checking so that users may not enter illegal entries, such as multiple decimal points [.]. Once the button is clicked, the value appends the TextBox control (txtValue) to show the user which numbers she has entered:

    Private Sub cmdButtons _
       (ByVal sender As System.Object, _
       ByVal e As System.EventArgs) _
       Handles cmd0.Click, cmd1.Click, cmd2.Click, _   
       cmd3.Click, cmd4.Click, cmd5.Click, _
       cmd6.Click, cmd7.Click, cmd8.Click, _
       cmd9.Click, cmdPt.Click
       If txtValue.Text.Length > 7 Or _ 
         (txtValue.Text.IndexOf("0") = 0 And _
         CType(sender, Button).Text = "0") Or _ 
         (txtValue.Text.IndexOf(".") >= 0 And _
         CType(sender, Button).Text = ".") Then
          ' remove multiple "."
        Else
          If txtValue.Text.IndexOf("0") = 0 Then
             txtValue.Text = ""
          End If
          txtValue.Text += CType( _
             sender, Button).Text
        End If
    End Sub

When the value in the TextBox control is changed, indicating that the user has entered another number, the conversion amount must be recalculated. The UpdateValue() event handles recalculation when any of these events occur:

• The selection in ComboBox1 is changed (the currency to convert from).
  
• The selection in ComboBox2 is changed (the currency to convert to).

• The value in the TextBox control is changed.

Here is the UpdateValue() event:

  Private Sub UpdateValue _
        (ByVal sender As System.Object, _
         ByVal e As System.EventArgs) _
         Handles ComboBox1.SelectedIndexChanged, _ 
         txtValue.TextChanged, _
         ComboBox2.SelectedIndexChanged
      If ready And txtValue.Text <> "" Then
       Try
        lblResult.Text = CSng(txtValue.Text) * _
              CSng(ComboBox2.SelectedValue) / _
              CSng(ComboBox1.SelectedValue)
        Catch ex As Exception
          'do nothing
        End Try
      End If
  End Sub

The cmdBackSpace_Click( ) method is invoked when the Back Space button is clicked. It is used to remove the last digit that was entered:

  Private Sub cmdBackSpace_Click _
             (ByVal sender As System.Object, _
             ByVal e As System.EventArgs) _
             Handles cmdBackSpace.Click
    ' length must be greater than 1 for deleting
    If txtValue.Text.Length > 0 Then
        txtValue.Text = Mid(txtValue.Text, 1, _
                        txtValue.Text.Length - 1)
    End If
    ' set to 0 if length is zero
    If txtValue.Text.Length = 0 Then
         txtValue.Text = 0
      End If
  End Sub

The Clear button resets the TextBox control to the value 0:

  Private Sub cmdClear_Click _
            (ByVal sender As System.Object, _
            ByVal e As System.EventArgs) _
            Handles cmdClear.Click
    txtValue.Text = 0
  End Sub

The “=” button adds the current conversion into the ListBox control:

  Private Sub cmdEq_Click _
            (ByVal sender As System.Object, _
            ByVal e As System.EventArgs) _
            Handles cmdEq.Click
      Dim str As String
      str = txtValue.Text & ComboBox1.Text & " = " _
            & lblResult.Text & ComboBox2.Text 
      ListBox1.Items.Add(str)
  End Sub

The Clear History menu item simply clears the items in the ListBox control:

  Private Sub MenuItem2_Click _
              (ByVal sender As System.Object, _
              ByVal e As System.EventArgs) _
              Handles MenuItem2.Click
      ListBox1.Items.Clear()
  End Sub

The second tab page control for the sample application allows the user to change the exchange rate of each currency. In production, this would frequently be done via the web service, which would grab the latest rates. In this case, users can override the rates included in the Rates.xmlfile. When ComboBox3 control is selected, the rate for the respective currency is displayed:

  Private Sub ComboBox3_SelectedIndexChanged _
            (ByVal sender As System.Object, _
             ByVal e As System.EventArgs) _
             Handles ComboBox3.SelectedIndexChanged
    txtRate.Text = ds.Tables("Currency").Rows _  
                   (ComboBox3.SelectedIndex).Item _
                   ("Rate").ToString
  End Sub

The Update button will cause all the dataset objects to be updated and reflect the newly entered rate. It will then call the saveRate( )method to save the changes to the Rates.xmlfile:

  Private Sub cmdUpdate_Click _
              (ByVal sender As System.Object, _
              ByVal e As System.EventArgs) _
              Handles cmdUpdate.Click
     ds.Tables("Currency").Rows _
              (ComboBox3.SelectedIndex).Item _
              ("Rate") = txtRate.Text
      saveRate(ds)
  End Sub

Finally, the saveRate() method saves the new exchange rates into the file:

  Public Sub saveRate(ByVal ds As DataSet)
      ds.WriteXml(FILENAME)
      MsgBox("Rates saved.")
  End Sub

That’s it! Press F5 in Visual Studio to run the application. Figure 3-5 shows the Currency Converter in action.

A currency converter is just an example of the types of things you can do with .NET Compact Framework. If a currency converter is not what you need, you can extrapolate the logic and tactics shown in this project in thousands of other ways. For example, you’ve learned:

• How to use various built-in controls in .NET Compact Framework

• How to use datasets to manipulate XML documents

• How to use web services to retrieve information

• How to use XML documents as data storage

• How to program your controls to handle events 
                                             
                Figure 3-5.  Using the Currency Converter

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This article is taken from chapter three of the book Programming the .NET Compact Framework, written by Wei-Meng Lee (O'Reilly, 2005; ISBN: 0596004338). Check it out at your favorite bookstore. Buy this book now.