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By Jeff Prosise February 27, 2002 A Web Forms Currency ConverterFigure 5-9 shows a Web form that performs currency conversions using exchange rates stored in an XML file. To see it in action, copy Converter.aspx and Rates.xml, which are listed in Figures 5-10 and 5-11, to \Inetpub\wwwroot and type http://localhost/converter.aspx in your browser's address bar. Then pick a currency, enter an amount in U.S. dollars, and click the Convert button to convert dollars to the currency of your choice. Here are some points of interest regarding the source code:
Notice how easily Converter.aspx reads XML from Rates.xml. It doesn't parse any XML; it simply calls ReadXml on a DataSet and provides an XML file name. ReadXml parses the file and initializes the DataSet with the file's contents. Each <Rate> element in the XML file becomes a row in the DataSet, and each row, in turn, contains fields named Currency and Exchange. Enumerating all the currency types is a simple matter of enumerating the DataSet's rows and reading each row's Currency field. Retrieving the exchange rate for a given currency is almost as easy. OnConvert uses DataTable.Select to query the DataSet for all rows matching the currency type. Then it reads the Exchange field from the row returned and converts it to a decimal value with Convert.ToDecimal. One reason I decided to use a DataSet to read the XML file is that a simple change would enable the Web form to read currencies and exchange rates from a database. Were Converter.aspx to open the XML file and parse it using the FCL's XML classes, more substantial changes would be required to incorporate database input. A word of caution regarding this Web form: Don't use it to perform real currency conversions! The exchange rates in Rates.xml were accurate when I wrote them, but they'll be outdated by the time you read this. Unless you devise an external mechanism for updating Rates.xml in real time, consider the output from Converter.aspx to be for educational purposes only.
Converter.aspx
<%@ Import Namespace=System.Data %>
<form>
<body>
<h1>Currency Converter</h1>
<hr>
<form runat="server">
Target Currency<br>
<asp:ListBox ID="Currencies" Width="256" RunAt="server" /><br>
<br>
Amount in U.S. Dollars<br>
<asp:TextBox ID="USD" Width="256" RunAt="server" /><br>
<br>
<asp:Button Text="Convert" ID="ConvertButton" Width="256"
RunAt="server" /><br>
<br>
<asp:Label ID="Output" RunAt="server" />
</form>
</body>
</form>
<script language="C#" runat="server">
void Page_Init (Object sender, EventArgs e)
{
// Wire the Convert button to OnConvert
ConvertButton.Click += new EventHandler (OnConvert);
}
void Page_Load (Object sender, EventArgs e)
{
// If this isn't a postback, initialize the ListBox
if (!IsPostBack) {
DataSet ds = new DataSet ();
ds.ReadXml (Server.MapPath ("Rates.xml"));
foreach (DataRow row in ds.Tables[0].Rows)
Currencies.Items.Add (row["Currency"].ToString ());
Currencies.SelectedIndex = 0;
}
}
void OnConvert (Object sender, EventArgs e)
{
// Perform the conversion and display the results
try {
decimal dollars = Convert.ToDecimal (USD.Text);
DataSet ds = new DataSet ();
ds.ReadXml (Server.MapPath ("Rates.xml"));
DataRow[] rows = ds.Tables[0].Select ("Currency = '" +
Currencies.SelectedItem.Text + "'");
decimal rate = Convert.ToDecimal (rows[0]["Exchange"]);
decimal amount = dollars * rate;
Output.Text = amount.ToString ("f2");
}
catch (FormatException) {
Output.Text = "Error";
}
}
</script>
Figure 5-10. Currency converter source code.
Rates.xml
<?xml version="1.0" encoding="UTF-8"?>
<Rates>
<Rate>
<Currency>British Pound</Currency>
<Exchange>0.698544</Exchange>
</Rate>
<Rate>
<Currency>Canadian Dollar</Currency>
<Exchange>1.57315</Exchange>
</Rate>
<Rate>
<Currency>French Franc</Currency>
<Exchange>7.32593</Exchange>
</Rate>
<Rate>
<Currency>German Mark</Currency>
<Exchange>2.18433</Exchange>
</Rate>
<Rate>
<Currency>Italian Lira</Currency>
<Exchange>2162.67</Exchange>
</Rate>
<Rate>
<Currency>Japanese Yen</Currency>
<Exchange>122.742</Exchange>
</Rate>
<Rate>
<Currency>Mexican Peso</Currency>
<Exchange>9.22841</Exchange>
</Rate>
<Rate>
<Currency>Swiss Franc</Currency>
<Exchange>1.64716</Exchange>
</Rate>
</Rates>
Figure 5-11. XML file used by Converter.aspx.
Editor's Note: This sample chapter continues in the article titled, Web Forms: Code-Behind Programming. About the AuthorJeff Prosisemakes his living programming Microsoft .NET and teaching others how to do the same. A former engineer who discovered after college that programming is immeasurably more fun than designing lifting fixtures and computing loads on mounting brackets, today Jeff travels the world for Wintellect, the company he co-founded in 2000, helping other companies get up to speed on Microsoft .NET. Programming Microsoft .NET is Jeff's ninth book.
Programming Microsoft .NET. Copyright 2002, Jeff Prosise. Reproduced by permission of Microsoft Press. All rights reserved. # # # |
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