A Localizable Currency TextBox

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Introduction

My attempts to find a suitable control for displaying and editing correctly formatted currency values that could be bound to a decimal value were in vain, so I set about creating my first custom web control.

Note: The download also includes a PercentBox and NumberBox control which use the same principles.

Control Requirements

The first step was to define the essential requirements for the control.

• Allow databinding to a decimal value.
• Display the value as currency text, formatted in accordance with the current culture.
• Display negative values in a different colour.
• Allow only valid data entry.
• Perform the work associated with validating input and formatting currency values on the client side.

Starting Out

While the control renders as a text box <INPUT type="text" ...>, inheriting from System.Web.UI.WebControls.TextBox would have required many properties to be hidden, so the control inherits from System.Web.UI.WebControls.WebControl and implements IPostBackDataHandler.

Properties

Eleven public properties are added to the base class to control its appearance:

• Alignment: Sets the alignment of text in the control (the default is Right).
• Amount: The decimal value to be bound to the control. The value is stored in ViewState and the implementation of IPostBackDataHandler ensures the AmountChanged event is raised when appropriate.

  [
  DefaultValue(typeof(decimal),"0"),
  Category("Appearance"),
  Bindable(true),
  Description("The amount displayed in the control")
  ]
  public decimal Amount
  {
    get
    {
      // Check if a value has been assigned
      object amount = ViewState["amount"];
      if (amount == null)
        // Return the default
        return 0M;
      else
        // Return the value
        return (decimal)amount;
    }
    set
    {
      ViewState["amount"] = value;
      // Set the text colour
      if (value < 0)
        base.ForeColor = NegativeColor;
      else
        base.ForeColor = PositiveColor;
    }
  }

• MinAmount: Sets the minimum amount which can be entered by the user.
• MaxAmount: Sets the maximum amount which can be entered by the user.
• NegativeColor: Sets the colour to display if the Amount is negative (<0).

  [
  DefaultValue(typeof(Color),"Red"),
  Category("Appearance"),
  Bindable(true),
  Description("The colour of negative currency values")
  ]
  public Color NegativeColor
  {
    get
    {
      return _NegativeColor;
    }
    set
    {
      _NegativeColor = value;
      // Set the controls ForeColor if appropriate
      if (Amount < 0)
        base.ForeColor = value;
    }
  }

• OnBlur, OnFocus and OnKeyPress: The control outputs JavaScript code for these HTML events, and these properties allow additional scripts to be assigned.
• PositiveColor: Sets the colour to display if the Amount is positive.
• Precision: The number of decimal places that the formatted amount will display.
• Text: A readonly property which returns the Amount formatted in accordance with the current culture.

In addition, the base class ForeColor property is hidden to implement it as readonly (because its colour is set using the PositiveColor and NegativeColor properties).

Rendering the Control

A WebControl, by default, renders as <SPAN>. To render as a textbox <INPUT>, we must first override TagKey.

protected override HtmlTextWriterTag TagKey
{
  get
  {
    return HtmlTextWriterTag.Input;
  }
}

and then add attributes by overriding AddAttributesToRender.

protected override void AddAttributesToRender(HtmlTextWriter writer)
{
  base.AddAttributesToRender(writer);
  // Add attributes necessary to display an text control
  writer.AddAttribute(HtmlTextWriterAttribute.Type, "text");
  writer.AddAttribute(HtmlTextWriterAttribute.Name, UniqueID);
  writer.AddAttribute(HtmlTextWriterAttribute.Value, Text);
  writer.AddStyleAttribute("text-align", Alignment.ToString());
  // Add user defined attributes to control colour formatting
  writer.AddAttribute("negativeColor", NegativeColor.Name);
  if (PositiveColor != Color.Empty)
  {
    writer.AddAttribute("positiveColor", PositiveColor.Name);
  }
  // Add client side event handlers
  writer.AddAttribute("onkeypress", "EnsureNumeric()");
  writer.AddAttribute("onfocus", "FormatAsDecimal(this)");
  writer.AddAttribute("onblur", "FormatAsCurrency(this)");
}

Two user defined attributes are added to the rendered control, positiveColor and negativeColor. These two attributes are read by the client-side script to set the colour of the text depending on its value. In addition, three client side events are assigned to script functions as discussed below.

Client Side Scripts

The base class OnPreRender method is overridden to get the current culture's NumberFormat. Then it is checked if the scripts have been registered, and if not, code is called to construct them using the NumberFormat, and finally register them.

protected override void OnPreRender(EventArgs e)
{
  // Get the number format of the current culture
  NumberFormatInfo format = Thread.CurrentThread.CurrentCulture.NumberFormat;
  // Register the EnsureNumeric script
  if (!Page.IsClientScriptBlockRegistered("EnsureNumeric"))
  {
    Page.RegisterClientScriptBlock("EnsureNumeric",EnsureNumericScript(format));
  }
  ...........

Five script functions are registered.

• EnsureNumeric() is assigned to the OnKeyPress event and limits keyboard entry to valid input.
• FormatCurrencyAsDecimal() is assigned to the OnFocus event. It calls the CurrencyToDecimal() function and formats its text based on the returned value.
• FormatDecimalAsCurrency() is assigned to the OnBlur event. It calls the DecimalToCurrency() function and formats its text based on the returned value.
• CurrencyToDecimal() parses a currency string to a decimal value.
• DecimalToCurrency() formats a decimal value as a currency string.

The code used to generate these scripts is too lengthy to be included here but the download includes a htm file explaining the methodology I used.

Using the Control

• Download and build the Web Control Library project.
• Create a new ASP.NET Web Application project and add the control to the toolbox.
• Drag the control onto a form and set some properties.
• In the ASP.NET Web Application project Web.Config file, modify the globalization section to set a specific culture (e.g., culture="fr-FR" (for French-France) or culture="id-ID" (for Indonesian-Indonesia) etc.). Note that you must use a specific culture, not a neutral culture.

  <globalization
    requestEncoding="utf-8"
    responseEncoding="utf-8"
    culture="fr-FR"
  />
  

• Run the project.
• The Amount property will be displayed in accordance with the culture you specify in the Web.Config file. Tab into the control and the text will be parsed to a decimal value.
• Enter a numeric value and tab out. The text will be formatted back to a currency string with the colour set in accordance with PositiveColor or NegativeColor.

Note that when entering data, only numbers (0-9), the minus sign and the decimal separator (if appropriate) can be entered. Some languages such as Indonesian do not have decimal digits, in which case a decimal separator cannot be entered. Note also that the decimal separator for many languages (European) is a comma, not a decimal point.

History

• v1.0 - 08 June 2005 - Created.
• v1.1 - 04 August 2005
  • Additional properties for MinAmount, MaxAmount, OnBlur, OnFocus, OnKeyPress and Precision added.
  • Scripts amended.
  • NumberBox and PercentBox added to the project.
• v1.2 - 20 September 2005
  • Scripts amended to validate minimum and maximum values.