Introduction

This article shall describe the construction of a custom control used to display currency values. The control is intended for use in Win Forms application development. The purpose of control is to allow the user to enter numeric values which are subsequently converted to and displayed as currency (as is indicated in figure 1 (below)).

The user of the control may drop any number of the currency controls into a form to display numeric values as currency; the control itself is merely an extended textbox.  By placing the code necessary to support the functionality into the format of a custom control, the need for writing the code necessary to support the display of currency for each textbox used is eliminated.



Figure 1:  The VB Currency Control in Use. 

Getting Started

The solution contains two projects. The first project is called "UseCurrencyControl" and the second project is called "CurrencyControl_VB". The first project is provided to demonstrate use of the control in a Win Forms application. The second project contains the actual custom control which is called "CurrencyText".  



Figure 2:  Solution Explorer with Both Projects Visible.

The Custom Control Project

Code:  Currency Text Control

The custom control's code contains all of the necessary methods used to restrict user input and to display the values into a textbox as a currency value. The control contains only the default imports as shown in the following:

Imports System

Imports System.Collections.Generic

Imports System.ComponentModel

Imports System.Data

Imports System.Drawing

Imports System.Text

Imports System.Windows.Forms 

Public Class CurrencyText

    Inherits TextBox

Following the imports statements the class declaration is configured to inherit from the standard textbox control. By inheriting from the textbox control, all of the functionality of the control is included in the custom control. After declaring the class, the next section of code is used to declare a private member variable which, through a public property, is used to store the current dollar value. Maintaining the dollar value as an internal variable is necessary to support edits to the underlying value after the value has been converted to a formatted string. It further allows the user to perform calculations against this value when, for example, the contents of multiple controls are added together. Note that, in the constructor show in the following code, the controls causes validation property is set to true.

Refer to the annotation for a description of the intent of each section of the sample code. 

''' <summary>

''' Declare a local variable used to contain

''' the current dollar value

''' </summary>

''' <remarks></remarks>

Private mDollarValue As Decimal 

''' <summary>

''' Declare public property used to get/set current dollar value

''' </summary>

''' <value></value>

''' <returns></returns>

''' <remarks></remarks>

Public Property DollarValue() As Decimal

    Get

        Return mDollarValue

    End Get

    Set(ByVal value As Decimal)

        mDollarValue = value

    End Set

End Property 

''' <summary>

''' Default constructor; sets dollar value to zero

''' when control is created

''' </summary>

''' <remarks></remarks>

Public Sub New()

    ' This call is required by the Windows Form Designer.

    InitializeComponent()

    Me.CausesValidation = True

    DollarValue = 0

End Sub 

''' <summary>

''' Default OnPaint event

''' </summary>

''' <param name="e"></param>

''' <remarks></remarks>

Protected Overrides Sub OnPaint(ByVal e As 

System.Windows.Forms.PaintEventArgs)

    MyBase.OnPaint(e)

End Sub 

The next section of code in the control is used to handle the keypress event. The purpose of the keypress event handler is to limit the user to keying in numbers and decimal points. The handler also allows the users to use the backspace button and other control characters when editing the contents of the currency text box. This code also limits the user to entering in only a single decimal point. As shown below, the event handler will also prevent the user from keying in a decimal point as the first character in the string; this may or may not be necessary and you can comment out this part if that functionality is not desired. 

''' <summary>

''' Key press event handler or text; limits user inputs to numbers,

''' a single decimal point, and control characters to permit edits

''' to the contents of the box (e.g., using the backspace key)

''' </summary>

''' <param name="sender"></param>

''' <param name="e"></param>

''' <remarks></remarks>

Private Sub CurrencyText_KeyPress(ByVal sender As System.Object, _

ByVal e As System.Windows.Forms.KeyPressEventArgs) Handles

MyBase.KeyPress

    ' allows only numbers, decimals and control characters

    If Not Char.IsDigit(e.KeyChar) And Not Char.IsControl(e.KeyChar) _

    And e.KeyChar <> "." Then

        e.Handled = True

    End If

    ' allows only a single decimal point

    If e.KeyChar = "." And Me.Text.Contains(".") Then

        e.Handled = True

    End If

    ' denies entry of point without leading value (e.g., zero)

    ' disable this if you don't want to insist on a leading zero

    ' or other value

    If e.KeyChar = "." And Me.Text.Length < 1 Then

        e.Handled = True

    End If

End Sub 

After the keypress event handler, the next section of code is used to handle the "validated" event for the control. This handler will attempt to convert the contents of the textbox to a properly formatted currency value whenever the control is validated (also recall that in the constructor, the control's causes validation property was set to true).  

''' <summary>

''' Update display to show decimal as currency whenver it is validated

''' </summary>

''' <param name="sender"></param>

''' <param name="e"></param>

''' <remarks></remarks>

Private Sub CurrencyText_Validated(ByVal sender As System.Object, ByVal e

As System.EventArgs) Handles MyBase.Validated

    Try

        Dim dTmp As Decimal

        dTmp = Convert.ToDecimal(Me.Text.ToString())

        Me.Text = dTmp.ToString("C")

    Catch

        ' skip it

    End Try

End Sub 

Next up is the  code used to handle the control's click event. In the case of this control, the intent is to convert the value displayed in the textbox back to the actual underlying control value as it stored in control (the actual decimal value). If the current dollar value property is zero, the application will clear the textbox (so that the user does not end up with the cursor trailing the zero). If the dollar value property does contain a value, the cursor is positioned at the end of that value. 

''' <summary>

''' Revert back to the display of numbers only

''' whenever the user clicks in the box for

''' editing purposes

''' </summary>

''' <param name="sender"></param>

''' <param name="e"></param>

''' <remarks></remarks>

Private Sub CurrencyText_Click(ByVal sender As System.Object, _

ByVal e As System.EventArgs) Handles MyBase.Click

    Me.Text = DollarValue.ToString()

    If Me.Text = "0.00" Then

        Me.Clear()

    End If

    Me.SelectionStart = Me.Text.Length

End Sub

The next section of code after the click event handler is the code used to handle the text changed event for the control. In this instance, whenever the value in the control is changed, the dollar value property is updated to hold the latest value. Since all computations and edits are based upon the dollar value property, it is necessary to constantly update this value to reflect the content of the text box. 

''' <summary>

''' Update the dollar value each time the value is changed

''' </summary>

''' <param name="sender"></param>

''' <param name="e"></param>

''' <remarks></remarks>

Private Sub CurrencyText_TextChanged(ByVal sender As System.Object, ByVal

e As System.EventArgs) Handles MyBase.TextChanged

    Try

        Dim strValue = Me.Text

        If (Me.Text.Length = 0) Then

            DollarValue = 0

        Else

            ' remove space and set allowed formatting

            DollarValue = Decimal.Parse(Me.Text.Replace(" ", ""), _

            Globalization.NumberStyles.AllowThousands Or _

            Globalization.NumberStyles.AllowDecimalPoint Or _

            Globalization.NumberStyles.AllowCurrencySymbol)

        End If

    Catch

        ' skip it for formatting

    End Try 

End Sub 

Code:  Use Currency Control Test Project

This project is used to test the custom control. The project contains a single Windows form. The form contains four of the custom controls and a button. The demo application is used to show the controls in use; the application allows the user to enter three currency values and then, in a button click event handler, the fourth value is calculated and displayed.

The code for this form is as follows: (refer to the annotation for specific descriptions of the functionality). 

''' <summary>

''' This form class is intended to demonstrate the use of the

''' CurrencyControl; it is not intended to serve as an actual

''' tax program although the Federal Government would likely be

''' very happy for people to use this program as the basis for

''' determining their tax liability - It is very likely that you will

''' experience a much happier tax return if you use TurboTax or TaxCut, or

''' if you hire the services of a tax specialist.

''' </summary>

''' <remarks></remarks>

Public Class Form1 

    ''' <summary>

    ''' Sum up the damages for this year and share the good news

    ''' with the user

    ''' </summary>

    ''' <param name="sender"></param>

    ''' <param name="e"></param>

    ''' <remarks></remarks>

    Private Sub btnCalcTaxes_Click(ByVal sender As System.Object, _

    ByVal e As System.EventArgs) Handles btnCalcTaxes.Click

        ' Need to know how much they earned, don't care if they paid any

        ' taxes though

        If Me.ctxWages.DollarValue = 0.0 Then

            MessageBox.Show("We need to know how much you made last year,

            really.", "Missing Money")

            Return

        End If

        ' Set some local variables of the correct type and

        ' collect their values from the form

        Dim decFedTax As Decimal = ctxFedTax.DollarValue

        Dim decStateTax As Decimal = ctxStateTax.DollarValue

        ' Do the math

        Dim decDeductions As Decimal = decFedTax + decStateTax

        Dim decOwedAmount As Decimal = ctxWages.DollarValue - decDeductions

        ' Display the results - since the amount owed's textbox does not

        ' get validated now, format the output so that it will immediately 

        ' display as currency

        Me.ctxOwedAmount.Text = decOwedAmount

        ctxOwedAmount.Text = decOwedAmount.ToString("C") 

    End Sub 

    ''' <summary>

    ''' Exit the application and close open forms

    ''' </summary>

    ''' <param name="sender"></param>

    ''' <param name="e"></param>

    ''' <remarks></remarks>

    Private Sub btnExit_Click(ByVal sender As System.Object, _

    ByVal e As System.EventArgs) Handles btnExit.Click

        Me.Dispose()

    End Sub 

End Class 

Aside from the button click event handler, there was no additional code added to the form to speak of. The button click event handler adds the dollar value properties for the state and federal taxes paid from the two custom controls used to collect those values; that total is then subtracted from the amount earned which is in turn gathered from the custom control used to collect the wages and tips. After subtracting the taxes paid from the money made, the result is dropped into the control used to show the amount due and the user is instructed to send in all of the rest of their earning less the amount already paid in taxes. I think the old 1040-EZ tax joke went something like, "How much did you make?", "Send it in". 

Summary.