How to: Create a Numeric Text Box

.NET Framework Developer's Guide

Updated: May 2009

You can create a custom control derived from TextBox so that it accepts only numeric input. This example defines the NumericTextBox class and shows how to place it on the form.

Note:
For an example of this control that strictly constrains user input, see How to: Control User Input in a Numeric Text Box.

The code in this example verifies the current character whenever a key is pressed. Another way to implement a numeric text box is to use pattern matching to verify the entire input string. For more information, see IsMatch.

To derive a class from TextBox

Add the NumericTextBox class to your project.

Visual Basic

Public Class NumericTextBox
    Inherits TextBox
    Private SpaceOK As Boolean = False

    ' Restricts the entry of characters to digits (including hex),
    ' the negative sign, the e decimal point, and editing keystrokes (backspace).
    Protected Overrides Sub OnKeyPress(ByVal e As KeyPressEventArgs)
        MyBase.OnKeyPress(e)

        Dim numberFormatInfo As NumberFormatInfo = System.Globalization.CultureInfo.CurrentCulture.NumberFormat
        Dim decimalSeparator As String = numberFormatInfo.NumberDecimalSeparator
        Dim groupSeparator As String = numberFormatInfo.NumberGroupSeparator
        Dim negativeSign As String = numberFormatInfo.NegativeSign

        Dim keyInput As String = e.KeyChar.ToString()

        If [Char].IsDigit(e.KeyChar) Then
            ' Digits are OK
        ElseIf keyInput.Equals(decimalSeparator) OrElse keyInput.Equals(groupSeparator) OrElse keyInput.Equals(negativeSign) Then
            ' Decimal separator is OK
        ElseIf e.KeyChar = vbBack Then
            ' Backspace key is OK
            '    else if ((ModifierKeys & (Keys.Control | Keys.Alt)) != 0)
            '    {
            '     // Let the edit control handle control and alt key combinations
            '    }
        ElseIf Me.SpaceOK AndAlso e.KeyChar = " "c Then

        Else
            ' Consume this invalid key and beep.
            e.Handled = True
        End If

    End Sub


    Public ReadOnly Property IntValue() As Integer
        Get
            Return Int32.Parse(Me.Text)
        End Get
    End Property


    Public ReadOnly Property DecimalValue() As Decimal
        Get
            Return [Decimal].Parse(Me.Text)
        End Get
    End Property


    Public Property AllowSpace() As Boolean

        Get
            Return Me.SpaceOK
        End Get
        Set(ByVal value As Boolean)
            Me.SpaceOK = value
        End Set
    End Property
End Class

public class NumericTextBox : TextBox
{
    bool allowSpace = false;

    // Restricts the entry of characters to digits (including hex), the negative sign,
    // the decimal point, and editing keystrokes (backspace).
    protected override void OnKeyPress(KeyPressEventArgs e)
    {
        base.OnKeyPress(e);

        NumberFormatInfo numberFormatInfo = System.Globalization.CultureInfo.CurrentCulture.NumberFormat;
        string decimalSeparator = numberFormatInfo.NumberDecimalSeparator;
        string groupSeparator = numberFormatInfo.NumberGroupSeparator;
        string negativeSign = numberFormatInfo.NegativeSign;

        string keyInput = e.KeyChar.ToString();

        if (Char.IsDigit(e.KeyChar))
        {
            // Digits are OK
        }
        else if (keyInput.Equals(decimalSeparator) || keyInput.Equals(groupSeparator) ||
         keyInput.Equals(negativeSign))
        {
            // Decimal separator is OK
        }
        else if (e.KeyChar == '\b')
        {
            // Backspace key is OK
        }
        //    else if ((ModifierKeys & (Keys.Control | Keys.Alt)) != 0)
        //    {
        //     // Let the edit control handle control and alt key combinations
        //    }
        else if (this.allowSpace && e.KeyChar == ' ')
        {

        }
        else
        {
            // Consume this invalid key and beep
            e.Handled = true;
            //    MessageBeep();
        }
    }

    public int IntValue
    {
        get
        {
            return Int32.Parse(this.Text);
        }
    }

    public decimal DecimalValue
    {
        get
        {
            return Decimal.Parse(this.Text);
        }
    }

    public bool AllowSpace
    {
        set
        {
            this.allowSpace = value;
        }

        get
        {
            return this.allowSpace;
        }
    }
}

To add the NumericTextBox control to the form

Add the following code to the form's constructor or Load event.

Visual Basic

' Create an instance of NumericTextBox.
Dim NumericTextBox1 As NumericTextBox = New NumericTextBox()
NumericTextBox1.Parent = Me


' Draw the bounds of the NumericTextBox.
NumericTextBox1.Bounds = New Rectangle(5, 5, 150, 100)

// Create an instance of NumericTextBox.
NumericTextBox numericTextBox1 = new NumericTextBox();
numericTextBox1.Parent = this;

//Draw the bounds of the NumericTextBox.
numericTextBox1.Bounds = new Rectangle(5, 5, 150, 100);

Add an InputPanel component to your form for user input into the NumericTextBox. For a Smartphone application, you can specify a numeric InputMode.

Compiling the Code

This example requires references to the following namespaces:

Tasks

How to: Set Smartphone Input Modes

How to: Use the InputPanel Component

Concepts

Custom Control Development

.NET Compact Framework How-to Topics

Change History

Date	History	Reason
May 2009	Added information about regular expressions.	Customer feedback.

Date

History

Reason

May 2009

Added information about regular expressions.

Customer feedback.