How to: Create a Numeric Text Box

Article
06/18/2009

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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.

This example checks 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.

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.

' 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:

How to: Create a Numeric Text Box

To derive a class from TextBox

To add the NumericTextBox control to the form

Compiling the Code

See Also

Tasks

Concepts

Additional resources