How to: Create a Numeric Text Box

Article
11/16/2012

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.

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

        If groupSeparator = CChar(160).ToString() Then
            .groupSeparator = " " 
        End If 

        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;

        // Workaround for groupSeparator equal to non-breaking space 
        if (groupSeparator == ((char)160).ToString())
        {
            groupSeparator = " ";
        }

        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:

Change History

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

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

Change History

Additional resources