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| Title | Make a TextBox that allows only digits in VB.NET |
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| Description | This example shows how to make a TextBox that allows only digits in VB.NET. It creates a TextBox subclass that sets its extended style to allow only digits. It also overrides the class's WindowProc to ignore paste messages. |
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| Keywords | TextBox, lower case, VB.NET |
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| Categories | Controls, VB.NET |
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The program creates a DigitTextBox class that is a subclass of TextBox. The class's constructor uses GetWindowLong and SetWindowLong to set the control's ES_NUMBER extended style bit. (It's too bad the TextBox class doesn't have GetExtendedStyle and SetExtendedStyle methods.)
This class also overrides the TextBox's WndProc subroutine to inspect Windows messsages. It ignores any WM_PASTE and WM_CONTEXTMENU messages so the user cannot press Ctrl-V or use the TextBox's context menu to paste non-numeric text into the control.
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Private Declare Function SetWindowLong Lib "user32" Alias _
"SetWindowLongA" (ByVal hWnd As IntPtr, ByVal nIndex As _
Int32, ByVal dwNewint32 As Int32) As Int32
Private Declare Function GetWindowLong Lib "user32" Alias _
"GetWindowLongA" (ByVal hWnd As IntPtr, ByVal nIndex As _
Int32) As Int32
Private Const GWL_STYLE As Int32 = (-16)
Private Const ES_NUMBER As Int32 = &H2000
Private Class DigitTextBox
Inherits TextBox
Public Sub New()
' Get the current style.
Dim style As Integer = _
GetWindowLong(Me.Handle, GWL_STYLE)
' Add ES_NUMBER to the style.
SetWindowLong(Me.Handle, GWL_STYLE, _
style Or ES_NUMBER)
End Sub
' Override the WndProc and ignore the
' WM_CONTEXTMENU and WM_PASTE messages.
Protected Overrides Sub WndProc(ByRef m As _
System.Windows.Forms.Message)
Const WM_CONTEXTMENU As Int32 = &H7B
Const WM_PASTE As Int32 = &H302
If (m.Msg <> WM_PASTE) And (m.Msg <> _
WM_CONTEXTMENU) Then
Me.DefWndProc(m)
End If
End Sub
End Class
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When the form loads, it creates a new DigitTextBox control and initializes its properties.
The program also sets the ES_NUMBER property on TextBox2 so you can see the difference. While you cannot type a non-digit into TextBox2, you can copy and paste one into it.
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Private Sub Form1_Load(ByVal sender As System.Object, ByVal _
e As System.EventArgs) Handles MyBase.Load
' Create the DigitTextBox.
DigitTextBox1 = New DigitTextBox
DigitTextBox1.Name = "DigitTextBox1"
DigitTextBox1.Location = New Point(TextBox1.Left, _
Label3.Top)
DigitTextBox1.Size = TextBox1.Size
DigitTextBox1.TabIndex = 2
DigitTextBox1.Text = ""
DigitTextBox1.Font = TextBox1.Font
Me.Controls.Add(DigitTextBox1)
' Set the numeric style for TextBox2.
' Get the current style.
Dim style As Integer = _
GetWindowLong(TextBox2.Handle, GWL_STYLE)
' Add ES_NUMBER to the style.
SetWindowLong(TextBox2.Handle, GWL_STYLE, _
style Or ES_NUMBER)
End Sub
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