Only allow users to enter a number in textboxes. Display and error message if a number is not entered.

Question

I am doing an imperial to metric length converter and I am struggling to create an error message if a number is not inputted. Does anyone have any suggestions? Here is the code: 

Public Class Form1
    Dim inchesVal, inchesMm, inchesCm, inchesM, inchesKm, footVal, footMm, footCm,
        footM, footKm, yardsVal, yardsMm, yardsCm, yardsM, yardsKm, milesVal, milesM, milesKm As Integer
    Dim strTextInput As String
    Dim decTextInput As Decimal
    Dim KeyChar As Char


    Private Sub inchCon_Click() Handles inchCon.Click
        inchesVal = Val(inchVal.Text)
        inchesMm = Val(inchMm.Text)
        inchesCm = Val(inchCm.Text)
        inchesM = Val(inchM.Text)
        inchesKm = Val(inchKm.Text)
        inchMm.Text = inchVal.Text / 0.03937
        inchCm.Text = inchVal.Text / 0.3937
        inchM.Text = inchVal.Text / 39.37
        inchKm.Text = inchVal.Text / 3937
        strTextInput = inchVal.Text
     
    End Sub

    Private Sub feetCon_Click() Handles feetCon.Click
        footVal = Val(feetVal.Text)
        footMm = Val(feetMm.Text)
        footCm = Val(feetCm.Text)
        footM = Val(feetM.Text)
        footKm = Val(feetKm.Text)
        feetMm.Text = feetVal.Text / 0.0032808
        feetCm.Text = feetVal.Text / 0.032808
        feetM.Text = feetVal.Text / 3.2808
        feetKm.Text = feetVal.Text / 3280.8
    End Sub

    Private Sub yardCon_Click() Handles yardCon.Click
        yardsVal = Val(yardVal.Text)
        yardsMm = Val(yardMm.Text)
        yardsCm = Val(yardCm.Text)
        yardsM = Val(yardM.Text)
        yardsKm = Val(yardKm.Text)
        yardMm.Text = yardVal.Text / 0.0010936
        yardCm.Text = yardVal.Text / 0.010936
        yardM.Text = yardVal.Text / 1.0936
        yardKm.Text = yardVal.Text / 1093.6
    End Sub

    Private Sub mileCon_Click() Handles mileCon.Click
        milesVal = Val(mileVal.Text)
        milesM = Val(mileM.Text)
        milesKm = Val(mileKm.Text)
        mileM.Text = mileVal.Text / 0.00062137
        mileKm.Text = mileVal.Text / 0.62137
    End Sub

    Private Sub btnClear_Click(sender As System.Object, e As System.EventArgs) Handles btnClear.Click

        inchVal.Text = ""
        inchMm.Text = ""
        inchCm.Text = ""
        inchM.Text = ""
        inchKm.Text = ""

        feetVal.Text = ""
        feetMm.Text = ""
        feetCm.Text = ""
        feetM.Text = ""
        feetKm.Text = ""

        yardVal.Text = ""
        yardMm.Text = ""
        yardCm.Text = ""
        yardM.Text = ""
        yardKm.Text = ""

        mileVal.Text = ""
        mileM.Text = ""
        mileKm.Text = ""
    End Sub

Answer 1

Hello,

First off I would suggest you set Option Strict On under Project properties which afterwards there will be serveral errors in the code above to fix. Second, instead of using Val use TryParse

TryParse example, you should not be using Integers.

Dim FeetValue As Decimal = 0
If Decimal.TryParse(feetVal.Text, FeetValue) Then
    ' Okay to use feetVal.Text
Else
    ' Not okay to use feelval.text as decimal
End If

For checking if a TextBox Text is empty the following class when added to your project then add the class to the form allows an error message to be shown when a value for a specfic TextBox is empty. This class works similar to a ToolTip.

Screenshot where the second TextBox has been setup to not allow empty entries. The tooltip text is set by you the developer.

Imports System.ComponentModel
Imports System.Windows.Forms
<Drawing.ToolboxBitmap(GetType(IsRequiredProvider), "your_image.bmp"), _
System.ComponentModel.DesignerCategoryAttribute("Code"), _
 ProvideProperty("IsRequiredMessage", GetType(Control)), _
 ProvideProperty("AllowEmptyValue", GetType(Control)), _
 ProvideProperty("AllowEmptyErrorMessage", GetType(Control))> _
Public Class SimpleTextBoxProvider
    Inherits System.ComponentModel.Component
    Implements IExtenderProvider
    Friend htProvidedProperties As New Hashtable
    Private MissingMessages As New Hashtable
    Private mEnabled As Boolean = True
    Public Sub New(ByVal Container As System.ComponentModel.IContainer)
        MyClass.New()
        Container.Add(Me)
    End Sub
    Public Sub New()
        MyBase.New()
        InitializeComponent()
    End Sub
    Protected Overloads Overrides Sub Dispose(ByVal disposing As Boolean)
        If disposing Then
            If Not (components Is Nothing) Then
                components.Dispose()
            End If
        End If
        MyBase.Dispose(disposing)
    End Sub
    Private components As System.ComponentModel.IContainer
    Friend WithEvents errOutOfRange As System.Windows.Forms.ErrorProvider
    <System.Diagnostics.DebuggerStepThrough()> Private Sub InitializeComponent()
        Me.errOutOfRange = New System.Windows.Forms.ErrorProvider
        Me.errOutOfRange.BlinkStyle = System.Windows.Forms.ErrorBlinkStyle.NeverBlink
    End Sub
    Private Class ValidatorProperties
        Public AllowEmptyValue As Boolean = True
        Public AllowEmptyErrorMessage As String = "Can not be empty"
    End Class
    Private Function GetControlFromHashtable(ByVal ctrl As Control) As ValidatorProperties
        If htProvidedProperties.Contains(ctrl) Then
            Return DirectCast(htProvidedProperties(ctrl), ValidatorProperties)
        Else
            Dim ProvidedProperties As New ValidatorProperties
            'AddHandler CType(ctrl, TextBox).KeyPress, AddressOf This_KeyPress
            htProvidedProperties.Add(ctrl, ProvidedProperties)
            Return ProvidedProperties
        End If
    End Function
    <Category("Behavior"), _
     DefaultValue(True)> _
    Public Property Enabled() As Boolean
        Get
            Return mEnabled
        End Get
        Set(ByVal Value As Boolean)
            mEnabled = Value
        End Set
    End Property
    Public Function CanExtend(ByVal client_control As Object) As Boolean Implements IExtenderProvider.CanExtend
        Return (TypeOf client_control Is TextBox)
    End Function
    <Category("Validation")> _
    <Description("Determine if text can be left empty")> _
    Public Function GetAllowEmptyValue(ByVal ctrl As Control) As Boolean
        Return CType(GetControlFromHashtable(ctrl).AllowEmptyValue, Boolean)
    End Function
    <Category("Validation")> _
    Public Sub SetAllowEmptyValue(ByVal ctrl As Control, ByVal value As Boolean)
        GetControlFromHashtable(ctrl).AllowEmptyValue = value
    End Sub
    <Category("Validation")> _
    <Description("Error message to display if not valid")> _
    Public Function GetAllowEmptyErrorMessage(ByVal ctrl As Control) As String
        Return CType(GetControlFromHashtable(ctrl).AllowEmptyErrorMessage, String)
    End Function
    <Category("Validation")> _
    Public Sub SetAllowEmptyErrorMessage(ByVal ctrl As Control, ByVal value As String)
        GetControlFromHashtable(ctrl).AllowEmptyErrorMessage = value
    End Sub
    <Category("Validation"), _
     DefaultValue("")> _
    Public Function GetIsRequiredMessage(ByVal client_control As Control) As String
        If MissingMessages.Contains(client_control) Then
            Return DirectCast(MissingMessages(client_control), String)
        Else
            Return Nothing
        End If
    End Function
    <Category("Validation"), _
     DefaultValue("")> _
    Public Sub SetIsRequiredMessage(ByVal client_control As Control, ByVal missing_message As String)
        If MissingMessages.Contains(client_control) Then
            MissingMessages.Remove(client_control)
            RemoveHandler client_control.Validating, AddressOf Client_Validating
        End If
        If Not (missing_message Is Nothing) Then
            If missing_message.Length > 0 Then
                MissingMessages.Add(client_control, missing_message)
                AddHandler client_control.Validating, AddressOf Client_Validating
            End If
        End If
    End Sub
    Private Sub Client_Validating(ByVal sender As Object, ByVal e As System.ComponentModel.CancelEventArgs)
        If Not mEnabled Then
            Exit Sub
        End If
        ValidateTextBox(DirectCast(sender, TextBox))
    End Sub
    Private Sub ValidateTextBox(ByVal sender As TextBox)
        If sender.Text.Trim.Length > 0 Then
            Me.errOutOfRange.SetError(sender, "")
        Else
            ' It's blank.
            Dim missing_message As String = ""
            Me.errOutOfRange.SetError(sender, DirectCast(MissingMessages(sender), String))
        End If
    End Sub
    Private Sub This_KeyPress(ByVal sender As Object, ByVal e As System.Windows.Forms.KeyPressEventArgs)
        If e.KeyChar = Microsoft.VisualBasic.ChrW(Keys.Return) Then
            SendKeys.Send("{TAB}")
            e.Handled = True
        End If
    End Sub
    Public Function HasError() As Boolean
        If Not mEnabled Then
            Return False
        End If
        ' Make sure all controls have been validated.
        For Each TB As TextBox In MissingMessages.Keys
            ValidateTextBox(TB)
        Next TB
        ' Find the first control in the tab order with an error.
        Dim FirstTextBox As TextBox = Nothing
        Dim FirstTabIndex As Integer = Integer.MaxValue
        For Each TB As TextBox In MissingMessages.Keys
            ' See if the control has an error.
            If errOutOfRange.GetError(TB).Length > 0 Then
                If TB.TabIndex < FirstTabIndex Then
                    FirstTextBox = TB
                    FirstTabIndex = TB.TabIndex
                End If
            End If
        Next
        ' See if any control has an error.
        If Not (FirstTextBox Is Nothing) Then
            FirstTextBox.Focus()
            Return True
        End If
        Return False
    End Function
End Class

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Answer 2

Hi,

 You could use the textbox KeyPress event to only allow numbers to be entered. This will only allow 0 - 9 and a decimal point to be typed in the textbox.

    Private Sub TextBox1_KeyPress(ByVal sender As Object, ByVal e As System.Windows.Forms.KeyPressEventArgs) Handles TextBox1.KeyPress
        Dim valid As String = "0123456789."
        If Not valid.Contains(e.KeyChar) Then
            e.Handled = True
            MessageBox.Show("Invalid character. Please enter numbers only.", "Invalid", MessageBoxButtons.OK, MessageBoxIcon.Error)
        End If
    End Sub

Answer 3

Since you have a lot of TextBoxes maybe this will work better for you.

Option Strict On

Public Class Form1

    Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
        Dim textBoxes = Me.Controls.OfType(Of TextBox)()
        For Each txt In textBoxes
            AddHandler txt.KeyPress, AddressOf txtKeyPress
        Next
    End Sub

    Private Sub txtKeyPress(sender As Object, e As KeyPressEventArgs)
        Dim valid As String = "0123456789."
        If Not valid.Contains(e.KeyChar) Then
            e.Handled = True
            MessageBox.Show("Invalid character. Please enter numbers only.", "Invalid", MessageBoxButtons.OK, MessageBoxIcon.Error)
        End If
    End Sub

End Class

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You've taught me everything I know but not everything you know.

Answer 4

If you do not have other textboxes other than the ones listed in your btnClear button event then this would make your btnClear code a little smaller.  :)

    Private Sub btnClear_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnClear.Click
        Dim textboxes = Me.Controls.OfType(Of TextBox)()
        For Each tb As TextBox In textboxes
            tb.Clear()
        Next
    End Sub

Answer 5

The code posted above will allow more than one decimal point. 

The following code will only permit input of digits with a single decimal point.  It will also allow the backspace key, and negative numbers.  The Enter key will move the focus to the next control in Tab order.  Use only the statements you need for your program:

	Private Sub TextBox1_KeyPress(ByVal sender As Object, ByVal e As System.Windows.Forms.KeyPressEventArgs) Handles TextBox1.KeyPress
		If Not Char.IsDigit(e.KeyChar) Then e.Handled = True
		If e.KeyChar = Chr(8) Then e.Handled = False 'allow Backspace
		If e.KeyChar = "-" And TextBox1.SelectionStart = 0 Then e.Handled = False 'allow negative number
		If e.KeyChar = "." And TextBox1.Text.IndexOf(".") = -1 Then e.Handled = False 'allow single decimal point
		If e.KeyChar = Chr(13) Then GetNextControl(TextBox1, True).Focus() 'Enter key moves to next control
	End Sub

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Solitaire

Answer 6

try this

if isnumeric(textbox.text) then

blablabla
elseif textbox1.text="" then


msgbox("Please enter something)
else

msgbox("only numberics are allow)
end if

Answer 7

When I type a letter into the textbox, the message box does not display. Instead an error displays which says 'Conversion from string "d" to type 'Double' is not valid.' This line of my code is then highlighted :  inchMm.Text = inchVal.Text / 0.03937. 

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Ryan Dodd

Answer 8

Thats because you need to ether use TryParse to convert it to a number or make the textbox so that only numbers can be used in it. Also you are trying to divide a string by a number. You will need to convert the text (String) to a number (Double) before doing any math functions to it and then convert it back to a string before assigning the number back to the textbox.

        Dim ival As Double = 0
        Double.TryParse(inchVal.Text, ival)

        inchMm.Text = (ival / 0.03937).ToString

Answer 9

I also forgot to tell you that you really should turn on Option Strict and Option Explicit. You can do that in the VB Options or by adding these two lines as the very first two lines of your code

Option Strict On
Option Explicit On

Public Class Form1

    'Your Program Code

End Class

This will put a little blue line under the code that has errors of type conversion. You will see a little red box on the end of the blue line that you can put the mouse over and that will make a red box with a (!) on it. Click on that red box and most of the time it will give you a choice of how you can fix the error. It is not correct 100% of the time but, i would say it is at least 90% of the time.

Answer 10

Using the KeyPress event doesn't work if the user pastes something into the textbox.

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"Those who use Application.DoEvents() have no idea what it does and those who know what it does never use it." JohnWein