Add a cancel button to input box VB - vb.net

Question

I have made a simple calculator and I would like to know if you put in a string instead of a double and if you press close or not. The program is a simple calculator where the menu is a box with buttons for addition, subtraction, multiplication, and division. I would like it if you enter a string a messagebox pops up that tells you to enter a number and when you enter an empty message or click cancel it takes you back to the start of the program. Here is the code
Public Class CalcForm
Property num1 As Double = Nothing
Property num2 As Double = Nothing
Private Sub CalcForm_Load(sender As Object, e As EventArgs) Handles MyBase.Load
End Sub
Private Sub Addition_Click(sender As Object, e As EventArgs) Handles Addition.Click
Getnum()
Dim Answer As Double = num1 + num2
MessageBox.Show("The answer is " & Answer, "Answer")
End Sub
Private Sub Subtraction_Click(sender As Object, e As EventArgs) Handles Subtraction.Click
Getnum()
Dim Answer As Double = num1 - num2
MessageBox.Show("The answer is " & Answer, "Answer")
End Sub
Private Sub Multiplication_Click(sender As Object, e As EventArgs) Handles Multiplication.Click
Getnum()
Dim Answer As Double = num1 * num2
MessageBox.Show("The answer is " & Answer, "Answer")
End Sub
Private Sub Division_Click(sender As Object, e As EventArgs) Handles Division.Click
Getnum()
Dim Answer As Double = num1 / num2
MessageBox.Show("The answer is " & Answer, "Answer")
End Sub
Private Sub Getnum()
num1 = InputBox("Enter your first number", "Number 1")
num2 = InputBox("Enter your second number", "Number 2")
End Sub
End Class
Also I don't care about the difference between an empty ok or a cancel. But since I use doubles do I have to make it a string then convert to an double? If I do, how do I detect if the string has letters and ask the user to re-enter their number?

Answer 1

Do you really need your message box?
If you follow Hans Passant's recommendations and start using TextBox, you could simply bind your variable properties and parse and format them.
Have a look at the example below, it is more lines of code but gets you an opportunity to look at binding parsing and formatting.
It is a simple form with your four buttons and 3 text boxes. User cannot "get out" of the TextBox value is not numeric. Should you go for this solution, I would recommend to set the TextBox property for the result to ReadOnly :)
Imports System.Math
Imports System.ComponentModel
Public Class Form1
Implements INotifyPropertyChanged
Public Event PropertyChanged As PropertyChangedEventHandler Implements INotifyPropertyChanged.PropertyChanged
Private Sub NotifyPropertyChanged(ByVal info As String)
RaiseEvent PropertyChanged(Me, New PropertyChangedEventArgs(info))
End Sub
Private _num1 As Double
Property num1 As Double
Get
Return _num1
End Get
Set(value As Double)
If (value <> _num1) Then
_num1 = value
NotifyPropertyChanged("num1")
End If
End Set
End Property
Private _num2 As Double
Property num2 As Double
Get
Return _num2
End Get
Set(value As Double)
If (value <> _num2) Then
_num2 = value
NotifyPropertyChanged("num2")
End If
End Set
End Property
Private _Answer As Double
Property Answer As Double
Get
Return _Answer
End Get
Set(value As Double)
If (value <> _Answer) Then
_Answer = value
NotifyPropertyChanged("Answer")
End If
End Set
End Property
Private Sub CalcForm_Load(sender As Object, e As EventArgs) Handles MyBase.Load
' TextBox for result
TextBox3.ReadOnly = True
Set_bindings()
End Sub
Private Sub Addition_Click(sender As Object, e As EventArgs) Handles Addition.Click
Answer = num1 + num2
End Sub
Private Sub Substraction_Click(sender As Object, e As EventArgs) Handles Substraction.Click
Answer = num1 - num2
End Sub
Private Sub Multiplication_Click(sender As Object, e As EventArgs) Handles Multiplication.Click
Answer = num1 * num2
End Sub
Private Sub Division_Click(sender As Object, e As EventArgs) Handles Division.Click
Answer = num1 / num2
End Sub
Private Sub Set_bindings()
Dim binH As Binding
TextBox1.DataBindings.Clear()
binH = New Binding("Text", Me, "num1", True, DataSourceUpdateMode.OnPropertyChanged)
AddHandler binH.Parse, AddressOf NumParser
AddHandler binH.Format, AddressOf NumFormatter
TextBox1.DataBindings.Add(binH)
TextBox2.DataBindings.Clear()
binH = New Binding("Text", Me, "num2", True, DataSourceUpdateMode.OnPropertyChanged)
AddHandler binH.Parse, AddressOf NumParser
AddHandler binH.Format, AddressOf NumFormatter
TextBox2.DataBindings.Add(binH)
TextBox3.DataBindings.Clear()
binH = New Binding("Text", Me, "Answer", True, DataSourceUpdateMode.Never)
AddHandler binH.Parse, AddressOf NumParser
AddHandler binH.Format, AddressOf NumFormatter
TextBox3.DataBindings.Add(binH)
End Sub
Private Sub NumParser(ByVal sender As Object, ByVal e As ConvertEventArgs)
If IsNumeric(e.Value) Then
e.Value = e.Value
End If
End Sub
Private Sub NumFormatter(ByVal sender As Object, ByVal e As ConvertEventArgs)
e.Value = FormatNum(e.Value)
End Sub
Private Function FormatNum(ByVal x As Double) As String
Dim sFormat As String
sFormat = ""
Select Case Abs(x)
Case 1 To 1000
sFormat = "##0.000"
Case Is > 10000
sFormat = "0.000E+00"
Case Is = 0
sFormat = "#0.0"
Case Is < 0.001
sFormat = "0.000E-00"
Case Is < 1
sFormat = "#0.0000"
End Select
FormatNum = Format(x, sFormat)
End Function
End Class