Как сделать вычисления внутри диалогового окна, а не по сообщению

Imports System.Runtime.InteropServices
Imports System.Text
 
Public Class Form1
 
    <DllImport("user32.dll", SetLastError:=True, CharSet:=CharSet.Auto)> _
    Private Shared Function SetWindowText(ByVal hwnd As IntPtr, ByVal lpString As String) As Boolean
    End Function
    <DllImport("user32.dll", SetLastError:=True, CharSet:=CharSet.Auto)> _
    Private Shared Function GetWindowText(ByVal hwnd As IntPtr, ByVal lpString As StringBuilder, ByVal cch As Integer) As Integer
    End Function
    <DllImport("user32.dll", SetLastError:=True, CharSet:=CharSet.Auto)> _
    Private Shared Function FindWindow( _
     ByVal lpClassName As String, _
     ByVal lpWindowName As String) As IntPtr
    End Function
    <DllImport("user32.dll", SetLastError:=True, CharSet:=CharSet.Auto)> _
    Private Shared Function FindWindowEx(ByVal parentHandle As IntPtr, _
                      ByVal childAfter As IntPtr, _
                      ByVal lclassName As String, _
                      ByVal windowTitle As String) As IntPtr
    End Function
    <DllImport("user32.dll", SetLastError:=True, CharSet:=CharSet.Auto)> _
    Private Shared Function GetWindowTextLength(ByVal hwnd As IntPtr) As Integer
    End Function
 
    Dim Title As String = "INPUT EDIT"
    Dim TextLabel As String = "ENTER TEXT!"
    Dim StartText As String = ""
 
    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        InputBox(TextLabel, Title, StartText)
    End Sub
 
    Dim HwndLabel As IntPtr
    Dim HwndTextBox As IntPtr
 
    Private Sub Timer1_Tick(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Timer1.Tick
        Dim HWND As IntPtr = FindWindow(vbNullString, Title)
        If HWND <> 0 Then
            Dim HwndLabel As IntPtr = FindWindowEx(HWND, 0, vbNullString, TextLabel)
            Dim HwndTextBox As IntPtr = FindWindowEx(HWND, 0, vbNullString, StartText)
            If HwndTextBox <> 0 Then
                Dim A As String = GetText(HwndTextBox)
                If NumBool(A) = False Then
                    SetWindowText(HwndLabel, "ERROR!")
                    TextLabel = "ERROR!"
                Else
                    SetWindowText(HwndLabel, A & "+" & A & "=" & Val(A) + Val(A))
                    TextLabel = A & "+" & A & "=" & Val(A) + Val(A)
                End If
            End If
        Else
            HwndLabel = 0
            HwndTextBox = 0
        End If
    End Sub
 
    Public Function GetText(ByVal hWnd As IntPtr) As String
        Dim length As Integer
        If hWnd.ToInt32 = 0 Then
            Return Nothing
        End If
        length = GetWindowTextLength(hWnd)
        If length = 0 Then
            Return Nothing
        End If
        Dim sb As New System.Text.StringBuilder("", length)
 
        GetWindowText(hWnd, sb, sb.Capacity + 1)
        Return sb.ToString()
    End Function
 
    Public Function NumBool(ByVal Text As String) As Boolean
        NumBool = True
        If Text = vbNullString Then
            Return False
        End If
        For i = 0 To Text.Length - 1 Step 1
            If IsNumeric(Text(i)) = False Then
                Return False
            End If
        Next
    End Function
 
End Class