Public Class Form1

    Private Sub Button1_Click(sender As System.Object, e As System.EventArgs) Handles Button1.Click
        ListBox1.Items.Clear()
        If TextBox1.BackColor = Color.Red Then
            ListBox1.Items.Add("Keine gültige Zahl")
            Exit Sub
        End If
        Dim time As Double
        Try
            time = threeplusn(Convert.ToDouble(TextBox1.Text), 0)
        Catch Exc As System.OverflowException
            ListBox1.Items.Add("=======================")
            ListBox1.Items.Add(TextBox1.Text & " ist wahrscheinlich keine wunderbare Zahl, wir wissen es nicht genau")
            Exit Sub
        End Try

        If Not time = -1 Then
            ListBox1.Items.Add("=======================")
            ListBox1.Items.Add(TextBox1.Text & " ist eine wunderbare Zahl | berechnet nach " & time & " Schritten")
        Else
            ListBox1.Items.Add("=======================")
            ListBox1.Items.Add(TextBox1.Text & " ist wahrscheinlich keine wunderbare Zahl, wir wissen es nicht genau")
        End If
    End Sub

    Public Function threeplusn(n As Double, time As Integer)
        If time > 1000 Then Return -1
        Dim nold As Double = n
        If Not n Mod 2 = 1 Then
            n = n / 2
            ListBox1.Items.Add("n = " & nold.ToString & " g -> " & n)
        Else
            n = 3 * n + 1
            ListBox1.Items.Add("n = " & nold.ToString & " u -> " & n)
        End If
        If n = 1 Then Return time Else Return threeplusn(n, time + 1)
    End Function

    Private Sub TextBox1_TextChanged(sender As System.Object, e As System.EventArgs) Handles TextBox1.TextChanged
        For i = 0 To TextBox1.Text.Length - 1
            Select Case (TextBox1.Text(i))
                Case "1", "2", "3", "4", "5", "6", "7", "8", "9", "0" : TextBox1.BackColor = Color.Green
                Case Else
                    TextBox1.BackColor = Color.Red
                    Exit For
            End Select
        Next
        If TextBox1.Text.Length < 1 Then TextBox1.BackColor = Color.Red
        If TextBox1.Text.Length = 1 And TextBox1.Text(0) = "0" Then TextBox1.BackColor = Color.Red

    End Sub
End Class