Module Module1
 
    Private Enum OpType
        unknown
        addition
        subtraktion
        multiplikation
        division
    End Enum
 
 
    Sub Main()
 
        Do
        Loop While Dialog()
    End Sub
 
 
    Private Function Dialog() As Boolean
        Dim ot As OpType = OpType.unknown
 
        Dim myinput As String = Console.ReadLine()
        If String.IsNullOrWhiteSpace(myinput) Then
            Console.WriteLine("Fehlende Eingabe - Programm beendet") : Return _
              False
        End If
 
        Dim ind As Integer = myinput.IndexOf("+")
        If ind > 0 Then
            ot = OpType.addition
        Else
            ind = myinput.IndexOf("-")
            If ind > 0 Then ot = OpType.subtraktion
        End If
 
        If ot = OpType.unknown Then
            Console.WriteLine("Keine bekannte Rechenoperation")
            Return True
        End If
 
        Dim arg1, arg2, erg As Single
 
        If Not Single.TryParse(myinput.Substring(0, ind), arg1) Then
            Console.WriteLine("Ungeeignetes 1. Argument")
            Return True
        End If
 
        If Not Single.TryParse(myinput.Substring(ind + 1), arg2) Then
            Console.WriteLine("Ungeeignetes 2. Argument")
            Return True
        End If
 
        Try
            If ot = OpType.addition Then
                erg = arg1 + arg2
            Else
                erg = arg1 - arg2
            End If
            Console.WriteLine(CStr(Math.Round(erg, 3)))
        Catch
            Console.WriteLine("Überlauf")
            'Blöd: Singles lösen keinen Überlauf aus / besser Decimal nutzen
        End Try
        Return True
 
    End Function
End Module