String Calculator Kata, VB, complete code

Public Function AddMulti(ByVal numbers As String) As Integer
    Dim sum As Integer
    Dim numcopy() As String
    Dim StringSeperators() As String = {"\n", ","}

    If numbers.Contains("//") Then
        Dim delim As Char = numbers(2)
        ReDim StringSeperators(1)
        StringSeperators(0) = delim
        numbers = numbers.Substring(5)
    End If
    numcopy = numbers.Split(StringSeperators, StringSplitOptions.None)

    If numcopy = String.Empty Then
        Return 0
    End If

    If numbers.Contains("-") Then
        Dim message As String = "Negative values are invalid for Add function, found "
        Dim negatives() As String
        ReDim negatives(numbers.Length)
        Dim y As Integer = 0
        For x As Integer = 0 To numcopy.Length - 1 Step 1
            If numcopy(x).First = "-" Then
                negatives(y) = numcopy(x).ToString
                message += negatives(y).ToString + ","
                y += 1
            End If

        Next
        MsgBox(message.Trim(","), MsgBoxStyle.Information, "message")

        Throw New ArgumentException(message)
    End If

    If numcopy.Any((Function(x) String.IsNullOrEmpty(x))) Then
        Throw New ArgumentException()
    End If

    For x As Integer = 0 To numcopy.Length - 1 Step 1
        sum += numcopy(x)
    Next

   Return sum

End Function

' using lambda expressions (bobby's example)
 Public Function AddLambda(ByVal numbers As String) As Integer
    If String.IsNullOrEmpty(numbers) Then
        Return 0
    End If
    Return numbers.Split(","c).[Select](Function(x) Integer.Parse(x)).Sum()

 End Function