vb.net Arithmetic operation resulted in an overflow

Question

I am getting following error:

Arithmetic operation resulted in an overflow

on the code below:

Public Function DoEncode(ByVal aPassword As String, ByVal aPassword2 As String) As String
        Dim ascii As New ASCIIEncoding()
        Dim b As Byte() = ascii.GetBytes(aPassword)
        Dim b2 As Byte() = ascii.GetBytes(aPassword2)

        Dim iDiff As UInt32 = b2(0) - b(0)
        For i As Integer = 0 To b.Length - 1
            b(i) += CByte(iDiff)
        Next

        Return ascii.GetString(b)
    End Function

I have used the function in my login form:

Private Sub OK_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles OK.Click

        If UsernameTextBox.Text.Trim() = "" Then
            MessageBox.Show("Please enter your user id", "User ID", MessageBoxButtons.OK, MessageBoxIcon.[Stop])
            Return
        End If

        If PasswordTextBox.Text.Trim() = "" Then
            MessageBox.Show("Please enter your password", "User ID", MessageBoxButtons.OK, MessageBoxIcon.[Stop])
            Return
        End If

        If O.AppMan.DoLogin(UsernameTextBox.Text, PasswordTextBox.Text) Then
            DialogResult = System.Windows.Forms.DialogResult.OK
        End If
    End Sub

Upper code is used in:

Public Class TApplicationManager
    Public Class O
        Public Shared AppMan As New TApplicationManager()
    End Class
    Public Function DoEncode(ByVal aPassword As String, ByVal aPassword2 As String) As String
        Dim ascii As New ASCIIEncoding()
        Dim b As Byte() = ascii.GetBytes(aPassword)
        Dim b2 As Byte() = ascii.GetBytes(aPassword2)

        Dim iDiff As UInt32 = b2(0) - b(0)
        For i As Integer = 0 To b.Length - 1
            b(i) += CByte(iDiff)
        Next

        Return ascii.GetString(b)
    End Function

    Public Function DoLogin(ByVal strUser As String, ByVal strPwd As String) As Boolean
        Dim dtbUser As New DataTable()
        Dim dtbUserCare As New DataTable()
        Dim UserName As String
        Dim UserPass As String
        Dim cnn As New MySqlConnection(My.Settings.sys_dbConnectionString)
        Dim strLoginCare As String = "SELECT * FROM tblusers where Username = '" + strUser + "'"
        Dim daUser As New MySqlDataAdapter(strLoginCare, cnn)
        dtbUser.Clear()
        daUser.Fill(dtbUser)

        If dtbUser.Rows.Count = 0 Then
            MessageBox.Show("User does not exist", "User status", MessageBoxButtons.OK, MessageBoxIcon.[Stop])
            Return False
        End If

        Dim drwUser As DataRow = dtbUser.Rows(0)
        UserPass = drwUser("Password").ToString()
        UserName = drwUser("Username").ToString()

        Dim daUserCare As New MySqlDataAdapter(strLoginCare, cnn)
        dtbUserCare.Clear()
        daUserCare.Fill(dtbUserCare)

        If dtbUserCare.Rows.Count = 0 Then
            MessageBox.Show("Invalid user id", "User status", MessageBoxButtons.OK, MessageBoxIcon.[Stop])
            Return False
        End If

        Dim drwUserCare As DataRow = dtbUserCare.Rows(0)

        If drwUserCare("Password").ToString() <> DoEncode(strPwd, drwUserCare("Password").ToString()) Then
            MessageBox.Show("Invalid password. Please try again", "User status", MessageBoxButtons.OK, MessageBoxIcon.[Stop])
            Return False
        End If



        If drwUserCare("status").ToString() <> "Y" Then
            MessageBox.Show("User is not active", "User status", MessageBoxButtons.OK, MessageBoxIcon.[Stop])
            Return False
        End If

        UserPass = drwUserCare("Password").ToString()
        UserName = drwUserCare("Username").ToString()

        Return True
    End Function

End Class

Why am I getting this error?

Answer 1

Ok I'm going to take a stab it's this:

    Dim iDiff As UInt32 = b2(0) - b(0)
    For i As Integer = 0 To b.Length - 1
        b(i) += CByte(iDiff)
   Next

A) Uint32 can't be negative - and you are performing a subtraction between two bytes.

B) You are then casting that subtracted value to a Byte - bytes can't be negative, and from memory will throw this exception if you attempt to do so.

I don't actually understand what you're attempting to do in the above loop.

EDIT

As per your comment about this code working in c#, but not in VB, my answer still applies. The conversion is failing, because of either negative numbers or greater than 255.

c# does not automatically check for overflow errors where VB does.

You can disable this behaviour although changing the code to not overflow would be better in my opinion. You may get unexpected results.

See: https://msdn.microsoft.com/en-us/library/aa716336(v=vs.60).aspx

You can disable it by:

Project properties, enable Configuration Properties => Optimizations => Remove Integer Overflow Checks.

The same behaviour can be implemented in c# by wrapping the conversion / calculation in a 'checked' block.