Number Base Conversion

[Troy Lundin]

I have been using this code for a long time now and it has always been accurate. I recently tried to convert a large string from one base to another and some weird stuff happens.

When I convert from base 36 to base 10, the result is accurate.
When I convert the result back to base 36, it becomes inaccurate.

Example: "AAAAAAAAAAAAAAAA" (36 -> 10) 2273903174270400214335488
Reverse: 2273903174270400214335488 (10 -> 36) "AAAAAAAAAA9NGCU8"

As you can see, the result gets messed up somewhere. I have tried other example posted across the net with similar results.

Here is the function I am using.

    Public Function ConvertBase(ByVal ValueToConvert As String, ByVal FromBase As Int32, ByVal ToBase As Int32) As String
        Dim s As String = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ"

        Dim fromValue As New System.Text.StringBuilder(ValueToConvert.ToString)
        Dim power As Int32 = 0
        Dim result As BigInteger = 0

        While fromValue.Length > 0
            Dim index As Integer = s.IndexOf(fromValue(fromValue.Length - 1))

            If index < 0 Then Throw New FormatException("ValueToConvert contains an unsupported character.")
            If index > FromBase Then Throw New FormatException("ValueToConvert contains an invalid character for FromBase.")

            result += BigInteger.Multiply(index, BigInteger.Pow(FromBase, power))
            If result < 0 Then Throw New OverflowException()

            fromValue.Length -= 1
            power += 1
        End While

        If ToBase = 10 Then Return result.ToString

        Dim ret As New System.Text.StringBuilder
        While result > 0
            Dim index As Integer = CInt(result Mod ToBase)
            ret.Insert(0, s(index))
            result = result / ToBase
        End While
        Return ret.ToString
    End Function

Can anyone see what's wrong with it?

[Troy Lundin]

Okay, apparently converting a BigInteger to a string causes it to break?
When I return the result to a string it is accurate. When I convert that string to a BigInteger I get a different number. I am going to look into this a bit more.