Some Probem in converting code from c# to Vb.net

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C#

VB

I have 1 project in c# i am converted it into vb.net but i got some difficulty in it some part of code which is properly worked in C# but if we convert into VB.net then it gives error i give you both codes if possible then give me the right code please....

C#

static string GetCoreSound(string str, ref int totalCoreSoundCharacter)
        {
            string soundmap = "अबसदइफगहईजकलमनओपकरसतउववज़यज";
            string h_shift = GethShift(str,ref totalCoreSoundCharacter);
            if (h_shift == null)
            {
               int position = ((str[0]) - 'a');
                if (position < soundmap.Length && position >= 0)
                {
                    return "" + soundmap[position];
                }
                totalCoreSoundCharacter=1;
                return str;

            }
            else
            {
                return h_shift;
            }
        }

VB.net

Private Shared Function GetCoreSound(ByVal str As String, ByRef totalCoreSoundCharacter As Integer) As String
            Dim soundmap As String = "अबसदइफगहईजकलमनओपकरसतउववज़यज"
            Dim h_shift As String = GethShift(str, totalCoreSoundCharacter)
            If h_shift Is Nothing Then
                Dim position As Integer = ((str(0)) - "a"c)
                If position < soundmap.Length AndAlso position >= 0 Then
                    Return "" & soundmap(position)
                End If
                totalCoreSoundCharacter = 1

                Return str
            Else
                Return h_shift
            End If
        End Function

**************************************************
where the code is bold there is problem....
" Dim position As Integer = ((str(0)) - "a"c)"
this line is not working so please tell me proper..
--
With Regards

Apeksha Deshpande