As mentioned in my comment to the original, this code was written in response to a thread about how best to do this -- I think the thread was
in the Lounge, but it may have been in the C# forum.

First you need to have a set of digits, the following is my prefered set of digits for bases up to 64 -- the user may provide a different set as needs require.

namespace PIEBALD.Lib
{
public static partial class LibStr
{
public const string Digits
= "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz_$" ;
}
}

Then you need to convert the existing string to a number (I chose to use a long, but your needs may differ):

public static long
StringToLong
(
string Subject
,
int Base
,
string Digits
)
{
if ( Subject == null )
{
throw ( new System.ArgumentNullException
(
"Subject"
,
"Subject must not be null"
) ) ;
}
if ( Base < 2 )
{
throw ( new System.ArgumentException
(
"Base must not be less than 2"
,
"Base"
) ) ;
}
if ( Digits == null )
{
Digits = LibStr.Digits ;
}
if ( Digits.Length < Base )
{
throw ( new System.ArgumentException
(
"Not enough Digits were provided for the Base"
,
"Digits"
) ) ;
}
long result = 0 ;
int sign = 0 ;
int offset ;
string DIGITS = Digits.ToUpper() ;
foreach ( char ch in Subject )
{
offset = Digits.IndexOf ( ch ) ;
if ( ( offset == -1 ) || ( offset >= Base ) )
{
offset = DIGITS.IndexOf ( char.ToUpper ( ch ) ) ;
}
if ( ( offset != -1 ) && ( offset < Base ) )
{
result = result * Base + offset ;
if ( sign == 0 )
{
sign = 1 ;
}
}
else
{
if ( ( sign == 0 ) && ( ch == '-' ) )
{
sign = -1 ;
}
}
}
return ( result * sign ) ;
}

Then you can convert back to a string, in a different base or with a different set of characters.

public static string
LongToString
(
long Subject
,
int Base
,
string Digits
)
{
if ( Base < 2 )
{
throw ( new System.ArgumentException
(
"Base must not be less than 2"
,
"Base"
) ) ;
}
if ( Digits == null )
{
Digits = PIEBALD.Lib.LibStr.Digits ;
}
if ( Digits.Length < Base )
{
throw ( new System.ArgumentException
(
"Not enough Digits were provided for the Base"
,
"Digits"
) ) ;
}
System.Text.StringBuilder result = new System.Text.StringBuilder() ;
int sign = 1 ;
if ( Subject < 0 )
{
Subject *= sign = -1 ;
}
do
{
result.Insert ( 0 , Digits [ (int) ( Subject % Base ) ] ) ;
Subject /= Base ;
}
while ( Subject > 0 ) ;
if ( sign == -1 )
{
result.Insert ( 0 , '-' ) ;
}
return ( result.ToString() ) ;
}

I also have overloads for these methods that allow the caller to specify only the value or only the value and base -- in C# 4,
this could be done with optional parameters. I just noticed that the code assumes that uppercase digits are lower than lowercase digits.
If you then want to add separators, that should be done in another method and probably honor cultural differences.