How to convert a 39-decimal characters into hex in c#

Hi experts
I've a string representation of a 39-characters long decimal integer.
I need to convert this into hex format in C#.
eg. 340282366920938463463374607431768211455 (39 characters)
      -> ffffffffffffffffffffffffffffffff (32 characters)

By the same token, I need to be able to reverse the operation from hex to decimal.

I am using C# .NET 3.5 which hasn't got BigInteger.

Any advice please.  Thanks in advance.
dominicwongAsked:
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QuinnDexCommented:
the code for doing this in strings is below but if you want to work in numbers try intX

https://github.com/devoyster/IntXLib


decimal string to hex string

        var s = "843370923007003347112437570992242323";
        var result = new List<byte>();
        result.Add( 0 );
        foreach ( char c in s )
        {
            int val = (int)( c - '0' );
            for ( int i = 0 ; i < result.Count ; i++ )
            {
                int digit = result[i] * 10 + val;
                result[i] = (byte)( digit & 0x0F );
                val = digit >> 4;
            }
            if ( val != 0 )
                result.Add( (byte)val );
        }

        var hex = "";
        foreach ( byte b in result )
            hex = "0123456789ABCDEF"[ b ] + hex;

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hex string to decimal string

static string HexToDecimal(string hex)
{
    List<int> dec = new List<int> { 0 };   // decimal result

    foreach (char c in hex)
    {
        int carry = Convert.ToInt32(c.ToString(), 16);   
            // initially holds decimal value of current hex digit;
            // subsequently holds carry-over for multiplication

        for (int i = 0; i < dec.Count; ++i)
        {
            int val = dec[i] * 16 + carry;
            dec[i] = val % 10;
            carry = val / 10;
        }

        while (carry > 0)
        {
            dec.Add(carry % 10);
            carry /= 10;
        }
    }

    var chars = dec.Select(d => (char)('0' + d));
    var cArr = chars.Reverse().ToArray();
    return new string(cArr);
}

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Jacques Bourgeois (James Burger)PresidentCommented:
There is a BigInteger. It is "hidden" in the System.Numerics namespace, available after referencing the dll by the same name.

If you can get a string representation of your value, the following will do the job:

BigInteger bigInt = BigInteger.Parse ( "340282366920938463463374607431768211455" );
string bigHex = bigInt.ToString ("X");
bigInt = BigInteger.Parse ( bigHex, System.Globalization.NumberStyles.HexNumber );

Otherwise, give a look at the BigInteger Structure in the documentation, it is quite exhaustive.
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QuinnDexCommented:
BigInteger is part of .net 4, OP is working with .net 3.5
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dominicwongAuthor Commented:
Thanks QuinnDex. That works like a charm, and is exactly what I need.

Thanks also to JamesBurger for your help.
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