Restituire contenuto di un documento in binario Bas64

static public string ToBase64( byte[] data )
{
int length = data == null ? 0 : data.Length;
if (length == 0)
return String.Empty;

int padding = length % 3;
if (padding > 0)
padding = 3 - padding;
int blocks = (length - 1) / 3 + 1;

char[] s = new char[blocks * 4];

for (int i = 0; i < blocks; i++)
{
bool finalBlock = i == blocks - 1;
bool pad2 = false;
bool pad1 = false;
if (finalBlock)
{
pad2 = padding == 2;
pad1 = padding > 0;
}

int index = i * 3;
byte b1 = data[index];
byte b2 = pad2 ? (byte)0 : data[index + 1];
byte b3 = pad1 ? (byte)0 : data[index + 2];

byte temp1 = (byte)((b1 & 0xFC) >> 2);

byte temp = (byte)((b1 & 0x03) << 4);
byte temp2 = (byte)((b2 & 0xF0) >> 4);
temp2 += temp;

temp = (byte)((b2 & 0x0F) << 2);
byte temp3 = (byte)((b3 & 0xC0) >> 6);
temp3 += temp;

byte temp4 = (byte)(b3 & 0x3F);

index = i * 4;
s[index] = SixBitToChar( temp1 );
s[index+1] = SixBitToChar( temp2 );
s[index+2] = pad2 ? "=" : SixBitToChar( temp3 );
s[index+3] = pad1 ? "=" : SixBitToChar( temp4 );
}

return new string( s );
}

static private char SixBitToChar( byte b )
{
char c;
if (b < 26)
{
c = (char)((int)b + (int)'A');
}
else if (b < 52)
{
c = (char)((int)b - 26 + (int)'a');
}
else if (b < 62)
{
c = (char)((int)b - 52 + (int)'0');
}
else if (b == 62)
{
c = s_CharPlusSign;
}
else
{
c = s_CharSlash;
}
return c;
}

static private char s_CharPlusSign = '+';
static private char s_CharSlash = '/';

static public char CharPlusSign
{
get
{
return s_CharPlusSign;
}
set
{
s_CharPlusSign = value;
}
}

static public char CharSlash
{
get
{
return s_CharSlash;
}
set
{
s_CharSlash = value;
}
} 

[WebMethod]
public byte[] GetFile(int ID)
{
   //..
}

 [WebMethod]
        public byte[] GetFile(int ID)
        {
            string filePath = "";

            if (System.IO.File.Exists(filePath))
            {
                return System.IO.File.ReadAllBytes(filePath);
            }
            else
            {
                //Inserire una gestione di errore oppure ritornare un array vuoto                
            }
        }