private static byte[] ConstructMapBase64() {
byte[] mapBase64 = new byte[MaxValidChar + 1];
for ( int i = 0; i < mapBase64.Length; i++ ) {
mapBase64[i]= Invalid;
}
for ( int i = 0; i < CharsBase64.Length; i++ ) {
mapBase64[(int)CharsBase64[i]] = (byte)i;
}
return mapBase64;
}