I like using extension methods for conversions like this, even if they just wrap standard library methods. In the case of hexadecimal conversions, I use the following hand-tuned (i.e., fast) algorithms:
public static string ToHex(this byte[] bytes)
{
char[] c = new char[bytes.Length * 2];
byte b;
for(int bx = 0, cx = 0; bx < bytes.Length; ++bx, ++cx)
{
b = ((byte)(bytes[bx] >> 4));
c[cx] = (char)(b > 9 ? b + 0x37 + 0x20 : b + 0x30);
b = ((byte)(bytes[bx] & 0x0F));
c[++cx]=(char)(b > 9 ? b + 0x37 + 0x20 : b + 0x30);
}
return new string(c);
}
public static byte[] HexToBytes(this string str)
{
if (str.Length == 0 || str.Length % 2 != 0)
return new byte[0];
byte[] buffer = new byte[str.Length / 2];
char c;
for (int bx = 0, sx = 0; bx < buffer.Length; ++bx, ++sx)
{
// Convert first half of byte
c = str[sx];
buffer[bx] = (byte)((c > '9' ? (c > 'Z' ? (c - 'a' + 10) : (c - 'A' + 10)) : (c - '0')) << 4);
// Convert second half of byte
c = str[++sx];
buffer[bx] |= (byte)(c > '9' ? (c > 'Z' ? (c - 'a' + 10) : (c - 'A' + 10)) : (c - '0'));
}
return buffer;
}