Performance Analysis

Note: new leader as of 2015-08-20.

I ran each of the various conversion methods through some crude Stopwatch performance testing, a run with a random sentence (n=61, 1000 iterations) and a run with a Project Gutenburg text (n=1,238,957, 150 iterations). Here are the results, roughly from fastest to slowest. All measurements are in ticks (10,000 ticks = 1 ms) and all relative notes are compared to the [slowest] StringBuilder implementation. For the code used, see below or the test framework repo where I now maintain the code for running this.

Disclaimer

WARNING: Do not rely on these stats for anything concrete; they are simply a sample run of sample data. If you really need top-notch performance, please test these methods in an environment representative of your production needs with data representative of what you will use.

Results

• Lookup by byte unsafe (via CodesInChaos) (added to test repo by airbreather)
  • Text: 4,727.85 (105.2X)
  • Sentence: 0.28 (99.7X)
• Lookup by byte (via CodesInChaos)
  • Text: 10,853.96 (45.8X faster)
  • Sentence: 0.65 (42.7X faster)
• Byte Manipulation 2 (via CodesInChaos)
  • Text: 12,967.69 (38.4X faster)
  • Sentence: 0.73 (37.9X faster)
• Byte Manipulation (via Waleed Eissa)
  • Text: 16,856.64 (29.5X faster)
  • Sentence: 0.70 (39.5X faster)
• Lookup/Shift (via Nathan Moinvaziri)
  • Text: 23,201.23 (21.4X faster)
  • Sentence: 1.24 (22.3X faster)
• Lookup by nibble (via Brian Lambert)
  • Text: 23,879.41 (20.8X faster)
  • Sentence: 1.15 (23.9X faster)
• BitConverter (via Tomalak)
  • Text: 113,269.34 (4.4X faster)
  • Sentence: 9.98 (2.8X faster)
• {SoapHexBinary}.ToString (via Mykroft)
  • Text: 178,601.39 (2.8X faster)
  • Sentence: 10.68 (2.6X faster)
• {byte}.ToString("X2") (using foreach) (derived from Will Dean's answer)
  • Text: 308,805.38 (2.4X faster)
  • Sentence: 16.89 (2.4X faster)
• {byte}.ToString("X2") (using {IEnumerable}.Aggregate, requires System.Linq) (via Mark)
  • Text: 352,828.20 (2.1X faster)
  • Sentence: 16.87 (2.4X faster)
• Array.ConvertAll (using string.Join) (via Will Dean)
  • Text: 675,451.57 (1.1X faster)
  • Sentence: 17.95 (2.2X faster)
• Array.ConvertAll (using string.Concat, requires .NET 4.0) (via Will Dean)
  • Text: 752,078.70 (1.0X faster)
  • Sentence: 18.28 (2.2X faster)
• {StringBuilder}.AppendFormat (using foreach) (via Tomalak)
  • Text: 672,115.77 (1.1X faster)
  • Sentence: 36.82 (1.1X faster)
• {StringBuilder}.AppendFormat (using {IEnumerable}.Aggregate, requires System.Linq) (derived from Tomalak's answer)
  • Text: 718,380.63 (1.0X faster)
  • Sentence: 39.71 (1.0X faster)

Lookup tables have taken the lead over byte manipulation. Basically, there is some form of precomputing what any given nibble or byte will be in hex. Then, as you rip through the data, you simply look up the next portion to see what hex string it would be. That value is then added to the resulting string output in some fashion. For a long time byte manipulation, potentially harder to read by some developers, was the top-performing approach.

Your best bet is still going to be finding some representative data and trying it out in a production-like environment. If you have different memory constraints, you may prefer a method with fewer allocations to one that would be faster but consume more memory.

Testing Code

Feel free to play with the testing code I used. A version is included here but feel free to clone the repo and add your own methods. Please submit a pull request if you find anything interesting or want to help improve the testing framework it uses.

1. Add the new static method (Func<byte[], string>) to /Tests/ConvertByteArrayToHexString/Test.cs.
2. Add that method's name to the TestCandidates return value in that same class.
3. Make sure you are running the input version you want, sentence or text, by toggling the comments in GenerateTestInput in that same class.
4. Hit F5 and wait for the output (an HTML dump is also generated in the /bin folder).

static string ByteArrayToHexStringViaStringJoinArrayConvertAll(byte[] bytes) {
    return string.Join(string.Empty, Array.ConvertAll(bytes, b => b.ToString("X2")));
}
static string ByteArrayToHexStringViaStringConcatArrayConvertAll(byte[] bytes) {
    return string.Concat(Array.ConvertAll(bytes, b => b.ToString("X2")));
}
static string ByteArrayToHexStringViaBitConverter(byte[] bytes) {
    string hex = BitConverter.ToString(bytes);
    return hex.Replace("-", "");
}
static string ByteArrayToHexStringViaStringBuilderAggregateByteToString(byte[] bytes) {
    return bytes.Aggregate(new StringBuilder(bytes.Length * 2), (sb, b) => sb.Append(b.ToString("X2"))).ToString();
}
static string ByteArrayToHexStringViaStringBuilderForEachByteToString(byte[] bytes) {
    StringBuilder hex = new StringBuilder(bytes.Length * 2);
    foreach (byte b in bytes)
        hex.Append(b.ToString("X2"));
    return hex.ToString();
}
static string ByteArrayToHexStringViaStringBuilderAggregateAppendFormat(byte[] bytes) {
    return bytes.Aggregate(new StringBuilder(bytes.Length * 2), (sb, b) => sb.AppendFormat("{0:X2}", b)).ToString();
}
static string ByteArrayToHexStringViaStringBuilderForEachAppendFormat(byte[] bytes) {
    StringBuilder hex = new StringBuilder(bytes.Length * 2);
    foreach (byte b in bytes)
        hex.AppendFormat("{0:X2}", b);
    return hex.ToString();
}
static string ByteArrayToHexViaByteManipulation(byte[] bytes) {
    char[] c = new char[bytes.Length * 2];
    byte b;
    for (int i = 0; i < bytes.Length; i++) {
        b = ((byte)(bytes[i] >> 4));
        c[i * 2] = (char)(b > 9 ? b + 0x37 : b + 0x30);
        b = ((byte)(bytes[i] & 0xF));
        c[i * 2 + 1] = (char)(b > 9 ? b + 0x37 : b + 0x30);
    }
    return new string(c);
}
static string ByteArrayToHexViaByteManipulation2(byte[] bytes) {
    char[] c = new char[bytes.Length * 2];
    int b;
    for (int i = 0; i < bytes.Length; i++) {
        b = bytes[i] >> 4;
        c[i * 2] = (char)(55 + b + (((b - 10) >> 31) & -7));
        b = bytes[i] & 0xF;
        c[i * 2 + 1] = (char)(55 + b + (((b - 10) >> 31) & -7));
    }
    return new string(c);
}
static string ByteArrayToHexViaSoapHexBinary(byte[] bytes) {
    SoapHexBinary soapHexBinary = new SoapHexBinary(bytes);
    return soapHexBinary.ToString();
}
static string ByteArrayToHexViaLookupAndShift(byte[] bytes) {
    StringBuilder result = new StringBuilder(bytes.Length * 2);
    string hexAlphabet = "0123456789ABCDEF";
    foreach (byte b in bytes) {
        result.Append(hexAlphabet[(int)(b >> 4)]);
        result.Append(hexAlphabet[(int)(b & 0xF)]);
    }
    return result.ToString();
}
static readonly uint* _lookup32UnsafeP = (uint*)GCHandle.Alloc(_Lookup32, GCHandleType.Pinned).AddrOfPinnedObject();
static string ByteArrayToHexViaLookup32UnsafeDirect(byte[] bytes) {
    var lookupP = _lookup32UnsafeP;
    var result = new string((char)0, bytes.Length * 2);
    fixed (byte* bytesP = bytes)
    fixed (char* resultP = result) {
        uint* resultP2 = (uint*)resultP;
        for (int i = 0; i < bytes.Length; i++) {
            resultP2[i] = lookupP[bytesP[i]];
        }
    }
    return result;
}
static uint[] _Lookup32 = Enumerable.Range(0, 255).Select(i => {
    string s = i.ToString("X2");
    return ((uint)s[0]) + ((uint)s[1] << 16);
}).ToArray();
static string ByteArrayToHexViaLookupPerByte(byte[] bytes) {
    var result = new char[bytes.Length * 2];
    for (int i = 0; i < bytes.Length; i++)
    {
        var val = _Lookup32[bytes[i]];
        result[2*i] = (char)val;
        result[2*i + 1] = (char) (val >> 16);
    }
    return new string(result);
}
static string ByteArrayToHexViaLookup(byte[] bytes) {
    string[] hexStringTable = new string[] {
        "00", "01", "02", "03", "04", "05", "06", "07", "08", "09", "0A", "0B", "0C", "0D", "0E", "0F",
        "10", "11", "12", "13", "14", "15", "16", "17", "18", "19", "1A", "1B", "1C", "1D", "1E", "1F",
        "20", "21", "22", "23", "24", "25", "26", "27", "28", "29", "2A", "2B", "2C", "2D", "2E", "2F",
        "30", "31", "32", "33", "34", "35", "36", "37", "38", "39", "3A", "3B", "3C", "3D", "3E", "3F",
        "40", "41", "42", "43", "44", "45", "46", "47", "48", "49", "4A", "4B", "4C", "4D", "4E", "4F",
        "50", "51", "52", "53", "54", "55", "56", "57", "58", "59", "5A", "5B", "5C", "5D", "5E", "5F",
        "60", "61", "62", "63", "64", "65", "66", "67", "68", "69", "6A", "6B", "6C", "6D", "6E", "6F",
        "70", "71", "72", "73", "74", "75", "76", "77", "78", "79", "7A", "7B", "7C", "7D", "7E", "7F",
        "80", "81", "82", "83", "84", "85", "86", "87", "88", "89", "8A", "8B", "8C", "8D", "8E", "8F",
        "90", "91", "92", "93", "94", "95", "96", "97", "98", "99", "9A", "9B", "9C", "9D", "9E", "9F",
        "A0", "A1", "A2", "A3", "A4", "A5", "A6", "A7", "A8", "A9", "AA", "AB", "AC", "AD", "AE", "AF",
        "B0", "B1", "B2", "B3", "B4", "B5", "B6", "B7", "B8", "B9", "BA", "BB", "BC", "BD", "BE", "BF",
        "C0", "C1", "C2", "C3", "C4", "C5", "C6", "C7", "C8", "C9", "CA", "CB", "CC", "CD", "CE", "CF",
        "D0", "D1", "D2", "D3", "D4", "D5", "D6", "D7", "D8", "D9", "DA", "DB", "DC", "DD", "DE", "DF",
        "E0", "E1", "E2", "E3", "E4", "E5", "E6", "E7", "E8", "E9", "EA", "EB", "EC", "ED", "EE", "EF",
        "F0", "F1", "F2", "F3", "F4", "F5", "F6", "F7", "F8", "F9", "FA", "FB", "FC", "FD", "FE", "FF",
    };
    StringBuilder result = new StringBuilder(bytes.Length * 2);
    foreach (byte b in bytes) {
        result.Append(hexStringTable[b]);
    }
    return result.ToString();
}

Update (2010-01-13)

Added Waleed's answer to analysis. Quite fast.

Update (2011-10-05)

Added string.Concat Array.ConvertAll variant for completeness (requires .NET 4.0). On par with string.Join version.

Update (2012-02-05)

Test repo includes more variants such as StringBuilder.Append(b.ToString("X2")). None upset the results any. foreach is faster than {IEnumerable}.Aggregate, for instance, but BitConverter still wins.

Update (2012-04-03)

Added Mykroft's SoapHexBinary answer to analysis, which took over third place.

Update (2013-01-15)

Added CodesInChaos's byte manipulation answer, which took over first place (by a large margin on large blocks of text).

Update (2013-05-23)

Added Nathan Moinvaziri's lookup answer and the variant from Bri…

The Apache Commons Codec library has a Hex class for doing just this type of work.

import org.apache.commons.codec.binary.Hex;

String foo = "I am a string";
byte[] bytes = foo.getBytes();
System.out.println( Hex.encodeHexString( bytes ) );

Consider the hex() method of the bytes type on Python 3.5 and up:

>>> array_alpha = [ 133, 53, 234, 241 ]
>>> print(bytes(array_alpha).hex())
8535eaf1

EDIT: it's also much faster than hexlify (modified @falsetru's benchmarks above)

from timeit import timeit
N = 10000
print("bytearray + hexlify ->", timeit(
    'binascii.hexlify(data).decode("ascii")',
    setup='import binascii; data = bytearray(range(255))',
    number=N,
))
print("byte + hex          ->", timeit(
    'data.hex()',
    setup='data = bytes(range(255))',
    number=N,
))

Result:

bytearray + hexlify -> 0.011218150997592602
byte + hex          -> 0.005952142993919551

Using map() won't work if the input is of a type like Uint8Array: the result of map() is also Uint8Array which can't hold the results of string conversion.

function toHexString(byteArray) {
  var s = '0x';
  byteArray.forEach(function(byte) {
    s += ('0' + (byte & 0xFF).toString(16)).slice(-2);
  });
  return s;
}

printf does what we want here:

fun ByteArray.toHexString() : String {
    return this.joinToString("") {
        java.lang.String.format("%02x", it)
    }
}