下面还通过允许本机小写选项扩展了这里的优秀答案，并且还处理null或空输入，并使其成为扩展方法。

    /// <summary>
    /// Converts the byte array to a hex string very fast. Excellent job
    /// with code lightly adapted from 'community wiki' here: https://stackoverflow.com/a/14333437/264031
    /// (the function was originally named: ByteToHexBitFiddle). Now allows a native lowerCase option
    /// to be input and allows null or empty inputs (null returns null, empty returns empty).
    /// </summary>
    public static string ToHexString(this byte[] bytes, bool lowerCase = false)
    {
        if (bytes == null)
            return null;
        else if (bytes.Length == 0)
            return "";

        char[] c = new char[bytes.Length * 2];

        int b;
        int xAddToAlpha = lowerCase ? 87 : 55;
        int xAddToDigit = lowerCase ? -39 : -7;

        for (int i = 0; i < bytes.Length; i++) {

            b = bytes[i] >> 4;
            c[i * 2] = (char)(xAddToAlpha + b + (((b - 10) >> 31) & xAddToDigit));

            b = bytes[i] & 0xF;
            c[i * 2 + 1] = (char)(xAddToAlpha + b + (((b - 10) >> 31) & xAddToDigit));
        }

        string val = new string(c);
        return val;
    }

    public static string ToHexString(this IEnumerable<byte> bytes, bool lowerCase = false)
    {
        if (bytes == null)
            return null;
        byte[] arr = bytes.ToArray();
        return arr.ToHexString(lowerCase);
    }

为了方便以后复制和粘贴，将几个答案合并到一个类中：

/// <summary>
/// Extension methods to quickly convert byte array to string and back.
/// </summary>
public static class HexConverter
{
    /// <summary>
    /// Map values to hex digits
    /// </summary>
    private static readonly char[] HexDigits =
        {
            '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'A', 'B', 'C', 'D', 'E', 'F'
        };

    /// <summary>
    /// Map 56 characters between ['0', 'F'] to their hex equivalents, and set invalid characters
    /// such that they will overflow byte to fail conversion.
    /// </summary>
    private static readonly ushort[] HexValues =
        {
            0x0000, 0x0001, 0x0002, 0x0003, 0x0004, 0x0005, 0x0006, 0x0007, 0x0008, 0x0009, 0x0100, 0x0100, 0x0100, 0x0100, 0x0100, 0x0100, 0x0100,
            0x000A, 0x000B, 0x000C, 0x000D, 0x000E, 0x000F, 0x0100, 0x0100, 0x0100, 0x0100, 0x0100, 0x0100, 0x0100, 0x0100, 0x0100, 0x0100, 0x0100,
            0x0100, 0x0100, 0x0100, 0x0100, 0x0100, 0x0100, 0x0100, 0x0100, 0x0100, 0x0100, 0x0100, 0x0100, 0x0100, 0x0100, 0x0100, 0x000A, 0x000B,
            0x000C, 0x000D, 0x000E, 0x000F
        };

    /// <summary>
    /// Empty byte array 
    /// </summary>
    private static readonly byte[] Empty = new byte[0];

    /// <summary>
    /// Convert a byte array to a hexadecimal string.
    /// </summary>
    /// <param name="bytes">
    /// The input byte array.
    /// </param>
    /// <returns>
    /// A string of hexadecimal digits.
    /// </returns>
    public static string ToHexString(this byte[] bytes)
    {
        var c = new char[bytes.Length * 2];
        for (int i = 0, j = 0; i < bytes.Length; i++)
        {
            c[j++] = HexDigits[bytes[i] >> 4];
            c[j++] = HexDigits[bytes[i] & 0x0F];
        }

        return new string(c);
    }

    /// <summary>
    /// Parse a string of hexadecimal digits into a byte array.
    /// </summary>
    /// <param name="hexadecimalString">
    /// The hexadecimal string.
    /// </param>
    /// <returns>
    /// The parsed <see cref="byte[]"/> array.
    /// </returns>
    /// <exception cref="ArgumentException">
    /// The input string either contained invalid characters, or was of an odd length.
    /// </exception>
    public static byte[] ToByteArray(string hexadecimalString)
    {
        if (!TryParse(hexadecimalString, out var value))
        {
            throw new ArgumentException("Invalid hexadecimal string", nameof(hexadecimalString));
        }

        return value;
    }

    /// <summary>
    /// Parse a hexadecimal string to bytes
    /// </summary>
    /// <param name="hexadecimalString">
    /// The hexadecimal string, which must be an even number of characters.
    /// </param>
    /// <param name="value">
    /// The parsed value if successful.
    /// </param>
    /// <returns>
    /// True if successful.
    /// </returns>
    public static bool TryParse(string hexadecimalString, out byte[] value)
    {
        if (hexadecimalString.Length == 0)
        {
            value = Empty;
            return true;
        }

        if (hexadecimalString.Length % 2 != 0)
        {
            value = Empty;
            return false;
        }

        try
        {

            value = new byte[hexadecimalString.Length / 2];
            for (int i = 0, j = 0; j < hexadecimalString.Length; i++)
            {
                value[i] = (byte)((HexValues[hexadecimalString[j++] - '0'] << 4)
                                  | HexValues[hexadecimalString[j++] - '0']);
            }

            return true;
        }
        catch (OverflowException)
        {
            value = Empty;
            return false;
        }
    }
}

用@CodesInChaus补充答案（反向方法）

public static byte[] HexToByteUsingByteManipulation(string s)
{
    byte[] bytes = new byte[s.Length / 2];
    for (int i = 0; i < bytes.Length; i++)
    {
        int hi = s[i*2] - 65;
        hi = hi + 10 + ((hi >> 31) & 7);

        int lo = s[i*2 + 1] - 65;
        lo = lo + 10 + ((lo >> 31) & 7) & 0x0f;

        bytes[i] = (byte) (lo | hi << 4);
    }
    return bytes;
}

说明：

&0x0f还支持小写字母

hi=hi+10+（（hi>>31）&7）；与以下内容相同：

hi=ch-65+10+（（ch-65）>>31）&7）；

对于“0”9’与hi＝ch-65+10+7相同；其为hi＝ch-48（这是因为0xffffff&7）。

对于“A”F’为hi=ch-65+10；（这是因为0x00000000&7）。

对于“a”我们必须使用大数字，所以我们必须通过使用&0x0f使某些位为0，从默认版本中减去32。

65是“A”的代码

48是“0”的代码

7是ASCII表中“9”和“A”之间的字母数（…456789:；<=>？@ABCD…）。

对于插入SQL字符串（如果不使用命令参数）：

public static String ByteArrayToSQLHexString(byte[] Source)
{
    return = "0x" + BitConverter.ToString(Source).Replace("-", "");
}

如果您希望比BitConverter更灵活，但不希望使用那些笨重的90年代风格的显式循环，那么您可以这样做：

String.Join(String.Empty, Array.ConvertAll(bytes, x => x.ToString("X2")));

或者，如果您使用的是.NET 4.0：

String.Concat(Array.ConvertAll(bytes, x => x.ToString("X2")));

（后者来自对原帖子的评论。）

从.NET 5 RC2开始，您可以使用：

Convert.ToHexString（byte[]inArray），返回字符串和Convert.FromHexString（字符串s），返回字节[]。

可以使用跨度参数的重载。