Base62 编码与 Base64 编码类似，都用于数据内容编码。Base64 和 Base62 的目的在都在于将需要传输的内容进行编码，尤其是一些特殊字符（如不可见字符、传输时与协议头冲突的字符）。

与 Base64 不同的是，Base62 编码是由10个数字、26个大写英文字母和26个小写英文字母组成，不包含 Base64 中的符号：+，/和=。这让经过 Base62 编码的数据很适合在 URL 中进行传输。微信的扫码登录规定了 state 只能使用 a-zA-Z0-9 的参数值，这种场景下使用 Base62 就很有效。

public static class Base62
{
    private
    const string DefaultCharacterSet = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz";
    private
    const string InvertedCharacterSet = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";
    /// <summary>
    /// 堆代码 duidaima.com
    /// Encode a byte array with Base62
    /// </summary>
    /// <param name="original">Byte array</param>
    /// <param name="inverted">Use inverted character set</param>
    /// <returns>Base62 string</returns>
    public static string ToBase62(byte[] original, bool inverted = false)
        {
            var characterSet = inverted ? InvertedCharacterSet : DefaultCharacterSet;
            var arr = Array.ConvertAll(original, t => (int) t);
            var converted = BaseConvert(arr, 256, 62);
            var builder = new StringBuilder();
            foreach(var t in converted)
            {
                builder.Append(characterSet[t]);
            }
            return builder.ToString();
        }
        /// <summary>
        /// Decode a base62-encoded string
        /// </summary>
        /// <param name="base62">Base62 string</param>
        /// <param name="inverted">Use inverted character set</param>
        /// <returns>Byte array</returns>
    public static byte[] FromBase62(string base62, bool inverted = false)
    {
        if(string.IsNullOrWhiteSpace(base62))
        {
            throw new ArgumentNullException(nameof(base62));
        }
        var characterSet = inverted ? InvertedCharacterSet : DefaultCharacterSet;
        var arr = Array.ConvertAll(base62.ToCharArray(), characterSet.IndexOf);
        var converted = BaseConvert(arr, 62, 256);
        return Array.ConvertAll(converted, Convert.ToByte);
    }
    private static int[] BaseConvert(int[] source, int sourceBase, int targetBase)
    {
        var result = new List < int > ();
        var leadingZeroCount = Math.Min(source.TakeWhile(x => x == 0).Count(), source.Length - 1);
        int count;
        while((count = source.Length) > 0)
        {
            var quotient = new List < int > ();
            var remainder = 0;
            for(var i = 0; i != count; i++)
            {
                var accumulator = source[i] + remainder * sourceBase;
                var digit = accumulator / targetBase;
                remainder = accumulator % targetBase;
                if(quotient.Count > 0 || digit > 0)
                {
                    quotient.Add(digit);
                }
            }
            result.Insert(0, remainder);
            source = quotient.ToArray();
        }
        result.InsertRange(0, Enumerable.Repeat(0, leadingZeroCount));
        return result.ToArray();
    }
}

将字节数组转换为 Base62 字符串

var guid = Guid.Parse("40943a11e5434d2f869d4e566e48c4f2");
Console.WriteLine(Base62.ToBase62(guid.ToByteArray()));

输出：

WVdhFoDbSqBBYSKzptnko

将 Base62 字符串转换为字节数组

var str = "WVdhFoDbSqBBYSKzptnko";
var bytes = Base62.FromBase62(str);
var guid = new Guid(bytes);
Console.WriteLine(guid.ToString("N"));

输出：

40943a11e5434d2f869d4e566e48c4f2