获取使用大端格式 C# 进行编码的指南

问题描述 投票：0回答：3

我有一个不寻常的情况，我有一个使用二进制（16）主键的现有 MySQL 数据库，这些是现有 api 中使用的 UUID 的基础。

我的问题是，我现在想添加一个用 dotnet core 编写的替换 api，并且我遇到了编码问题，已在 here

进行了解释

具体来说，dotnet 中的 Guid 结构使用混合字节序格式，生成与现有 api 不同的字符串。由于显而易见的原因，这是不可接受的。

所以我的问题是：有没有一种优雅的方法来强制 Guid 结构完全使用大端格式进行编码？

如果没有，我可以写一个糟糕的 hack，但我想我应该首先检查 SO 社区的集体智慧！

c# mysql .net-core guid asp.net-core-webapi
3个回答
3
投票

不；据我所知，没有内置的方法可以实现这一点。是的，

Guid
目前只能称之为“疯狂字节序”实现。您需要获取 
Guid
排序的位（通过 
unsafe
或 
Guid.ToByteArray
），然后手动排序它们，找出要反转的块 - 这不是一个简单的 
Array.Reverse()
。所以：恐怕非常手动。我建议使用类似的指南

00010203-0405-0607-0809-0a0b0c0d0e0f

调试它；这给了你（我怀疑你知道）：

03-02-01-00-05-04-07-06-08-09-0A-0B-0C-0D-0E-0F

所以：

  • 反转4
  • 反转2
  • 反转2
  • 直8
1
投票

截至 2021 年 2023 年，仍然没有内置方法可以在 C# 中将 System.Guid

 转换为 MySQL 兼容的大端字符串。

这是我们在工作中遇到这个确切的 C# 混合字节序 Guid 问题时提出的扩展：

原版

public static string ToStringBigEndian(this Guid guid) { // allocate enough bytes to store Guid ASCII string Span<byte> result = stackalloc byte[36]; // set all bytes to 0xFF (to be able to distinguish them from real data) result.Fill(0xFF); // get bytes from guid Span<byte> buffer = stackalloc byte[16]; _ = guid.TryWriteBytes(buffer); int skip = 0; // iterate over guid bytes for (int i = 0; i < buffer.Length; i++) { // indices 4, 6, 8 and 10 will contain a '-' delimiter character in the Guid string. // --> leave space for those delimiters if (i is 4 or 6 or 8 or 10) { skip++; } // stretch high and low bytes of every single byte into two bytes (skipping '-' delimiter characters) result[(2 * i) + skip] = (byte)(buffer[i] >> 0x4); result[(2 * i) + 1 + skip] = (byte)(buffer[i] & 0x0Fu); } // iterate over precomputed byte array. // values 0x0 to 0xF are final hex values, but must be mapped to ASCII characters. // value 0xFF is to be mapped to '-' delimiter character. for (int i = 0; i < result.Length; i++) { // map bytes to ASCII values (a-f will be lowercase) ref byte b = ref result[i]; b = b switch { 0xFF => 0x2D, // Map 0xFF to '-' character < 0xA => (byte)(b + 0x30u), // Map 0x0 - 0x9 to '0' - '9' _ => (byte)(b + 0x57u) // Map 0xA - 0xF to 'a' - 'f' }; } // get string from ASCII encoded guid byte array return Encoding.ASCII.GetString(result); } 
它有点长，但除了它返回的大端字符串之外，它不进行堆分配，因此保证速度很快:)


2023 年更新：更快的版本


更少的分支 => 更少的分支错误预测 => 更少的管道停顿 => 更快。


public static string ToStringBigEndian(this Guid guid)
{
    // allocate enough bytes to store Guid ASCII string
    Span<byte> result = stackalloc byte[36];
    // get bytes from guid
    Span<byte> buffer = stackalloc byte[16];
    _ = guid.TryWriteBytes(buffer);
    int skip = 0;
    // iterate over guid bytes
    for (int i = 0; i < buffer.Length; i++)
    {
        // indices 4, 6, 8 and 10 will contain a '-' delimiter character in the Guid string.
        // --> leave space for those delimiters
        // we can check if i is even and i / 2 is >= 2 and <= 5 to determine if we are at one of those indices
        // 0xF...F if i is odd and 0x0...0 if i is even
        int isOddMask = -(i & 1);
        // 0xF...F if i / 2 is < 2 and 0x0...0 if i / 2 is >= 2
        int less2Mask = ((i >> 1) - 2) >> 31;
        // 0xF...F if i / 2 is > 5 and 0x0...0 if i / 2 is <= 5
        int greater5Mask = ~(((i >> 1) - 6) >> 31);
        // 0xF...F if i is even and 2 <= i / 2 <= 5 otherwise 0x0...0
        int skipIndexMask = ~(isOddMask | less2Mask | greater5Mask);
        // skipIndexMask will be 0xFFFFFFFF for indices 4, 6, 8 and 10 and 0x00000000 for all other indices
        // --> skip those indices
        skip += 1 & skipIndexMask;
        result[(2 * i) + skip] = ToHexCharBranchless(buffer[i] >>> 0x4);
        result[(2 * i) + skip + 1] = ToHexCharBranchless(buffer[i] & 0x0F);
    }
    // add dashes
    const byte dash = (byte)'-';
    result[8] = result[13] = result[18] = result[23] = dash;
    // get string from ASCII encoded guid byte array
    return Encoding.ASCII.GetString(result);
}

[MethodImpl(MethodImplOptions.AggressiveInlining)]
private static byte ToHexCharBranchless(int b) =>
    // b + 0x30 for [0-9] if 0 <= b <= 9 and b + 0x30 + 0x27 for [a-f] if 10 <= b <= 15
    (byte)(b + 0x30 + (0x27 & ~((b - 0xA) >> 31)));

基准测试结果表明性能提高了约 30%：



    



      

      
      
      

      

        

          

            

              
0
投票

              
            

          

        

        
转发自 
为什么 Guid.ToByteArray() 以这种方式对字节进行排序？

using System;

namespace rm.Extensions;

/// <summary>
/// Guid extensions.
/// </summary>
public static class GuidExtension
{
    /// <summary>
    /// Returns a 16-element byte array that contains the value of this instance
    /// matching its string representation (endian-agnostic).
    /// <para></para>
    /// See https://stackoverflow.com/questions/9195551/why-does-guid-tobytearray-order-the-bytes-the-way-it-does
    /// <remarks>
    /// <para></para>
    /// Note: The byte[] returned by <see cref="ToByteArrayMatchingStringRepresentation(Guid)"/> will not yield
    /// the original Guid with <see cref="Guid(byte[])"/> ctor.
    /// </remarks>
    /// </summary>
    public static byte[] ToByteArrayMatchingStringRepresentation(this Guid guid)
    {
        var bytes = guid.ToByteArray();
        TweakOrderOfGuidBytesToMatchStringRepresentation(bytes);
        return bytes;
    }

    /// <summary>
    /// Initializes a new instance of the <see cref="Guid"></see> structure by using the specified array of bytes
    /// matching its string representation (endian-agnostic).
    /// <para></para>
    /// See https://stackoverflow.com/questions/9195551/why-does-guid-tobytearray-order-the-bytes-the-way-it-does
    /// <remarks>
    /// <para></para>
    /// Note: The Guid returned by <see cref="ToGuidMatchingStringRepresentation(byte[])"/> will not yield
    /// the original byte[] with <see cref="Guid.ToByteArray()"/>.
    /// </remarks>
    /// </summary>
    public static Guid ToGuidMatchingStringRepresentation(this byte[] bytes)
    {
        _ = bytes ??
            throw new ArgumentNullException(nameof(bytes));
        if (bytes.Length != 16)
        {
            throw new ArgumentException("Length should be 16.", nameof(bytes));
        }
        TweakOrderOfGuidBytesToMatchStringRepresentation(bytes);
        return new Guid(bytes);
    }

    private static void TweakOrderOfGuidBytesToMatchStringRepresentation(byte[] guidBytes)
    {
        if (BitConverter.IsLittleEndian)
        {
            Array.Reverse(guidBytes, 0, 4);
            Array.Reverse(guidBytes, 4, 2);
            Array.Reverse(guidBytes, 6, 2);
        }
    }
}

来源：
https://github.com/rmandvikar/csharp-extensions/blob/dev/src/rm.Extensions/GuidExtension.cs
    



      

      
    

  

  

    
最新问题


  





  

    © www.soinside.com 2019 - 2024. All rights reserved.