BitConverter.GetBytes实现

BitConverter.GetBytes 方法以什么顺序返回字节数组

前言

我在“浅谈 ZipInteger”一文中的 ZipInteger 结构中使用了 BitConverter 类的 GetBytes 方法。当时我是假设 GetBytes 方法根据 IsLittleEndian 的值不同而按照不同的顺序返回字节数组。但是 MSDN 有关 BitConverter 类的文档中没有对此作出明确的说明。请参见我在 MSDN 论坛的一个贴子“请问 BitConverter.GetBytes 方法以什么顺序返回字节数组”：

在 MSDN 文档的“BigInteger 构造函数 (Byte[])”中提到：

value 数组中的各个字节应该为 little-endian 顺序，从最低序位字节到最高序位字节。 
将数值转换为字节数组的大多数方法，例如 BigInteger.ToByteArray 和 BitConverter.GetBytes，以 little-endian 顺序返回字节数组。

按照我的理解，该 MSDN 文档认为 BitConverter.GetBytes 方法总是以 little-endian 顺序返回字节数组，而不管 BitConverter.IsLittleEndian 的值如何。
而在 BitConverter 类的文档中并没有明确指出 BitConverter.GetBytes 方法应该以什么顺序返回字节数组。
不知我的理解是否正确。

该贴子并没有得到靠谱的答复。

测试程序

那么，我们写个程序来测试一下吧。下面就是 BitConverterTester.cs：

 

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using  System;   namespace  Skyiv.Tester {    static  class  BitConverterTester    {      static  void  Main()      {        Console.WriteLine( "      OS Version: "  + Environment.OSVersion);        Console.WriteLine( "     CLR Version: "  + Environment.Version);        Console.WriteLine( "  IsLittleEndian: "  + BitConverter.IsLittleEndian);        long  n = 0x1234567890ABCDEF;        double  d = 1;        Console.WriteLine(n.ToString( "X" ) + ": "  + BitConverter.ToString(BitConverter.GetBytes(n)));        Console.WriteLine(d.ToString( "F14" ) + ": "  + BitConverter.ToString(BitConverter.GetBytes(d)));      }    } }

 

这个程序在 Windows Server 2003 操作系统的 .NET Framework 4 环境下编译和运行：

C:\CS\BitConverterTester> csc BitConverterTester.cs
Microsoft(R) Visual C# 2010 编译器 4.0.30319.1 版
版权所有(C) Microsoft Corporation。保留所有权利。

C:\CS\BitConverterTester> BitConverterTester
      OS Version: Microsoft Windows NT 5.2.3790 Service Pack 2
     CLR Version: 4.0.30319.1
  IsLittleEndian: True
1234567890ABCDEF: EF-CD-AB-90-78-56-34-12
1.00000000000000: 00-00-00-00-00-00-F0-3F

C:\CS\BitConverterTester>

 

在 Ubuntu 10.10 操作系统的 Mono 2.8.2 环境下编译和运行：

ben@ben-m4000t:~/work/BitConverterTester$ dmcs BitConverterTester.cs
ben@ben-m4000t:~/work/BitConverterTester$ mono28 BitConverterTester.exe
      OS Version: Unix 2.6.35.24
     CLR Version: 4.0.30319.1
  IsLittleEndian: True
1234567890ABCDEF: EF-CD-AB-90-78-56-34-12
1.00000000000000: 00-00-00-00-00-00-F0-3F
ben@ben-m4000t:~/work/BitConverterTester$ 

 

这两次运行的结果都在预料之中，BitConverter 类的 GetBytes 方法以 Little-Endian 顺序返回字节数组。但是，在这两次运行中，IsLittleEndian 的值都为 True，所以还是没有解决我们的问题。

查看 Microsoft .NET Framework 4 中相关的源程序代码

祭出 Reflector 这个神器：

在 mscorlib.dll 的 System 命名空间下找到 BitConverter 类:

如上图如示，IsLittleEndian 是 BitConverter 类的静态只读字段。

如上图所示，在 BitConverter 类的静态构造函数中，直接把 IsLittleEndian 这个静态只读字段的值赋值为 true。由于我没有 Microsoft 实现 BitConverter 类的 C# 源程序代码，不知道是 Microsoft 的 C# 源程序中就是直接这样写呢，还是实际上是有根据平台来判断的，但是 C# 编译器在具体平台上优化了这段代码。

如上图所示，GetBytes(Int64) 方法也非常简单，直接通过不安全的指针转换就得到了相应的字节数组。学过 C 语言中的朋友想必非常熟悉这种做法。这下清楚了，BitConverter 类的 GetBytes 方法是根据 IsLittleEndian 的值的不同按照不同顺序来返回字节数组的。MSDN 文档在“BigInteger 构造函数 (Byte[]) ”中的相关说法：“例如 BigInteger.ToByteArray 和 BitConverter.GetBytes，以 little-endian 顺序返回字节数组”是不正确的。

继续看下去：

上图中的 ToInt64 方法也明确地根据 IsLittleEndian 的值采取不同的动作。其实，既然在静态构造函数中明确地给 IsLittleEndian 赋值为 true，这里也可以省略对 IsLittleEndian 的判断，直接按照 IsLittleEndian 的值为 true 去做就行了，可以稍微节省点代码，提高点速度。

我们看到，GetBytes(Double) 方法也非常简单。

查看 Mono 2.8.2 中相关的源代码

我们可以到 http://ftp.novell.com/pub/mono/sources/mono/ 下载 Mono 的源代码，然后按以下方法找到 BitConverter 类的源程序代码：

ben@ben-m4000t:~$ cd src/mono-2.8.2
ben@ben-m4000t:~/src/mono-2.8.2$ find . -name BitConverter.cs
./mcs/class/corlib/System/BitConverter.cs
ben@ben-m4000t:~/src/mono-2.8.2$

下面就是 BitConverter.cs：

 

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// // System.BitConverter.cs // // Author: //   Matt Kimball (matt@kimball.net) // // // Copyright (C) 2004 Novell, Inc (http://www.novell.com) // // Permission is hereby granted, free of charge, to any person obtaining // a copy of this software and associated documentation files (the // "Software"), to deal in the Software without restriction, including // without limitation the rights to use, copy, modify, merge, publish, // distribute, sublicense, and/or sell copies of the Software, and to // permit persons to whom the Software is furnished to do so, subject to // the following conditions: // // The above copyright notice and this permission notice shall be // included in all copies or substantial portions of the Software. // // THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, // EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF // MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND // NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE // LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION // OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION // WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE. //   using  System.Text;   namespace  System {      public      static      class  BitConverter      {          static  readonly  bool  SwappedWordsInDouble = DoubleWordsAreSwapped ();          public  static  readonly  bool  IsLittleEndian = AmILittleEndian ();            static  unsafe  bool  AmILittleEndian ()          {              // binary representations of 1.0:              // big endian: 3f f0 00 00 00 00 00 00              // little endian: 00 00 00 00 00 00 f0 3f              // arm fpa little endian: 00 00 f0 3f 00 00 00 00              double  d = 1.0;              byte  *b = ( byte *)&d;              return  (b [0] == 0);          }            static  unsafe  bool  DoubleWordsAreSwapped ()          {              // binary representations of 1.0:              // big endian: 3f f0 00 00 00 00 00 00              // little endian: 00 00 00 00 00 00 f0 3f              // arm fpa little endian: 00 00 f0 3f 00 00 00 00              double  d = 1.0;              byte  *b = ( byte *)&d;              return  b [2] == 0xf0;          }            public  static  long  DoubleToInt64Bits ( double  value)          {              return  ToInt64 (GetBytes (value), 0);          }            public  static  double  Int64BitsToDouble ( long  value)          {              return  ToDouble (GetBytes (value), 0);          }            internal  static  double  InternalInt64BitsToDouble ( long  value)          {              return  SwappableToDouble (GetBytes (value), 0);          }                    unsafe  static  byte [] GetBytes ( byte  *ptr, int  count)          {              byte  [] ret = new  byte  [count];                for  ( int  i = 0; i < count; i++) {                  ret [i] = ptr [i];              }                return  ret;          }            unsafe  public  static  byte [] GetBytes ( bool  value)          {              return  GetBytes (( byte  *) &value, 1);          }            unsafe  public  static  byte [] GetBytes ( char  value)          {              return  GetBytes (( byte  *) &value, 2);          }            unsafe  public  static  byte [] GetBytes ( short  value)          {              return  GetBytes (( byte  *) &value, 2);          }            unsafe  public  static  byte [] GetBytes ( int  value)          {              return  GetBytes (( byte  *) &value, 4);          }            unsafe  public  static  byte [] GetBytes ( long  value)          {              return  GetBytes (( byte  *) &value, 8);          }            [CLSCompliant ( false )]          unsafe  public  static  byte [] GetBytes ( ushort  value)          {              return  GetBytes (( byte  *) &value, 2);          }            [CLSCompliant ( false )]          unsafe  public  static  byte [] GetBytes ( uint  value)          {              return  GetBytes (( byte  *) &value, 4);          }            [CLSCompliant ( false )]          unsafe  public  static  byte [] GetBytes ( ulong  value)          {              return  GetBytes (( byte  *) &value, 8);          }            unsafe  public  static  byte [] GetBytes ( float  value)          {              return  GetBytes (( byte  *) &value, 4);          }            unsafe  public  static  byte [] GetBytes ( double  value)          {              if  (SwappedWordsInDouble) {                  byte [] data = new  byte  [8];                  byte  *p = ( byte *)&value;                  data [0] = p [4];                  data [1] = p [5];                  data [2] = p [6];                  data [3] = p [7];                  data [4] = p [0];                  data [5] = p [1];                  data [6] = p [2];                  data [7] = p [3];                  return  data;              } else  {                  return  GetBytes (( byte  *) &value, 8);              }          }            unsafe  static  void  PutBytes ( byte  *dst, byte [] src, int  start_index, int  count)          {              if  (src == null )                  throw  new  ArgumentNullException ( "value" );                if  (start_index < 0 || (start_index > src.Length - 1))                  throw  new  ArgumentOutOfRangeException ( "startIndex" , "Index was"                      + " out of range. Must be non-negative and less than the"                      + " size of the collection." );                // avoid integer overflow (with large pos/neg start_index values)              if  (src.Length - count < start_index)                  throw  new  ArgumentException ( "Destination array is not long"                      + " enough to copy all the items in the collection."                      + " Check array index and length." );                for  ( int  i = 0; i < count; i++)                  dst[i] = src[i + start_index];          }            unsafe  public  static  bool  ToBoolean ( byte [] value, int  startIndex)          {              if  (value == null )                  throw  new  ArgumentNullException ( "value" );                if  (startIndex < 0 || (startIndex > value.Length - 1))                  throw  new  ArgumentOutOfRangeException ( "startIndex" , "Index was"                      + " out of range. Must be non-negative and less than the"                      + " size of the collection." );                if  (value [startIndex] != 0)                  return  true ;                            return  false ;          }            unsafe  public  static  char  ToChar ( byte [] value, int  startIndex)          {              char  ret;                PutBytes (( byte  *) &ret, value, startIndex, 2);                return  ret;          }            unsafe  public  static  short  ToInt16 ( byte [] value, int  startIndex)          {              short  ret;                PutBytes (( byte  *) &ret, value, startIndex, 2);                return  ret;          }            unsafe  public  static  int  ToInt32 ( byte [] value, int  startIndex)          {              int  ret;                PutBytes (( byte  *) &ret, value, startIndex, 4);                return  ret;          }            unsafe  public  static  long  ToInt64 ( byte [] value, int  startIndex)          {              long  ret;                PutBytes (( byte  *) &ret, value, startIndex, 8);                return  ret;          }            [CLSCompliant ( false )]          unsafe  public  static  ushort  ToUInt16 ( byte [] value, int  startIndex)          {              ushort  ret;                PutBytes (( byte  *) &ret, value, startIndex, 2);                return  ret;          }            [CLSCompliant ( false )]          unsafe  public  static  uint  ToUInt32 ( byte [] value, int  startIndex)          {              uint  ret;                PutBytes (( byte  *) &ret, value, startIndex, 4);                return  ret;          }            [CLSCompliant ( false )]          unsafe  public  static  ulong  ToUInt64 ( byte [] value, int  startIndex)          {              ulong  ret;                PutBytes (( byte  *) &ret, value, startIndex, 8);                return  ret;          }            unsafe  public  static  float  ToSingle ( byte [] value, int  startIndex)          {              float  ret;                PutBytes (( byte  *) &ret, value, startIndex, 4);                return  ret;          }            unsafe  public  static  double  ToDouble ( byte [] value, int  startIndex)          {              double  ret;                if  (SwappedWordsInDouble) {                  byte * p = ( byte *)&ret;                  if  (value == null )                      throw  new  ArgumentNullException ( "value" );                    if  (startIndex < 0 || (startIndex > value.Length - 1))                      throw  new  ArgumentOutOfRangeException ( "startIndex" , "Index was"                          + " out of range. Must be non-negative and less than the"                          + " size of the collection." );                    // avoid integer overflow (with large pos/neg start_index values)                  if  (value.Length - 8 < startIndex)                      throw  new  ArgumentException ( "Destination array is not long"                          + " enough to copy all the items in the collection."                          + " Check array index and length." );                    p [0] = value [startIndex + 4];                  p [1] = value [startIndex + 5];                  p [2] = value [startIndex + 6];                  p [3] = value [startIndex + 7];                  p [4] = value [startIndex + 0];                  p [5] = value [startIndex + 1];                  p [6] = value [startIndex + 2];                  p [7] = value [startIndex + 3];                    return  ret;              }                PutBytes (( byte  *) &ret, value, startIndex, 8);                return  ret;          }            unsafe  internal  static  double  SwappableToDouble ( byte [] value, int  startIndex)          {              double  ret;                if  (SwappedWordsInDouble) {                  byte * p = ( byte *)&ret;                  if  (value == null )                      throw  new  ArgumentNullException ( "value" );                    if  (startIndex < 0 || (startIndex > value.Length - 1))                      throw  new  ArgumentOutOfRangeException ( "startIndex" , "Index was"                          + " out of range. Must be non-negative and less than the"                          + " size of the collection." );                    // avoid integer overflow (with large pos/neg start_index values)                  if  (value.Length - 8 < startIndex)                      throw  new  ArgumentException ( "Destination array is not long"                          + " enough to copy all the items in the collection."                          + " Check array index and length." );                    p [0] = value [startIndex + 4];                  p [1] = value [startIndex + 5];                  p [2] = value [startIndex + 6];                  p [3] = value [startIndex + 7];                  p [4] = value [startIndex + 0];                  p [5] = value [startIndex + 1];                  p [6] = value [startIndex + 2];                  p [7] = value [startIndex + 3];                    return  ret;              } else  if  (!IsLittleEndian) {                  byte * p = ( byte *)&ret;                  if  (value == null )                      throw  new  ArgumentNullException ( "value" );                    if  (startIndex < 0 || (startIndex > value.Length - 1))                      throw  new  ArgumentOutOfRangeException ( "startIndex" , "Index was"                          + " out of range. Must be non-negative and less than the"                          + " size of the collection." );                    // avoid integer overflow (with large pos/neg start_index values)                  if  (value.Length - 8 < startIndex)                      throw  new  ArgumentException ( "Destination array is not long"                          + " enough to copy all the items in the collection."                          + " Check array index and length." );                    p [0] = value [startIndex + 7];                  p [1] = value [startIndex + 6];                  p [2] = value [startIndex + 5];                  p [3] = value [startIndex + 4];                  p [4] = value [startIndex + 3];                  p [5] = value [startIndex + 2];                  p [6] = value [startIndex + 1];                  p [7] = value [startIndex + 0];                    return  ret;              }                PutBytes (( byte  *) &ret, value, startIndex, 8);                return  ret;          }                    public  static  string  ToString ( byte [] value)          {              if  (value == null )                  throw  new  ArgumentNullException ( "value" );                return  ToString (value, 0, value.Length);          }            public  static  string  ToString ( byte [] value, int  startIndex)          {              if  (value == null )                  throw  new  ArgumentNullException ( "value" );                return  ToString (value, startIndex, value.Length - startIndex);          }            public  static  string  ToString ( byte [] value, int  startIndex, int  length)          {              if  (value == null )                  throw  new  ArgumentNullException ( "byteArray" );                // The 4th and last clause (start_index >= value.Length)              // was added as a small fix to a very obscure bug.              // It makes a small difference when start_index is              // outside the range and length==0.              if  (startIndex < 0 || startIndex >= value.Length) {                  // special (but valid) case (e.g. new byte [0])                  if  ((startIndex == 0) && (value.Length == 0))                      return  String.Empty;                  throw  new  ArgumentOutOfRangeException ( "startIndex" , "Index was"                      + " out of range. Must be non-negative and less than the"                      + " size of the collection." );              }                if  (length < 0)                  throw  new  ArgumentOutOfRangeException ( "length" ,                      "Value must be positive." );                // note: re-ordered to avoid possible integer overflow              if  (startIndex > value.Length - length)                  throw  new  ArgumentException ( "startIndex + length > value.Length" );                if  (length == 0)                  return  string .Empty;                StringBuilder builder = new  StringBuilder(length * 3 - 1);              int  end = startIndex + length;                for  ( int  i = startIndex; i < end; i++) {                  if  (i > startIndex)                      builder.Append( '-' );                                    char  high = ( char )((value[i] >> 4) & 0x0f);                  char  low = ( char )(value[i] & 0x0f);                    if  (high < 10)                      high += '0' ;                  else  {                      high -= ( char ) 10;                      high += 'A' ;                  }                    if  (low < 10)                      low += '0' ;                  else  {                      low -= ( char ) 10;                      low += 'A' ;                  }                  builder.Append(high);                  builder.Append(low);              }                return  builder.ToString ();          }      } }

 

 

可以看出，在第 39 行通过第 41 到 50 行的 AmILittleEndian 方法给 IsLittleEndian 这个静态只读字段赋值。这个 AmILittleEndian 方法中的注释写得很明白，其实有三种不台的平台，除了 Big-Endian 和 Little-Endian 外，还有一种“arm fpa little endian”，在这里也归于 Little-Endian。但是在后面的方法中是要以不同的手段进行处理的。

另外，不象 Microsoft .NET Framework 4，Mono 中 BitConverter 类的各种重载的公有静态GetBytes 方法是统一调用第 78 到 87 行的私有静态 GetBytes 方法来实现其功能的。

从 Mono 中相关的源程序代码中也可以看出，BitConverter 类的 GetBytes 方法是根据 IsLittleEndian 的值的不同以不同的顺序返回字节数组的。

结论

总结一下，MSDN 文档在“BigInteger 构造函数 (Byte[]) ”中的相关说法：“例如 BigInteger.ToByteArray 和 BitConverter.GetBytes，以 little-endian 顺序返回字节数组”是不正确的。BitConverter 类的 GetBytes 方法是根据 IsLittleEndian 的值的不同以不同的顺序返回字节数组的。

分类:  .NET Framework