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：

 

?

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

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：

 

?

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442

// // 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。