[C#]MemoryStream.Dispose之后,为什么仍可以ToArray()?

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[C#]MemoryStream.Dispose之后,为什么仍可以ToArray()?

shy丶gril 2016-05-24 12:18:43 浏览2746
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目录

概述

MemoryStream分析

总结

概述

事件起因,一哥们在群里面贴出了类似下面这样的一段代码:

复制代码
 1     class Program
 2     {
 3         static void Main(string[] args)
 4         {
 5             byte[] buffer = File.ReadAllBytes("test.txt");     
 6             MemoryStream ms = new MemoryStream(buffer);
 7             ms.Dispose();
 8             Console.WriteLine(ms.ToArray().Length);
 9             Console.Read();
10         }
11     }
复制代码

先不去考究这段代码到底有没有什么意义,就代码而言,内存流释放之后,再去使用ms会有问题么?

运行结果:


在印象中非托管资源Dispose之后,应该会出现“无法访问已释放的资源”之类的异常吧,但是你真正的运行的时候,你会发现并没有错。真的怪了,没办法,出于好奇也就研究了一下。

那我们如果访问ms对象的其他的属性(ms.Length)会怎么样呢?

访问其它的方法它也会出现上面的异常。

这问题出来了,难道内存流的Dispose方法是选择性的释放?

在看MemoryStream分析之前,回顾一下托管与非托管资源的概念。

托管资源

一般是指被CLR控制的内存资源,这些资源的管理可以由CLR来控制,例如程序中分配(new)的对象,作用域内的变量等。

非托管资源

是CLR不能控制或者管理的部分,这些资源有很多,比如文件流,数据库的连接,系统的窗口句柄(Window内核对象(句柄))、字体、刷子、dc打印机资源等等……这些资源一般情况下不存在于Heap(内存中用于存储对象实例的地方)中。

C#的垃圾回收器:
    CLR为程序员提供的内存管理机制,使得程序员在编写代码时不需要显式的去释放自己使用的内存资源(这些在先前C和C++中是需要程序员自己去显式的释放的)。这种管理机制称为GC(garbage collection)。GC的作用是很明显的,当系统内存资源匮乏时,它就会被激发,然后自动的去释放那些没有被使用的托管资源(也就是程序员没有显式释放的对象)。对于那些非托管资源虽然垃圾回收器可以跟踪封装非托管资源的对象的生存期,但它不了解具体如何清理这些资源。还好.net提供了Finalize()方法,它允许在垃圾回收器回收该类资源时,适当的清理非托管资源。但是Finalize()会产生很多副作用。

释放非托管资源 

资源的释放一般是通过"垃圾回收器"自动完成的,但具体来说,仍有些需要注意的地方:
1、值类型和引用类型的引用其实是不需要什么"垃圾回收器"来释放内存的,因为当它们出了作用域后会自动释放所占内存,因为它们都保存在栈(Stack)中;
2、只有引用类型的引用所指向的对象实例才保存在堆(Heap)中,而堆因为是一个自由存储空间,所以它并没有像"栈"那样有生存期("栈"的元素弹出后就代表生存期结束,也就代表释放了内存),并且要注意的是,"垃圾回收器"只对这块区域起作用。
    然而,有些情况下,当需要释放非托管资源时,就必须通过写代码的方式来解决。

非托管资源的释放

当我们在类中封装了对非托管资源的操作时,我们就需要显式释放(Dispose),或隐式释放(Finalize)的释放这些资源。
Finalize一般情况下用于基类不带close方法或者不带Dispose显式方法的类,也就是说,在Finalize过程中我们需要隐式的去实现非托管资源的释放,然后系统会在Finalize过程完成后,自己的去释放托管资源。如果要实现Dispose方法,可以通过实现IDisposable接口,这样用户在使用这个类的同时就可以显示的执行Dispose方法,释放资源。

 (详细的内容可参考:http://blog.csdn.net/xiven/article/details/4951099

 

MemoryStream分析

对于这个问题,吃饭的时候一直很纠结,所以就查了一些这方面的资料,也试图反编译MemoryStream类,看看Dispose方法是如何实现。

 参考:

http://stackoverflow.com/questions/4274590/memorystream-close-or-memorystream-dispose

https://github.com/mono/mono/blob/137a5c40cfecff099f3b5e97c425663ed2e8505d/mcs/class/corlib/System.IO/MemoryStream.cs#L220

github上MemoryStream中的代码:

复制代码
  1 //
  2 // System.IO.MemoryStream.cs
  3 //
  4 // Authors:    Marcin Szczepanski (marcins@zipworld.com.au)
  5 //        Patrik Torstensson
  6 //        Gonzalo Paniagua Javier (gonzalo@ximian.com)
  7 //        Marek Safar (marek.safar@gmail.com)
  8 //
  9 // (c) 2001,2002 Marcin Szczepanski, Patrik Torstensson
 10 // (c) 2003 Ximian, Inc. (http://www.ximian.com)
 11 // Copyright (C) 2004 Novell, Inc (http://www.novell.com)
 12 // Copyright 2011 Xamarin, Inc (http://www.xamarin.com)
 13 //
 14 // Permission is hereby granted, free of charge, to any person obtaining
 15 // a copy of this software and associated documentation files (the
 16 // "Software"), to deal in the Software without restriction, including
 17 // without limitation the rights to use, copy, modify, merge, publish,
 18 // distribute, sublicense, and/or sell copies of the Software, and to
 19 // permit persons to whom the Software is furnished to do so, subject to
 20 // the following conditions:
 21 // 
 22 // The above copyright notice and this permission notice shall be
 23 // included in all copies or substantial portions of the Software.
 24 // 
 25 // THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
 26 // EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
 27 // MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
 28 // NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
 29 // LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
 30 // OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
 31 // WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
 32 //
 33 
 34 using System.Globalization;
 35 using System.Runtime.InteropServices;
 36 using System.Threading;
 37 #if NET_4_5
 38 using System.Threading.Tasks;
 39 #endif
 40 
 41 namespace System.IO
 42 {
 43     [Serializable]
 44     [ComVisible (true)]
 45     [MonoLimitation ("Serialization format not compatible with .NET")]
 46     public class MemoryStream : Stream
 47     {
 48         bool canWrite;
 49         bool allowGetBuffer;
 50         int capacity;
 51         int length;
 52         byte [] internalBuffer;
 53         int initialIndex;
 54         bool expandable;
 55         bool streamClosed;
 56         int position;
 57         int dirty_bytes;
 58 #if NET_4_5
 59         [NonSerialized]
 60         Task<int> read_task;
 61 #endif
 62 
 63         public MemoryStream () : this (0)
 64         {
 65         }
 66 
 67         public MemoryStream (int capacity)
 68         {
 69             if (capacity < 0)
 70                 throw new ArgumentOutOfRangeException ("capacity");
 71 
 72             canWrite = true;
 73 
 74             this.capacity = capacity;
 75             internalBuffer = new byte [capacity];
 76 
 77             expandable = true;
 78             allowGetBuffer = true;
 79         }
 80 
 81         public MemoryStream (byte [] buffer)
 82         {
 83             if (buffer == null)
 84                 throw new ArgumentNullException ("buffer");
 85             
 86             InternalConstructor (buffer, 0, buffer.Length, true, false);                        
 87         }
 88 
 89         public MemoryStream (byte [] buffer, bool writable)
 90         {
 91             if (buffer == null)
 92                 throw new ArgumentNullException ("buffer");
 93             
 94             InternalConstructor (buffer, 0, buffer.Length, writable, false);
 95         }
 96 
 97         public MemoryStream (byte [] buffer, int index, int count)
 98         {
 99             InternalConstructor (buffer, index, count, true, false);
100         }
101 
102         public MemoryStream (byte [] buffer, int index, int count, bool writable)
103         {
104             InternalConstructor (buffer, index, count, writable, false);
105         }
106 
107         public MemoryStream (byte [] buffer, int index, int count, bool writable, bool publiclyVisible)
108         {
109             InternalConstructor (buffer, index, count, writable, publiclyVisible);
110         }
111 
112         void InternalConstructor (byte [] buffer, int index, int count, bool writable, bool publicallyVisible)
113         {
114             if (buffer == null)
115                 throw new ArgumentNullException ("buffer");
116 
117             if (index < 0 || count < 0)
118                 throw new ArgumentOutOfRangeException ("index or count is less than 0.");
119 
120             if (buffer.Length - index < count)
121                 throw new ArgumentException ("index+count", 
122                                  "The size of the buffer is less than index + count.");
123 
124             canWrite = writable;
125 
126             internalBuffer = buffer;
127             capacity = count + index;
128             length = capacity;
129             position = index;
130             initialIndex = index;
131 
132             allowGetBuffer = publicallyVisible;
133             expandable = false;                
134         }
135 
136         void CheckIfClosedThrowDisposed ()
137         {
138             if (streamClosed)
139                 throw new ObjectDisposedException ("MemoryStream");
140         }
141         
142         public override bool CanRead {
143             get { return !streamClosed; }
144         }
145 
146         public override bool CanSeek {
147             get { return !streamClosed; }
148         }
149 
150         public override bool CanWrite {
151             get { return (!streamClosed && canWrite); }
152         }
153 
154         public virtual int Capacity {
155             get {
156                 CheckIfClosedThrowDisposed ();
157                 return capacity - initialIndex;
158             }
159 
160             set {
161                 CheckIfClosedThrowDisposed ();
162 
163                 if (!expandable)
164                     throw new NotSupportedException ("Cannot expand this MemoryStream");
165 
166                 if (value < 0 || value < length)
167                     throw new ArgumentOutOfRangeException ("value",
168                     "New capacity cannot be negative or less than the current capacity " + value + " " + capacity);
169 
170                 if (internalBuffer != null && value == internalBuffer.Length)
171                     return;
172 
173                 byte [] newBuffer = null;
174                 if (value != 0) {
175                     newBuffer = new byte [value];
176                     if (internalBuffer != null)
177                         Buffer.BlockCopy (internalBuffer, 0, newBuffer, 0, length);
178                 }
179 
180                 dirty_bytes = 0; // discard any dirty area beyond previous length
181                 internalBuffer = newBuffer; // It's null when capacity is set to 0
182                 capacity = value;
183             }
184         }
185 
186         public override long Length {
187             get {
188                 // LAMESPEC: The spec says to throw an IOException if the
189                 // stream is closed and an ObjectDisposedException if
190                 // "methods were called after the stream was closed".  What
191                 // is the difference?
192 
193                 CheckIfClosedThrowDisposed ();
194 
195                 // This is ok for MemoryStreamTest.ConstructorFive
196                 return length - initialIndex;
197             }
198         }
199 
200         public override long Position {
201             get {
202                 CheckIfClosedThrowDisposed ();
203                 return position - initialIndex;
204             }
205 
206             set {
207                 CheckIfClosedThrowDisposed ();
208                 if (value < 0)
209                     throw new ArgumentOutOfRangeException ("value",
210                                 "Position cannot be negative" );
211 
212                 if (value > Int32.MaxValue)
213                     throw new ArgumentOutOfRangeException ("value",
214                     "Position must be non-negative and less than 2^31 - 1 - origin");
215 
216                 position = initialIndex + (int) value;
217             }
218         }
219 
220         protected override void Dispose (bool disposing)
221         {
222             streamClosed = true;
223             expandable = false;
224         }
225 
226         public override void Flush ()
227         {
228             // Do nothing
229         }
230 
231         public virtual byte [] GetBuffer ()
232         {
233             if (!allowGetBuffer)
234                 throw new UnauthorizedAccessException ();
235 
236             return internalBuffer;
237         }
238 
239         public override int Read ([In,Out] byte [] buffer, int offset, int count)
240         {
241             if (buffer == null)
242                 throw new ArgumentNullException ("buffer");
243 
244             if (offset < 0 || count < 0)
245                 throw new ArgumentOutOfRangeException ("offset or count less than zero.");
246 
247             if (buffer.Length - offset < count )
248                 throw new ArgumentException ("offset+count",
249                                   "The size of the buffer is less than offset + count.");
250 
251             CheckIfClosedThrowDisposed ();
252 
253             if (position >= length || count == 0)
254                 return 0;
255 
256             if (position > length - count)
257                 count = length - position;
258 
259             Buffer.BlockCopy (internalBuffer, position, buffer, offset, count);
260             position += count;
261             return count;
262         }
263 
264         public override int ReadByte ()
265         {
266             CheckIfClosedThrowDisposed ();
267             if (position >= length)
268                 return -1;
269 
270             return internalBuffer [position++];
271         }
272 
273         public override long Seek (long offset, SeekOrigin loc)
274         {
275             CheckIfClosedThrowDisposed ();
276 
277             // It's funny that they don't throw this exception for < Int32.MinValue
278             if (offset > (long) Int32.MaxValue)
279                 throw new ArgumentOutOfRangeException ("Offset out of range. " + offset);
280 
281             int refPoint;
282             switch (loc) {
283             case SeekOrigin.Begin:
284                 if (offset < 0)
285                     throw new IOException ("Attempted to seek before start of MemoryStream.");
286                 refPoint = initialIndex;
287                 break;
288             case SeekOrigin.Current:
289                 refPoint = position;
290                 break;
291             case SeekOrigin.End:
292                 refPoint = length;
293                 break;
294             default:
295                 throw new ArgumentException ("loc", "Invalid SeekOrigin");
296             }
297 
298             // LAMESPEC: My goodness, how may LAMESPECs are there in this
299             // class! :)  In the spec for the Position property it's stated
300             // "The position must not be more than one byte beyond the end of the stream."
301             // In the spec for seek it says "Seeking to any location beyond the length of the 
302             // stream is supported."  That's a contradiction i'd say.
303             // I guess seek can go anywhere but if you use position it may get moved back.
304 
305             refPoint += (int) offset;
306             if (refPoint < initialIndex)
307                 throw new IOException ("Attempted to seek before start of MemoryStream.");
308 
309             position = refPoint;
310             return position;
311         }
312 
313         int CalculateNewCapacity (int minimum)
314         {
315             if (minimum < 256)
316                 minimum = 256; // See GetBufferTwo test
317 
318             if (minimum < capacity * 2)
319                 minimum = capacity * 2;
320 
321             return minimum;
322         }
323 
324         void Expand (int newSize)
325         {
326             // We don't need to take into account the dirty bytes when incrementing the
327             // Capacity, as changing it will only preserve the valid clear region.
328             if (newSize > capacity)
329                 Capacity = CalculateNewCapacity (newSize);
330             else if (dirty_bytes > 0) {
331                 Array.Clear (internalBuffer, length, dirty_bytes);
332                 dirty_bytes = 0;
333             }
334         }
335 
336         public override void SetLength (long value)
337         {
338             if (!expandable && value > capacity)
339                 throw new NotSupportedException ("Expanding this MemoryStream is not supported");
340 
341             CheckIfClosedThrowDisposed ();
342 
343             if (!canWrite) {
344                 throw new NotSupportedException (Locale.GetText 
345                     ("Cannot write to this MemoryStream"));
346             }
347 
348             // LAMESPEC: AGAIN! It says to throw this exception if value is
349             // greater than "the maximum length of the MemoryStream".  I haven't
350             // seen anywhere mention what the maximum length of a MemoryStream is and
351             // since we're this far this memory stream is expandable.
352             if (value < 0 || (value + initialIndex) > (long) Int32.MaxValue)
353                 throw new ArgumentOutOfRangeException ();
354 
355             int newSize = (int) value + initialIndex;
356 
357             if (newSize > length)
358                 Expand (newSize);
359             else if (newSize < length) // Postpone the call to Array.Clear till expand time
360                 dirty_bytes += length - newSize;
361 
362             length = newSize;
363             if (position > length)
364                 position = length;
365         }
366 
367         public virtual byte [] ToArray ()
368         {
369             int l = length - initialIndex;
370             byte[] outBuffer = new byte [l];
371 
372             if (internalBuffer != null)
373                 Buffer.BlockCopy (internalBuffer, initialIndex, outBuffer, 0, l);
374             return outBuffer; 
375         }
376 
377         public override void Write (byte [] buffer, int offset, int count)
378         {
379             if (buffer == null)
380                 throw new ArgumentNullException ("buffer");
381             
382             if (offset < 0 || count < 0)
383                 throw new ArgumentOutOfRangeException ();
384 
385             if (buffer.Length - offset < count)
386                 throw new ArgumentException ("offset+count",
387                                  "The size of the buffer is less than offset + count.");
388 
389             CheckIfClosedThrowDisposed ();
390 
391             if (!CanWrite)
392                 throw new NotSupportedException ("Cannot write to this stream.");
393 
394             // reordered to avoid possible integer overflow
395             if (position > length - count)
396                 Expand (position + count);
397 
398             Buffer.BlockCopy (buffer, offset, internalBuffer, position, count);
399             position += count;
400             if (position >= length)
401                 length = position;
402         }
403 
404         public override void WriteByte (byte value)
405         {
406             CheckIfClosedThrowDisposed ();
407             if (!canWrite)
408                 throw new NotSupportedException ("Cannot write to this stream.");
409 
410             if (position >= length) {
411                 Expand (position + 1);
412                 length = position + 1;
413             }
414 
415             internalBuffer [position++] = value;
416         }
417 
418         public virtual void WriteTo (Stream stream)
419         {
420             CheckIfClosedThrowDisposed ();
421 
422             if (stream == null)
423                 throw new ArgumentNullException ("stream");
424 
425             stream.Write (internalBuffer, initialIndex, length - initialIndex);
426         }
427 
428 #if NET_4_5
429 
430         public override Task CopyToAsync (Stream destination, int bufferSize, CancellationToken cancellationToken)
431         {
432             // TODO: Specialization but what for?
433             return base.CopyToAsync (destination, bufferSize, cancellationToken);
434         }
435 
436         public override Task FlushAsync (CancellationToken cancellationToken)
437         {
438             if (cancellationToken.IsCancellationRequested)
439                 return TaskConstants.Canceled;
440 
441             try {
442                 Flush ();
443                 return TaskConstants.Finished;
444             } catch (Exception ex) {
445                 return Task<object>.FromException (ex);
446             }
447         }
448 
449         public override Task<int> ReadAsync (byte[] buffer, int offset, int count, CancellationToken cancellationToken)
450         {
451             if (buffer == null)
452                 throw new ArgumentNullException ("buffer");
453 
454             if (offset < 0 || count < 0)
455                 throw new ArgumentOutOfRangeException ("offset or count less than zero.");
456 
457             if (buffer.Length - offset < count )
458                 throw new ArgumentException ("offset+count",
459                                              "The size of the buffer is less than offset + count.");
460             if (cancellationToken.IsCancellationRequested)
461                 return TaskConstants<int>.Canceled;
462 
463             try {
464                 count = Read (buffer, offset, count);
465 
466                 // Try not to allocate a new task for every buffer read
467                 if (read_task == null || read_task.Result != count)
468                     read_task = Task<int>.FromResult (count);
469 
470                 return read_task;
471             } catch (Exception ex) {
472                 return Task<int>.FromException (ex);
473             }
474         }
475 
476         public override Task WriteAsync (byte[] buffer, int offset, int count, CancellationToken cancellationToken)
477         {
478             if (buffer == null)
479                 throw new ArgumentNullException ("buffer");
480             
481             if (offset < 0 || count < 0)
482                 throw new ArgumentOutOfRangeException ();
483 
484             if (buffer.Length - offset < count)
485                 throw new ArgumentException ("offset+count",
486                                              "The size of the buffer is less than offset + count.");
487 
488             if (cancellationToken.IsCancellationRequested)
489                 return TaskConstants.Canceled;
490 
491             try {
492                 Write (buffer, offset, count);
493                 return TaskConstants.Finished;
494             } catch (Exception ex) {
495                 return Task<object>.FromException (ex);
496             }
497         }
498 #endif
499     }               
500 }
复制代码

通过上面的代码,你可以看出,很多方法和属性中都会有这样的一个方法的调用:

CheckIfClosedThrowDisposed ();

该方法的实现

1 void CheckIfClosedThrowDisposed ()
2     {
3                       if (streamClosed)
4         throw new ObjectDisposedException ("MemoryStream");
5     }

通过方法名可以清楚的知道该方法的作用:检查如果关闭了则抛出Dispose异常,通过方法体也可以清楚的知道该方法的作用。
你可以看一下ToArray方法

复制代码
1     public virtual byte [] ToArray ()
2         {
3             int l = length - initialIndex;
4             byte[] outBuffer = new byte [l];
5 
6             if (internalBuffer != null)
7                 Buffer.BlockCopy (internalBuffer, initialIndex, outBuffer, 0, l);
8             return outBuffer; 
9         }
复制代码

从代码中,也可以看出该方法并没有对流是否关闭进行检查。

总结

虽然上面的代码能说明为什么出现异常的原因,但是为什么要这样设计?为什么其他的方法会检测是否释放而ToArray方法反而放过呢?

功底有限,也只能到这地步,如果哪个园友有更深入的解释,可以给个合理的解释,为什么这样设计?

选择性的释放,那会不会就不安全了呢?

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转载:http://www.cnblogs.com/wolf-sun/p/3925477.html

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