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

原作者: [db:作者] 来自: [db:来源] 收藏 邀请

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:

 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     }

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:

  1 //
  2 // System.IO.MemoryStream.cs
  3 //
  4 // Authors:    Marcin Szczepanski ([email protected])
  5 //        Patrik Torstensson
  6 //        Gonzalo Paniagua Javier ([email protected])
  7 //        Marek Safar ([email protected])
  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 

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