.Net Framework 中的 Garbage Collection 会帮助程序员自动回收托管资源，这对类库的调用者而言，是个相当惬意的体验：可以在任何位置，任何时候，创建任何对象，GC 最后总是会兜底。

易地而处，当自己是类库提供者的时候，则需要如何才能提供这样良好的体验呢？

首先，.Net framework 里面哪些是托管的资源，哪些是非托管的资源？

基本上，在 .Net framework 里面的所有类，都是托管资源，包括各种各样的 stream(例如 FileStream, MemoryStream), database connection, components 等等。。

可以写一个简单的小程序验证：（以 FileStream 为例）

一个方法，在后台线程中监控文件是否正在被占用：

private static void MonitorFileStatus(string fileName)        {            Console.WriteLine("Start to monitor file: {0}", fileName);            Task.Factory.StartNew(() =>            {                while(true)                {                    bool isInUse = IsFileInUse(fileName);                    string messageFormat = isInUse ? "File {0} is in use." : "File {0} is released.";                    Console.WriteLine(messageFormat, fileName);                    Thread.Sleep(oneSeconds);                }            });        }        private static bool IsFileInUse(string fileName)        {            bool isInUse = true;            FileStream stream = null;            try            {                stream = File.Open(fileName, FileMode.Append, FileAccess.Write);                isInUse = false;            }            catch            {            }            finally            {                if (stream != null)                {                    stream.Dispose();                }            }            return isInUse;        }

再写一个占着文件不用的方法, FileStream 只是个局部变量，这个方法返回的时候，它应该被回收：

private static void OpenFile()        {            FileStream stream  = File.Open(TestFileName, FileMode.Append, FileAccess.Write);            Wait(fiveSeconds);        }

然后有一个收垃圾的 GC

private static void CallGC()        {            Console.WriteLine("Call GC.Collect...");            GC.Collect();        }

最后是一个必不可少的等待：

private static void Wait(TimeSpan time)        {            Console.WriteLine("Wait for {0} seconds...", time.TotalSeconds);            Thread.Sleep(time);        }

合并起来就是一个测试：

首先启动文件监视线程，然后打开文件不用。OpenFile 方法返回，预测 FileStream 被回收接着调用 GC， 看文件是否被释放了

private static void FileTest()        {            MonitorFileStatus(TestFileName);            OpenFile();            CallGC();            Wait(fiveSeconds);        }

运行结果，可见 GC 自动把 FileStream 自动回收。无须调用 Dispose 方法，也无须使用 using

那么，非托管资源包括哪些呢？

通常，涉及到 windows api 的 pinvoke，各种的 intptr 都是非托管资源。

例如，同样是打开文件，如果写成以下的样子，就包括了非托管资源

[Flags]    internal enum OpenFileStyle : uint    {        OF_CANCEL = 0x00000800,  // Ignored. For a dialog box with a Cancel button, use OF_PROMPT.        OF_CREATE = 0x00001000,  // Creates a new file. If file exists, it is truncated to zero (0) length.        OF_DELETE = 0x00000200,  // Deletes a file.        OF_EXIST = 0x00004000,  // Opens a file and then closes it. Used to test that a file exists        OF_PARSE = 0x00000100,  // Fills the OFSTRUCT structure, but does not do anything else.        OF_PROMPT = 0x00002000,  // Displays a dialog box if a requested file does not exist         OF_READ = 0x00000000,  // Opens a file for reading only.        OF_READWRITE = 0x00000002,  // Opens a file with read/write permissions.        OF_REOPEN = 0x00008000,  // Opens a file by using information in the reopen buffer.        // For MS-DOS–based file systems, opens a file with compatibility mode, allows any process on a         // specified computer to open the file any number of times.        // Other efforts to open a file with other sharing modes fail. This flag is mapped to the         // FILE_SHARE_READ|FILE_SHARE_WRITE flags of the CreateFile function.        OF_SHARE_COMPAT = 0x00000000,        // Opens a file without denying read or write access to other processes.        // On MS-DOS-based file systems, if the file has been opened in compatibility mode        // by any other process, the function fails.        // This flag is mapped to the FILE_SHARE_READ|FILE_SHARE_WRITE flags of the CreateFile function.        OF_SHARE_DENY_NONE = 0x00000040,        // Opens a file and denies read access to other processes.        // On MS-DOS-based file systems, if the file has been opened in compatibility mode,        // or for read access by any other process, the function fails.        // This flag is mapped to the FILE_SHARE_WRITE flag of the CreateFile function.        OF_SHARE_DENY_READ = 0x00000030,        // Opens a file and denies write access to other processes.        // On MS-DOS-based file systems, if a file has been opened in compatibility mode,        // or for write access by any other process, the function fails.        // This flag is mapped to the FILE_SHARE_READ flag of the CreateFile function.        OF_SHARE_DENY_WRITE = 0x00000020,        // Opens a file with exclusive mode, and denies both read/write access to other processes.        // If a file has been opened in any other mode for read/write access, even by the current process,        // the function fails.        OF_SHARE_EXCLUSIVE = 0x00000010,        // Verifies that the date and time of a file are the same as when it was opened previously.        // This is useful as an extra check for read-only files.        OF_VERIFY = 0x00000400,        // Opens a file for write access only.        OF_WRITE = 0x00000001    }    [StructLayout(LayoutKind.Sequential)]    internal struct OFSTRUCT    {        public byte cBytes;        public byte fFixedDisc;        public UInt16 nErrCode;        public UInt16 Reserved1;        public UInt16 Reserved2;        [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 128)]        public string szPathName;    }    class WindowsApi    {        [DllImport("kernel32.dll", BestFitMapping = false, ThrowOnUnmappableChar = true)]        internal static extern IntPtr OpenFile([MarshalAs(UnmanagedType.LPStr)]string lpFileName, out OFSTRUCT lpReOpenBuff, OpenFileStyle uStyle);        [DllImport("kernel32.dll", SetLastError = true)]        [ReliabilityContract(Consistency.WillNotCorruptState, Cer.Success)]        [SuppressUnmanagedCodeSecurity]        [return: MarshalAs(UnmanagedType.Bool)]        internal static extern bool CloseHandle(IntPtr hObject);    }

处理非托管资源，需要实现 IDisposable interface。原因有两个：

1. 不能依赖析构函数，因为异构函数的调用由 GC 决定。无法实时释放紧缺的资源。

2. 有一通用的处理原则：析构函数处理托管资源，IDisposable interface 处理托管与非托管资源。

如上述的例子，完成的实现代码如下：

public class UnmanagedFileHolder : IFileHolder, IDisposable    {        private IntPtr _handle;        private string _fileName;        public UnmanagedFileHolder(string fileName)        {            _fileName = fileName;        }        public void OpenFile()        {            Console.WriteLine("Open file with windows api.");            OFSTRUCT info;            _handle = WindowsApi.OpenFile(_fileName, out info, OpenFileStyle.OF_READWRITE);        }        #region IDisposable Support        private bool disposed = false;        protected virtual void Dispose(bool disposing)        {            if (!disposed)            {                if (disposing)                {                    // no managed resource                }                WindowsApi.CloseHandle(_handle);                _handle = IntPtr.Zero;                disposed = true;            }        }        ~UnmanagedFileHolder()        {            Dispose(false);        }        public void Dispose()        {            Dispose(true);            GC.SuppressFinalize(this);        }        #endregion    }

如果同一个类里面既有托管资源，也有非托管资源，那样应该怎么办呢？

可以依照下面的模式：

class HybridPattern : IDisposable    {        private bool _disposed = false;        ~HybridPattern()        {            Dispose(false);        }        protected void Dispose(bool disposing)        {            if (_disposed)            {                return;            }            if (disposing)            {                // Code to dispose the managed resources of the class                // internalComponent1.Dispose();            }            // Code to dispose the un-managed resources of the class            // CloseHandle(handle);            // handle = IntPtr.Zero;            _disposed = true;        }        public void Dispose()        {            Dispose(true);            GC.SuppressFinalize(this);        }    }

以下为完整的例子，有托管的 FileStream, 以及非托管的 Handler

public class HybridHolder : IFileHolder, IDisposable    {        private string _unmanagedFile;        private string _managedFile;        private IntPtr _handle;        private FileStream _stream;        public HybridHolder(string unmanagedFile, string managedFile)        {            _unmanagedFile = unmanagedFile;            _managedFile = managedFile;        }        public void OpenFile()        {            Console.WriteLine("Open file with windows api.");            OFSTRUCT info;            _handle = WindowsApi.OpenFile(_unmanagedFile, out info, OpenFileStyle.OF_READWRITE);            Console.WriteLine("Open file with .Net libray.");            _stream = File.Open(_managedFile, FileMode.Append, FileAccess.Write);        }        #region IDisposable Support        private bool disposed = false;        protected virtual void Dispose(bool disposing)        {            if (!disposed)            {                //Console.WriteLine("string is null? {0}", _stream == null);                if (disposing && _stream != null)                {                    Console.WriteLine("Clean up managed resource.");                    _stream.Dispose();                }                Console.WriteLine("Clean up unmanaged resource.");                WindowsApi.CloseHandle(_handle);                _handle = IntPtr.Zero;                disposed = true;            }        }        ~HybridHolder()        {            Dispose(false);        }        public void Dispose()        {            Dispose(true);            GC.SuppressFinalize(this);        }        #endregion    }

最后，如果是没有实现 IDisposable interface 的类呢？ 例如 byte[], StringBuilder

完全不要插手干预它们的回收， GC 做得很好。

尝试过在析构函数中把一个庞大的 byte[] 设置为 null，唯一的结果是导致它的回收被延迟到下一次 GC 周期。原因也很简单，每一次引用到会导致它的引用树上的计数加一。。

完整代码见 Github: