SafeHandle Classe

Definição

public ref class SafeHandle abstract : IDisposable

public ref class SafeHandle abstract : System::Runtime::ConstrainedExecution::CriticalFinalizerObject, IDisposable

[System.Security.SecurityCritical]
public abstract class SafeHandle : IDisposable

public abstract class SafeHandle : System.Runtime.ConstrainedExecution.CriticalFinalizerObject, IDisposable

[System.Security.SecurityCritical]
public abstract class SafeHandle : System.Runtime.ConstrainedExecution.CriticalFinalizerObject, IDisposable

[<System.Security.SecurityCritical>]
type SafeHandle = class
    interface IDisposable

type SafeHandle = class
    inherit CriticalFinalizerObject
    interface IDisposable

[<System.Security.SecurityCritical>]
type SafeHandle = class
    inherit CriticalFinalizerObject
    interface IDisposable

Public MustInherit Class SafeHandle
Implements IDisposable

Public MustInherit Class SafeHandle
Inherits CriticalFinalizerObject
Implements IDisposable

Herança

Object

SafeHandle

Herança

Object

CriticalFinalizerObject

SafeHandle

Derivado

Microsoft.Win32.SafeHandles.SafeAccessTokenHandle

Microsoft.Win32.SafeHandles.SafeFileHandle

Microsoft.Win32.SafeHandles.SafeHandleMinusOneIsInvalid

Microsoft.Win32.SafeHandles.SafeHandleZeroOrMinusOneIsInvalid

Microsoft.Win32.SafeHandles.SafeWaitHandle

Mais…

Atributos

SecurityCriticalAttribute

Implementações

IDisposable

Exemplos

O seguinte exemplo de código cria um handle seguro personalizado para um handle de ficheiro do sistema operativo, derivando de SafeHandleZeroOrMinusOneIsInvalid. Lê bytes de um ficheiro e mostra os seus valores hexadecimais. Também contém um harness de teste de falhas que faz com que a thread aborte, mas o valor do handle é libertado. Ao usar um IntPtr para representar handles, o handle é ocasionalmente vazado devido ao aborto assíncrono da thread.

Vai precisar de um ficheiro de texto na mesma pasta da aplicação compilada. Assumindo que chama a aplicação "HexViewer", a utilização na linha de comandos é:

HexViewer <filename> -Fault

Opcionalmente, especifique -Fault tentar intencionalmente vazar a alavanca abortando o thread numa determinada janela. Use a ferramenta Windows Perfmon.exe para monitorizar as contagens de controlos enquanto injetam falhas.

using System;
using System.Runtime.InteropServices;
using System.IO;
using System.ComponentModel;
using System.Security;
using System.Threading;
using Microsoft.Win32.SafeHandles;
using System.Runtime.ConstrainedExecution;
using System.Security.Permissions;

namespace SafeHandleDemo
{
    internal class MySafeFileHandle : SafeHandleZeroOrMinusOneIsInvalid
    {
        // Create a SafeHandle, informing the base class
        // that this SafeHandle instance "owns" the handle,
        // and therefore SafeHandle should call
        // our ReleaseHandle method when the SafeHandle
        // is no longer in use.
        private MySafeFileHandle()
            : base(true)
        {
        }
        [ReliabilityContract(Consistency.WillNotCorruptState, Cer.MayFail)]
        override protected bool ReleaseHandle()
        {
            // Here, we must obey all rules for constrained execution regions.
            return NativeMethods.CloseHandle(handle);
            // If ReleaseHandle failed, it can be reported via the
            // "releaseHandleFailed" managed debugging assistant (MDA).  This
            // MDA is disabled by default, but can be enabled in a debugger
            // or during testing to diagnose handle corruption problems.
            // We do not throw an exception because most code could not recover
            // from the problem.
        }
    }

    [SuppressUnmanagedCodeSecurity()]
    internal static class NativeMethods
    {
        // Win32 constants for accessing files.
        internal const int GENERIC_READ = unchecked((int)0x80000000);

        // Allocate a file object in the kernel, then return a handle to it.
        [DllImport("kernel32", SetLastError = true, CharSet = CharSet.Unicode)]
        internal extern static MySafeFileHandle CreateFile(String fileName,
           int dwDesiredAccess, System.IO.FileShare dwShareMode,
           IntPtr securityAttrs_MustBeZero, System.IO.FileMode dwCreationDisposition,
           int dwFlagsAndAttributes, IntPtr hTemplateFile_MustBeZero);

        // Use the file handle.
        [DllImport("kernel32", SetLastError = true)]
        internal extern static int ReadFile(MySafeFileHandle handle, byte[] bytes,
           int numBytesToRead, out int numBytesRead, IntPtr overlapped_MustBeZero);

        // Free the kernel's file object (close the file).
        [DllImport("kernel32", SetLastError = true)]
        [ReliabilityContract(Consistency.WillNotCorruptState, Cer.MayFail)]
        internal extern static bool CloseHandle(IntPtr handle);
    }

    // The MyFileReader class is a sample class that accesses an operating system
    // resource and implements IDisposable. This is useful to show the types of
    // transformation required to make your resource wrapping classes
    // more resilient. Note the Dispose and Finalize implementations.
    // Consider this a simulation of System.IO.FileStream.
    public class MyFileReader : IDisposable
    {
        // _handle is set to null to indicate disposal of this instance.
        private MySafeFileHandle _handle;

        public MyFileReader(String fileName)
        {
            // Security permission check.
            String fullPath = Path.GetFullPath(fileName);
            new FileIOPermission(FileIOPermissionAccess.Read, fullPath).Demand();

            // Open a file, and save its handle in _handle.
            // Note that the most optimized code turns into two processor
            // instructions: 1) a call, and 2) moving the return value into
            // the _handle field.  With SafeHandle, the CLR's platform invoke
            // marshaling layer will store the handle into the SafeHandle
            // object in an atomic fashion. There is still the problem
            // that the SafeHandle object may not be stored in _handle, but
            // the real operating system handle value has been safely stored
            // in a critical finalizable object, ensuring against leaking
            // the handle even if there is an asynchronous exception.

            MySafeFileHandle tmpHandle;
            tmpHandle = NativeMethods.CreateFile(fileName, NativeMethods.GENERIC_READ,
                FileShare.Read, IntPtr.Zero, FileMode.Open, 0, IntPtr.Zero);

            // An async exception here will cause us to run our finalizer with
            // a null _handle, but MySafeFileHandle's ReleaseHandle code will
            // be invoked to free the handle.

            // This call to Sleep, run from the fault injection code in Main,
            // will help trigger a race. But it will not cause a handle leak
            // because the handle is already stored in a SafeHandle instance.
            // Critical finalization then guarantees that freeing the handle,
            // even during an unexpected AppDomain unload.
            Thread.Sleep(500);
            _handle = tmpHandle;  // Makes _handle point to a critical finalizable object.

            // Determine if file is opened successfully.
            if (_handle.IsInvalid)
                throw new Win32Exception(Marshal.GetLastWin32Error(), fileName);
        }

        public void Dispose()  // Follow the Dispose pattern - public nonvirtual.
        {
            Dispose(disposing: true);
            GC.SuppressFinalize(this);
        }

        // No finalizer is needed. The finalizer on SafeHandle
        // will clean up the MySafeFileHandle instance,
        // if it hasn't already been disposed.
        // However, there may be a need for a subclass to
        // introduce a finalizer, so Dispose is properly implemented here.
        protected virtual void Dispose(bool disposing)
        {
            // Note there are three interesting states here:
            // 1) CreateFile failed, _handle contains an invalid handle
            // 2) We called Dispose already, _handle is closed.
            // 3) _handle is null, due to an async exception before
            //    calling CreateFile. Note that the finalizer runs
            //    if the constructor fails.
            if (_handle != null && !_handle.IsInvalid)
            {
                // Free the handle
                _handle.Dispose();
            }
            // SafeHandle records the fact that we've called Dispose.
        }

        public byte[] ReadContents(int length)
        {
            if (_handle.IsInvalid)  // Is the handle disposed?
                throw new ObjectDisposedException("FileReader is closed");

            // This sample code will not work for all files.
            byte[] bytes = new byte[length];
            int numRead = 0;
            int r = NativeMethods.ReadFile(_handle, bytes, length, out numRead, IntPtr.Zero);
            // Since we removed MyFileReader's finalizer, we no longer need to
            // call GC.KeepAlive here.  Platform invoke will keep the SafeHandle
            // instance alive for the duration of the call.
            if (r == 0)
                throw new Win32Exception(Marshal.GetLastWin32Error());
            if (numRead < length)
            {
                byte[] newBytes = new byte[numRead];
                Array.Copy(bytes, newBytes, numRead);
                bytes = newBytes;
            }
            return bytes;
        }
    }

    static class Program
    {
        // Testing harness that injects faults.
        private static bool _printToConsole = false;
        private static bool _workerStarted = false;

        private static void Usage()
        {
            Console.WriteLine("Usage:");
            // Assumes that application is named HexViewer"
            Console.WriteLine("HexViewer <fileName> [-fault]");
            Console.WriteLine(" -fault Runs hex viewer repeatedly, injecting faults.");
        }

        private static void ViewInHex(Object fileName)
        {
            _workerStarted = true;
            byte[] bytes;
            using (MyFileReader reader = new MyFileReader((String)fileName))
            {
                bytes = reader.ReadContents(20);
            }  // Using block calls Dispose() for us here.

            if (_printToConsole)
            {
                // Print up to 20 bytes.
                int printNBytes = Math.Min(20, bytes.Length);
                Console.WriteLine("First {0} bytes of {1} in hex", printNBytes, fileName);
                for (int i = 0; i < printNBytes; i++)
                    Console.Write("{0:x} ", bytes[i]);
                Console.WriteLine();
            }
        }

        static void Main(string[] args)
        {
            if (args.Length == 0 || args.Length > 2 ||
                args[0] == "-?" || args[0] == "/?")
            {
                Usage();
                return;
            }

            String fileName = args[0];
            bool injectFaultMode = args.Length > 1;
            if (!injectFaultMode)
            {
                _printToConsole = true;
                ViewInHex(fileName);
            }
            else
            {
                Console.WriteLine("Injecting faults - watch handle count in perfmon (press Ctrl-C when done)");
                int numIterations = 0;
                while (true)
                {
                    _workerStarted = false;
                    Thread t = new Thread(new ParameterizedThreadStart(ViewInHex));
                    t.Start(fileName);
                    Thread.Sleep(1);
                    while (!_workerStarted)
                    {
                        Thread.Sleep(0);
                    }
                    t.Abort();  // Normal applications should not do this.
                    numIterations++;
                    if (numIterations % 10 == 0)
                        GC.Collect();
                    if (numIterations % 10000 == 0)
                        Console.WriteLine(numIterations);
                }
            }
        }
    }
}

Observações

Para mais informações sobre esta API, consulte Observações Suplementares da API para o SafeHandle.

Notas para Implementadores

Para criar uma classe derivada de SafeHandle, deve saber como criar e libertar um handle de sistema operativo. Este processo é diferente para diferentes tipos de handle porque alguns usam a função CloseHandle , enquanto outros usam funções mais específicas, como UnmapViewOfFile ou FindClose. Por esta razão, deve criar uma classe derivada de SafeHandle para cada tipo de handle do sistema operativo que pretende envolver num handle seguro.

Quando herdas de SafeHandle, deves sobrepor os seguintes membros: IsInvalid e ReleaseHandle().

Deve também fornecer um construtor público sem parâmetros que chame o construtor base com um valor que represente um valor de handle inválido, e um Boolean valor que indique se o handle nativo pertence ao SafeHandle e, consequentemente, deve ser libertado quando este SafeHandle for descartado.

Ver também

• Microsoft.Win32.SafeHandles
• CriticalHandle
• CriticalFinalizerObject