SafeHandle Klasse

Definition

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

Vererbung

Object

SafeHandle

Vererbung

Object

CriticalFinalizerObject

SafeHandle

Abgeleitet

Microsoft.Win32.SafeHandles.SafeAccessTokenHandle

Microsoft.Win32.SafeHandles.SafeFileHandle

Microsoft.Win32.SafeHandles.SafeHandleMinusOneIsInvalid

Microsoft.Win32.SafeHandles.SafeHandleZeroOrMinusOneIsInvalid

Microsoft.Win32.SafeHandles.SafeMemoryMappedFileHandle

Mehr …

Attribute

SecurityCriticalAttribute

Implementiert

IDisposable

Beispiele

Im folgenden Codebeispiel wird ein benutzerdefiniertes sicheres Handle für ein Betriebssystemdateihandle erstellt, das von abgeleitet wird SafeHandleZeroOrMinusOneIsInvalid. Es liest Bytes aus einer Datei und zeigt deren Hexadezimalwerte an. Es enthält auch eine Fehlertest-Harness, die bewirkt, dass der Thread abgebrochen wird, aber der Handlewert freigegeben wird. Bei Verwendung eines IntPtr zur Darstellung von Handles wird das Handle gelegentlich aufgrund des asynchronen Threadabbruchs kompromittiert.

Sie benötigen eine Textdatei im gleichen Ordner wie die kompilierte Anwendung. Wenn Sie der Anwendung den Namen "HexViewer" geben, lautet die Befehlszeilenverwendung wie folgt:

HexViewer <filename> -Fault

Geben Sie optional an -Fault , um absichtlich zu versuchen, das Handle zu kompromittieren, indem Sie den Thread in einem bestimmten Fenster abbrechen. Verwenden Sie das Windows-Perfmon.exe-Tool, um die Anzahl der Handles beim Einschleusen von Fehlern zu überwachen.

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);
                }
            }
        }
    }
}

Hinweise

Weitere Informationen zu dieser API finden Sie unter Ergänzende API-Hinweise für SafeHandle.

Hinweise für Ausführende

Um eine von SafeHandleabgeleitete Klasse zu erstellen, müssen Sie wissen, wie Sie ein Betriebssystemhandle erstellen und freigeben. Dieser Prozess unterscheidet sich bei verschiedenen Handletypen, da einige die Funktion [CloseHandle](/windows/win32/api/handleapi/nf-handleapi-closehandle) verwenden, während andere spezifischere Funktionen wie [UnmapViewOfFile](/windows/win32/api/memoryapi/nf-memoryapi-unmapviewoffile) oder [FindClose](/windows/win32/api/fileapi/nf-fileapi-findclose) verwenden. Aus diesem Grund müssen Sie eine abgeleitete Klasse von für jeden Handletyp des SafeHandle Betriebssystems erstellen, den Sie in einem sicheren Handle umschließen möchten.

Wenn Sie von SafeHandle erben, müssen die folgenden Member überschrieben werden: IsInvalid und ReleaseHandle().

Sie sollten auch einen öffentlichen parameterlosen Konstruktor bereitstellen, der den Basiskonstruktor mit einem Wert aufruft, der einen ungültigen Handle-Wert darstellt, und einen Boolean Wert, der angibt, ob das systemeigene Handle im Besitz von SafeHandle ist und folglich freigegeben werden sollte, wenn dieses SafeHandle freigegeben wurde.

Konstruktoren

Felder

Eigenschaften

Methoden

Gilt für:

Weitere Informationen

• Microsoft.Win32.SafeHandles
• CriticalHandle
• CriticalFinalizerObject