SafeHandle 類別

定義

表示作業系統控制代碼的包裝函式類別 (Wrapper Class)。Represents a wrapper class for operating system handles. 這個類別必須被繼承。This class must be inherited.

public ref class SafeHandle abstract : System::Runtime::ConstrainedExecution::CriticalFinalizerObject, IDisposable
[System.Security.SecurityCritical]
public abstract class SafeHandle : System.Runtime.ConstrainedExecution.CriticalFinalizerObject, IDisposable
type SafeHandle = class
    inherit CriticalFinalizerObject
    interface IDisposable
Public MustInherit Class SafeHandle
Inherits CriticalFinalizerObject
Implements IDisposable
繼承
衍生
屬性
實作

範例

下列程式碼範例會建立作業系統檔案控制代碼的自訂安全控制碼,衍生自 SafeHandleZeroOrMinusOneIsInvalidThe following code example creates a custom safe handle for an operating system file handle, deriving from SafeHandleZeroOrMinusOneIsInvalid. 它會從檔案讀取位元組,並顯示其十六進位值。It reads bytes from a file and displays their hexadecimal values. 它也包含錯誤測試控管,會導致執行緒中止,但控制碼值會釋出。It also contains a fault testing harness that causes the thread to abort, but the handle value is freed. 當使用 IntPtr 來代表控制碼時,此控制碼偶爾會因為非同步執行緒中止而洩漏。When using an IntPtr to represent handles, the handle is occasionally leaked due to the asynchronous thread abort.

您在與編譯的應用程式相同的資料夾中,將會需要一個文字檔。You will need a text file in the same folder as the compiled application. 假設您將應用程式命名為 "HexViewer",則命令列使用方式為:Assuming that you name the application "HexViewer", the command line usage is:

HexViewer <filename> -Fault

選擇性地指定 -Fault,藉由中止特定視窗中的執行緒,故意嘗試流失控制碼。Optionally specify -Fault to intentionally attempt to leak the handle by aborting the thread in a certain window. 當插入錯誤時,請使用 Windows Perfmon 工具來監視控制碼計數。Use the Windows Perfmon.exe tool to monitor handle counts while injecting faults.

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

namespace SafeHandleDemo
{
    [SecurityPermission(SecurityAction.InheritanceDemand, UnmanagedCode = true)]
    [SecurityPermission(SecurityAction.Demand, UnmanagedCode = true)]
    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(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.
        // Howerver, there may be a need for a subclass to
        // introduce a finalizer, so Dispose is properly implemented here.
        [SecurityPermission(SecurityAction.Demand, UnmanagedCode = true)]
        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.
        }

        [SecurityPermission(SecurityAction.Demand, UnmanagedCode = true)]
        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 HexViwer"
            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);
                }
            }
        }
    }
}

備註

SafeHandle 類別提供控制碼資源的關鍵結束,以防止垃圾收集,並由 Windows 回收處理常式來參考非預期的非受控物件。The SafeHandle class provides critical finalization of handle resources, preventing handles from being reclaimed prematurely by garbage collection and from being recycled by Windows to reference unintended unmanaged objects.

本主題包含下列章節:This topic includes the following sections:

為什麼要選擇 SafeHandle? Why SafeHandle?
SafeHandle 的作用 What SafeHandle does
衍生自 SafeHandle 的類別Classes derived from SafeHandle

為什麼要選擇 SafeHandle?Why SafeHandle?

在 .NET Framework 版本2.0 之前,所有作業系統控制碼只能封裝在 IntPtr managed 包裝函式物件中。Before the .NET Framework version 2.0, all operating system handles could only be encapsulated in the IntPtr managed wrapper object. 雖然這是與機器碼互通的便利方法,但非同步例外狀況可能會流失控制碼,例如執行緒意外中止或堆疊溢位。While this was a convenient way to interoperate with native code, handles could be leaked by asynchronous exceptions, such as a thread aborting unexpectedly or a stack overflow. 這些非同步例外是清除作業系統資源的障礙,而且幾乎可以在應用程式中的任何位置進行。These asynchronous exceptions are an obstacle to cleaning up operating system resources, and they can occur almost anywhere in your app.

雖然 Object.Finalize 方法的覆寫允許在物件進行垃圾收集時清除非受控資源,但是在某些情況下,可在平台叫用呼叫中執行方法時,由垃圾收集來回收可終結的物件。Although overrides to the Object.Finalize method allow cleanup of unmanaged resources when an object is being garbage collected, in some circumstances, finalizable objects can be reclaimed by garbage collection while executing a method within a platform invoke call. 如果完成項釋放傳遞給該平台叫用呼叫的控制碼,可能會導致處理損毀。If a finalizer frees the handle passed to that platform invoke call, it could lead to handle corruption. 當您的方法在平台叫用呼叫期間遭到封鎖(例如讀取檔案)時,也可以回收控制碼。The handle could also be reclaimed while your method is blocked during a platform invoke call, such as while reading a file.

更嚴重的是,由於 Windows 會積極回收控制碼,因此可以回收控制碼,並指向可能包含敏感性資料的另一個資源。More critically, because Windows aggressively recycles handles, a handle could be recycled and point to another resource that might contain sensitive data. 這就是所謂的回收攻擊,可能會損毀資料,而且會造成安全性威脅。This is known as a recycle attack and can potentially corrupt data and be a security threat.

SafeHandle 的功能What SafeHandle does

SafeHandle 類別簡化了其中幾個物件存留期問題,並與平台叫用整合,讓作業系統資源不會流失。The SafeHandle class simplifies several of these object lifetime issues, and is integrated with platform invoke so that operating system resources are not leaked. SafeHandle 類別藉由指派和釋放控制碼而不中斷,來解析物件存留期的問題。The SafeHandle class resolves object lifetime issues by assigning and releasing handles without interruption. 它包含重要的完成項,可確保控制碼已關閉,並保證在非預期的 AppDomain 卸載期間執行,即使平台叫用呼叫被假設為損毀狀態也一樣。It contains a critical finalizer that ensures that the handle is closed and is guaranteed to run during unexpected AppDomain unloads, even in cases when the platform invoke call is assumed to be in a corrupted state.

因為 SafeHandle 繼承自 CriticalFinalizerObject,所以所有非關鍵性的完成項都會在任何重要的完成項之前呼叫。Because SafeHandle inherits from CriticalFinalizerObject, all the noncritical finalizers are called before any of the critical finalizers. 在相同的垃圾收集行程期間,針對不再存留的物件呼叫完成項。The finalizers are called on objects that are no longer live during the same garbage collection pass. 例如,FileStream 物件可以執行正常的完成項,以清除現有的緩衝資料,而不會有處理常式流失或回收的風險。For example, a FileStream object can run a normal finalizer to flush out existing buffered data without the risk of the handle being leaked or recycled. 在關鍵和非關鍵的完成項之間,這項非常弱的順序並不適用于一般用途。This very weak ordering between critical and noncritical finalizers is not intended for general use. 它主要是為了協助遷移現有的程式庫,藉由允許這些程式庫使用 SafeHandle 而不改變其語義。It exists primarily to assist in the migration of existing libraries by allowing those libraries to use SafeHandle without altering their semantics. 此外,關鍵完成項和它所呼叫的任何專案(例如 SafeHandle.ReleaseHandle() 方法)都必須在限制執列區域中。Additionally, the critical finalizer and anything it calls, such as the SafeHandle.ReleaseHandle() method, must be in a constrained execution region. 這會對可在完成項的呼叫圖形中寫入的程式碼施加條件約束。This imposes constraints on what code can be written within the finalizer's call graph.

平台叫用作業會自動遞增 SafeHandle 所封裝之控制碼的參考計數,並在完成時將其遞減。Platform invoke operations automatically increment the reference count of handles encapsulated by a SafeHandle and decrement them upon completion. 這可確保不會意外回收或關閉控制碼。This ensures that the handle will not be recycled or closed unexpectedly.

您可以在 SafeHandle 類別的函式中,為 ownsHandle 引數提供值,以指定基礎句 SafeHandle 柄的擁有權。You can specify ownership of the underlying handle when constructing SafeHandle objects by supplying a value to the ownsHandle argument in the SafeHandle class constructor. 這會控制 SafeHandle 物件是否會在處置物件之後釋放控制碼。This controls whether the SafeHandle object will release the handle after the object has been disposed. 這適用于具有什麼嗎存留期需求的控制碼,或使用其存留期由其他人控制的控制碼。This is useful for handles with peculiar lifetime requirements or for consuming a handle whose lifetime is controlled by someone else.

衍生自 SafeHandle 的類別Classes derived from SafeHandle

SafeHandle 是作業系統控制碼的抽象包裝函式類別。SafeHandle is an abstract wrapper class for operating system handles. 從這個類別衍生並不容易。Deriving from this class is difficult. 請改用 Microsoft.Win32.SafeHandles 命名空間中的衍生類別,這些類別可為下列各項提供安全的控制代碼:Instead, use the derived classes in the Microsoft.Win32.SafeHandles namespace that provide safe handles for the following:

給實施者的注意事項

若要建立衍生自 SafeHandle的類別,您必須知道如何建立和釋放作業系統控制碼。To create a class derived from SafeHandle, you must know how to create and free an operating system handle. 不同的控制碼類型會有不同的進程,因為有些會使用CloseHandle函式,而其他則會使用更特定的函式,例如UnmapViewOfFileFindCloseThis process is different for different handle types because some use the CloseHandle function, while others use more specific functions such as UnmapViewOfFile or FindClose. 基於這個理由,您必須針對要在安全控制碼中包裝的每個作業系統控制碼類型,建立 SafeHandle 的衍生類別。For this reason, you must create a derived class of SafeHandle for each operating system handle type that you want to wrap in a safe handle.

當您繼承自 SafeHandle 時,您必須覆寫下列成員:IsInvalidReleaseHandle()When you inherit from SafeHandle, you must override the following members: IsInvalid and ReleaseHandle().

您也應該提供無參數的函式,以使用代表無效控制碼值的值呼叫基底函式,以及 Boolean 值,指出原生控制碼是否由 SafeHandle 所擁有,因此應該在處置該 SafeHandle 時加以釋放。You should also provide a parameterless constructor that calls the base constructor with a value that represent an invalid handle value, and a Boolean value indicating whether the native handle is owned by the SafeHandle and consequently should be freed when that SafeHandle has been disposed.

建構函式

SafeHandle()
SafeHandle(IntPtr, Boolean)

使用指定的無效控制代碼值,初始化 SafeHandle 類別的新執行個體。Initializes a new instance of the SafeHandle class with the specified invalid handle value.

欄位

handle

指定要包裝的控制代碼。Specifies the handle to be wrapped.

屬性

IsClosed

取得值,指出控制代碼是否已關閉。Gets a value indicating whether the handle is closed.

IsInvalid

在衍生類別中覆寫時,取得值以指出這個控制代碼值是否無效。When overridden in a derived class, gets a value indicating whether the handle value is invalid.

方法

Close()

標記要釋出和釋放資源的控制代碼。Marks the handle for releasing and freeing resources.

DangerousAddRef(Boolean)

手動遞增 SafeHandle 執行個體上的參考計數器。Manually increments the reference counter on SafeHandle instances.

DangerousGetHandle()

傳回 handle 欄位的值。Returns the value of the handle field.

DangerousRelease()

手動遞減 SafeHandle 執行個體上的參考計數器。Manually decrements the reference counter on a SafeHandle instance.

Dispose()

釋放 SafeHandle 類別所使用的所有資源。Releases all resources used by the SafeHandle class.

Dispose(Boolean)

釋放 SafeHandle 類別所使用的 Unmanaged 資源,指定是否要執行一般處置作業。Releases the unmanaged resources used by the SafeHandle class specifying whether to perform a normal dispose operation.

Equals(Object)

判斷指定的物件是否等於目前的物件。Determines whether the specified object is equal to the current object.

(繼承來源 Object)
Finalize()

釋放與控制代碼相關的所有資源。Frees all resources associated with the handle.

GetHashCode()

做為預設雜湊函式。Serves as the default hash function.

(繼承來源 Object)
GetType()

取得目前執行個體的 TypeGets the Type of the current instance.

(繼承來源 Object)
MemberwiseClone()

建立目前 Object 的淺層複製。Creates a shallow copy of the current Object.

(繼承來源 Object)
ReleaseHandle()

在衍生類別中覆寫時,執行釋放控制代碼所需的程式碼。When overridden in a derived class, executes the code required to free the handle.

SetHandle(IntPtr)

將控制代碼設定為指定的既有控制代碼。Sets the handle to the specified pre-existing handle.

SetHandleAsInvalid()

將控制代碼標記為不再使用。Marks a handle as no longer used.

ToString()

傳回代表目前物件的字串。Returns a string that represents the current object.

(繼承來源 Object)

安全性

InheritanceDemand
適用於繼承者的完全信任的。for full trust for inheritors. 部分信任的程式碼無法繼承這個成員。This member cannot be inherited by partially trusted code.

SecurityCriticalAttribute
需要完全信任立即呼叫者。requires full trust for the immediate caller. 部分信任或透明的程式碼無法使用這個成員。This member cannot be used by partially trusted or transparent code.

適用於

另請參閱