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MSDN Library

SafeHandle Class

Represents a wrapper class for operating system handles. This class must be inherited.

Namespace:  System.Runtime.InteropServices
Assemblies:   mscorlib (in mscorlib.dll)
  System.Runtime.Handles (in System.Runtime.Handles.dll)

'Declaration
<SecurityPermissionAttribute(SecurityAction.InheritanceDemand, UnmanagedCode := True)> _
Public MustInherit Class SafeHandle _
	Inherits CriticalFinalizerObject _
	Implements IDisposable

The SafeHandle type exposes the following members.

  NameDescription
Protected methodSafeHandleInitializes a new instance of the SafeHandle class.
Protected methodSupported by Portable Class LibrarySafeHandle(IntPtr, Boolean)Initializes a new instance of the SafeHandle class with the specified invalid handle value.
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  NameDescription
Public propertySupported by Portable Class LibraryIsClosedGets a value indicating whether the handle is closed.
Public propertySupported by Portable Class LibraryIsInvalidWhen overridden in a derived class, gets a value indicating whether the handle value is invalid.
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  NameDescription
Public methodCloseMarks the handle for releasing and freeing resources.
Public methodSupported by Portable Class LibraryDangerousAddRefManually increments the reference counter on SafeHandle instances.
Public methodSupported by Portable Class LibraryDangerousGetHandleReturns the value of the handle field.
Public methodSupported by Portable Class LibraryDangerousReleaseManually decrements the reference counter on a SafeHandle instance.
Public methodSupported by Portable Class LibraryDisposeReleases all resources used by the SafeHandle class.
Protected methodSupported by Portable Class LibraryDispose(Boolean)Releases the unmanaged resources used by the SafeHandle class specifying whether to perform a normal dispose operation.
Public methodSupported by Portable Class LibraryEquals(Object)Determines whether the specified object is equal to the current object. (Inherited from Object.)
Protected methodSupported by Portable Class LibraryFinalizeFrees all resources associated with the handle. (Overrides CriticalFinalizerObject.Finalize.)
Public methodSupported by Portable Class LibraryGetHashCodeServes as the default hash function. (Inherited from Object.)
Public methodSupported by Portable Class LibraryGetTypeGets the Type of the current instance. (Inherited from Object.)
Protected methodSupported by Portable Class LibraryMemberwiseCloneCreates a shallow copy of the current Object. (Inherited from Object.)
Protected methodSupported by Portable Class LibraryReleaseHandleWhen overridden in a derived class, executes the code required to free the handle.
Protected methodSupported by Portable Class LibrarySetHandleSets the handle to the specified pre-existing handle.
Public methodSupported by Portable Class LibrarySetHandleAsInvalidMarks a handle as no longer used.
Public methodSupported by Portable Class LibraryToStringReturns a string that represents the current object. (Inherited from Object.)
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  NameDescription
Protected fieldSupported by Portable Class LibraryhandleSpecifies the handle to be wrapped.
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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.

This topic includes the following sections:

Why SafeHandle?
What SafeHandle does
Classes derived from SafeHandle

Why SafeHandle?

Before the .NET Framework version 2.0, all operating system handles could only be encapsulated in the IntPtr managed wrapper object. 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. These asynchronous exceptions are an obstacle to cleaning up operating system resources, and they can occur almost anywhere in your app.

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. If a finalizer frees the handle passed to that platform invoke call, it could lead to handle corruption. The handle could also be reclaimed while your method is blocked during a platform invoke call, such as while reading a file.

More critically, because Windows aggressively recycles handles, a handle could be recycled and point to another resource that might contain sensitive data. This is known as a recycle attack and can potentially corrupt data and be a security threat.

What SafeHandle does

The SafeHandle class simplifies several of these object lifetime issues, and is integrated with platform invoke so that operating system resources are not leaked. The SafeHandle class resolves object lifetime issues by assigning and releasing handles without interruption. 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.

Because SafeHandle inherits from CriticalFinalizerObject, all the noncritical finalizers are called before any of the critical finalizers. The finalizers are called on objects that are no longer live during the same garbage collection pass. 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. This very weak ordering between critical and noncritical finalizers is not intended for general use. It exists primarily to assist in the migration of existing libraries by allowing those libraries to use SafeHandle without altering their semantics. Additionally, the critical finalizer and anything it calls, such as the SafeHandle.ReleaseHandle method, must be in a constrained execution region. This imposes constraints on what code can be written within the finalizer's call graph.

Platform invoke operations automatically increment the reference count of handles encapsulated by a SafeHandle and decrement them upon completion. This ensures that the handle will not be recycled or closed unexpectedly.

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. This controls whether the SafeHandle object will release the handle after the object has been disposed. This is useful for handles with peculiar lifetime requirements or for consuming a handle whose lifetime is controlled by someone else.

Classes derived from SafeHandle

SafeHandle is an abstract wrapper class for operating system handles. Deriving from this class is difficult. Instead, use the derived classes in the Microsoft.Win32.SafeHandles namespace that provide safe handles for the following:

Notes to Inheritors

To create a class derived from SafeHandle, you must know how to create and free an operating system handle. This process is different for different handle types because some use the CloseHandle function, while others use more specific functions such as UnmapViewOfFile or FindClose. 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.

Important noteImportant

Writing your own classes derived from SafeHandle is an advanced programming feature. The .NET Framework provides a set of prewritten classes derived from SafeHandle in the Microsoft.Win32.SafeHandles namespace. These classes are designed to provide common functionality supporting file and operating system handles.

When you inherit from SafeHandle, you must override the following members: IsInvalid and ReleaseHandle.

You should also provide a default 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.

The following code example creates a custom safe handle for an operating system file handle, deriving from SafeHandleZeroOrMinusOneIsInvalid. It reads bytes from a file and displays their hexadecimal values. It also contains a fault testing harness that causes the thread to abort, but the handle value is freed. When using an IntPtr to represent handles, the handle is occasionally leaked due to the asynchronous thread abort.

You will need a text file in the same folder as the compiled application. Assuming that you name the application "HexViewer", the command line usage is:

HexViewer <filename> -Fault

Optionally specify -Fault to intentionally attempt to leak the handle by aborting the thread in a certain window. Use the Windows Perform.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);
                }
            }

        }
    }
}

.NET Framework

Supported in: 4.6, 4.5, 4, 3.5, 3.0, 2.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

.NET for Windows Phone apps

Supported in: Windows Phone 8.1, Windows Phone Silverlight 8.1, Windows Phone Silverlight 8

Portable Class Library

Supported in: Portable Class Library

  • InheritanceDemand 

    for full trust for inheritors. This member cannot be inherited by partially trusted code.

  • SecurityCriticalAttribute 

    requires full trust for the immediate caller. This member cannot be used by partially trusted or transparent code.

Any public static (Shared in Visual Basic) members of this type are thread safe. Any instance members are not guaranteed to be thread safe.
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