C# 中的安全句柄

Question

什么是安全手柄？它与 IntPtr 有何不同？我什么时候应该使用一个？它有什么优势？

Answer 1

另一种看待它的方式：使用 SafeHandle，您应该几乎永远不需要编写另一个终结器。

Answer 2

在托管代码从非托管代码接收 IntPtr 时，应尽可能使用 SafeHandle 的派生类。虽然 SafeHandle 类的名称、一般用途甚至文档都暗示它只应该用于包含 Windows 操作系统句柄、一些内部 .NET 框架类，例如 Microsoft.Win32.SafeHandles.SafeLocalAllocHandle 以及派生的那些来自公开可用的抽象类System.Runtime.InteropServices.SafeBuffer 也使用它来保证释放其他非托管资源，例如动态分配的结构和数组。总的来说，我认为当 IntPtr 从非托管代码返回到托管代码时创建此类的派生类是一种很好的做法，即使它不需要清理。

SafeHandle 的既定目的是保证即使世界即将结束（例如，正在卸载 AppDomain 或发生 StackOverflowException），.NET 框架也应绝对确保调用 SafeHandle 的终结器以关闭或解除分配被包装的 IntPtr 引用的非托管实体。 SafeHandle 类通过继承CriticalFinalizerObject 类来实现这一点。然而，从这个类继承确实给继承者带来了在调用终结器时不完全搞砸进程状态的义务，这可能是为什么它不经常用于 Windows 操作系统句柄以外的实体的原因。 .NET 框架还提供了一些弱终结排序，因此可以安全地与任何不继承自 CriticalFinalizerObject 的类的终结器中的 SafeHandle 对象进行交互，但需要这样做的情况应该很少见。

理想情况下，还应该使用 SafeHandle 派生类通过在派生类中封装预期功能来更安全地与非托管实体引用进行交互。一个从 SafeHandle 继承的编写良好的类应该有一个特定的目的，并且应该提供足以防止任何开发人员出于该目的使用它的方法，而无需直接与其包含的 IntPtr 交互。添加此类方法还可以让其他开发人员清楚地了解非托管方法调用的结果将用于托管上下文中的用途。即使不需要清理非托管方法通过在类的构造函数中调用 base(false) 返回的指针，也可以使用从 SafeHandle 继承的类。

下面的两个示例使用派生自 SafeHandle 的类来安全地清理对非托管实体的引用并封装与非托管实体相关的功能。 The first example is a more traditional scenario in which a user token returned by LogonUser is wrapped by an instance of the SafeTokenHandle class.当对象被释放或完成时，此类将在令牌上调用 CloseHandle。它还包括一个名为 GetWindowsIdentity 的方法，该方法为用户令牌所代表的用户返回一个 WindowsIdentity 对象。第二个示例使用 Windows 内置函数 CommandLineToArgvW 解析命令行。此函数返回一个指向包含连续内存块的数组的指针，该内存块可以通过一次调用 LocalFree 来释放。 SafeLocalAllocWStrArray 类（继承自 SafeLocalAllocArray 类，该类也在本示例中定义）将在对象被释放或最终确定时调用数组上的 LocalFree。它还包括一个将非托管数组的内容复制到托管数组的函数。

static class Examples
{
    static void Example1_SafeUserToken()
    {
        const string user = "SomeLocalUser";
        const string domain = null;
        const string password = "ExamplePassword";
        NativeMethods.SafeTokenHandle userToken;
        WindowsIdentity identity;

        NativeMethods.LogonUser(user, domain, password, NativeMethods.LogonType.LOGON32_LOGON_INTERACTIVE, NativeMethods.LogonProvider.LOGON32_PROVIDER_DEFAULT, out userToken);

        using (userToken)
        {
            // get a WindowsIdentity object for the user
            // WindowsIdentity will duplicate the token, so it is safe to free the original token after this is called
            identity = userToken.GetWindowsIdentity();
        }

        // impersonate the user
        using (identity)
        using (WindowsImpersonationContext impersonationContext = identity.Impersonate())
        {
            Console.WriteLine("I'm running as {0}!", Thread.CurrentPrincipal.Identity.Name);
        }
    }

    static void Example2_SafeLocalAllocWStrArray()
    {
        const string commandLine = "/example /command";
        int argc;
        string[] args;

        using (NativeMethods.SafeLocalAllocWStrArray argv = NativeMethods.CommandLineToArgvW(commandLine, out argc))
        {
            // CommandLineToArgvW returns NULL on failure; since SafeLocalAllocWStrArray inherits from
            // SafeHandleZeroOrMinusOneIsInvalid, it will see this value as invalid
            // if that happens, throw an exception containing the last Win32 error that occurred
            if (argv.IsInvalid)
            {
                int lastError = Marshal.GetHRForLastWin32Error();
                throw new Win32Exception(lastError, "An error occurred when calling CommandLineToArgvW.");
            }

            // the one unsafe aspect of this is that the developer calling this function must be trusted to
            // pass in an array of length argc or specify the length of the copy as the value of argc
            // if the developer does not do this, the array may end up containing some garbage or an
            // AccessViolationException could be thrown
            args = new string[argc];
            argv.CopyTo(args);
        }

        for (int i = 0; i < args.Length; ++i)
        {
            Console.WriteLine("Argument {0}: {1}", i, args[i]);
        }
    }
}

/// <summary>
/// P/Invoke methods and helper classes used by this example.
/// </summary>
internal static class NativeMethods
{
    // documentation: http://msdn.microsoft.com/en-us/library/windows/desktop/aa378184(v=vs.85).aspx
    [DllImport("advapi32.dll", SetLastError = true, CharSet = CharSet.Unicode)]
    public static extern bool LogonUser(string lpszUsername, string lpszDomain, string lpszPassword, LogonType dwLogonType, LogonProvider dwLogonProvider, out SafeTokenHandle phToken);

    // documentation: http://msdn.microsoft.com/en-us/library/windows/desktop/ms724211(v=vs.85).aspx
    [DllImport("kernel32.dll", SetLastError = true)]
    public static extern bool CloseHandle(IntPtr handle);

    // documentation: http://msdn.microsoft.com/en-us/library/windows/desktop/bb776391(v=vs.85).aspx
    [DllImport("shell32.dll", CharSet = CharSet.Unicode, SetLastError = true)]
    public static extern SafeLocalAllocWStrArray CommandLineToArgvW(string lpCmdLine, out int pNumArgs);

    // documentation: http://msdn.microsoft.com/en-us/library/windows/desktop/aa366730(v=vs.85).aspx
    [DllImport("kernel32.dll", SetLastError = true)]
    public static extern IntPtr LocalFree(IntPtr hLocal);

    /// <summary>
    /// Wraps a handle to a user token.
    /// </summary>
    public class SafeTokenHandle : SafeHandleZeroOrMinusOneIsInvalid
    {
        /// <summary>
        /// Creates a new SafeTokenHandle. This constructor should only be called by P/Invoke.
        /// </summary>
        private SafeTokenHandle()
            : base(true)
        {
        }

        /// <summary>
        /// Creates a new SafeTokenHandle to wrap the specified user token.
        /// </summary>
        /// <param name="arrayPointer">The user token to wrap.</param>
        /// <param name="ownHandle"><c>true</c> to close the token when this object is disposed or finalized,
        /// <c>false</c> otherwise.</param>
        public SafeTokenHandle(IntPtr handle, bool ownHandle)
            : base(ownHandle)
        {
            this.SetHandle(handle);
        }

        /// <summary>
        /// Provides a <see cref="WindowsIdentity" /> object created from this user token. Depending
        /// on the type of token, this can be used to impersonate the user. The WindowsIdentity
        /// class will duplicate the token, so it is safe to use the WindowsIdentity object created by
        /// this method after disposing this object.
        /// </summary>
        /// <returns>a <see cref="WindowsIdentity" /> for the user that this token represents.</returns>
        /// <exception cref="InvalidOperationException">This object does not contain a valid handle.</exception>
        /// <exception cref="ObjectDisposedException">This object has been disposed and its token has
        /// been released.</exception>
        public WindowsIdentity GetWindowsIdentity()
        {
            if (this.IsClosed)
            {
                throw new ObjectDisposedException("The user token has been released.");
            }
            if (this.IsInvalid)
            {
                throw new InvalidOperationException("The user token is invalid.");
            }

            return new WindowsIdentity(this.handle);
        }

        /// <summary>
        /// Calls <see cref="NativeMethods.CloseHandle" /> to release this user token.
        /// </summary>
        /// <returns><c>true</c> if the function succeeds, <c>false otherwise</c>. To get extended
        /// error information, call <see cref="Marshal.GetLastWin32Error"/>.</returns>
        protected override bool ReleaseHandle()
        {
            return NativeMethods.CloseHandle(this.handle);
        }
    }

    /// <summary>
    /// A wrapper around a pointer to an array of Unicode strings (LPWSTR*) using a contiguous block of
    /// memory that can be freed by a single call to LocalFree.
    /// </summary>
    public sealed class SafeLocalAllocWStrArray : SafeLocalAllocArray<string>
    {
        /// <summary>
        /// Creates a new SafeLocalAllocWStrArray. This constructor should only be called by P/Invoke.
        /// </summary>
        private SafeLocalAllocWStrArray()
            : base(true)
        {
        }

        /// <summary>
        /// Creates a new SafeLocalallocWStrArray to wrap the specified array.
        /// </summary>
        /// <param name="handle">The pointer to the unmanaged array to wrap.</param>
        /// <param name="ownHandle"><c>true</c> to release the array when this object
        /// is disposed or finalized, <c>false</c> otherwise.</param>
        public SafeLocalAllocWStrArray(IntPtr handle, bool ownHandle)
            : base(ownHandle)
        {
            this.SetHandle(handle);
        }

        /// <summary>
        /// Returns the Unicode string referred to by an unmanaged pointer in the wrapped array.
        /// </summary>
        /// <param name="index">The index of the value to retrieve.</param>
        /// <returns>the value at the position specified by <paramref name="index" /> as a string.</returns>
        protected override string GetArrayValue(int index)
        {
            return Marshal.PtrToStringUni(Marshal.ReadIntPtr(this.handle + IntPtr.Size * index));
        }
    }

    // This class is similar to the built-in SafeBuffer class. Major differences are:
    // 1. This class is less safe because it does not implicitly know the length of the array it wraps.
    // 2. The array is read-only.
    // 3. The type parameter is not limited to value types.
    /// <summary>
    /// Wraps a pointer to an unmanaged array of objects that can be freed by calling LocalFree.
    /// </summary>
    /// <typeparam name="T">The type of the objects in the array.</typeparam>
    public abstract class SafeLocalAllocArray<T> : SafeHandleZeroOrMinusOneIsInvalid
    {
        /// <summary>
        /// Creates a new SafeLocalArray which specifies that the array should be freed when this
        /// object is disposed or finalized.
        /// <param name="ownsHandle"><c>true</c> to reliably release the handle during the finalization phase;
        /// <c>false</c> to prevent reliable release (not recommended).</param>
        /// </summary>
        protected SafeLocalAllocArray(bool ownsHandle)
            : base(ownsHandle)
        {
        }

        /// <summary>
        /// Converts the unmanaged object referred to by <paramref name="valuePointer" /> to a managed object
        /// of type T.
        /// </summary>
        /// <param name="index">The index of the value to retrieve.</param>
        /// <returns>the value at the position specified by <paramref name="index" /> as a managed object of
        /// type T.</returns>
        protected abstract T GetArrayValue(int index);

        // 
        /// <summary>
        /// Frees the wrapped array by calling LocalFree.
        /// </summary>
        /// <returns><c>true</c> if the call to LocalFree succeeds, <c>false</c> if the call fails.</returns>
        protected override bool ReleaseHandle()
        {
            return (NativeMethods.LocalFree(this.handle) == IntPtr.Zero);
        }

        /// <summary>
        /// Copies the unmanaged array to the specified managed array.
        /// 
        /// It is important that the length of <paramref name="array"/> be less than or equal to the length of
        /// the unmanaged array wrapped by this object. If it is not, at best garbage will be read and at worst
        /// an exception of type <see cref="AccessViolationException" /> will be thrown.
        /// </summary>
        /// <param name="array">The managed array to copy the unmanaged values to.</param>
        /// <exception cref="ObjectDisposedException">The unmanaged array wrapped by this object has been
        /// freed.</exception>
        /// <exception cref="InvalidOperationException">The pointer to the unmanaged array wrapped by this object
        /// is invalid.</exception>
        /// <exception cref="ArgumentNullException"><paramref name="array"/> is null.</exception>
        public void CopyTo(T[] array)
        {
            if (array == null)
            {
                throw new ArgumentNullException("array");
            }

            this.CopyTo(array, 0, array.Length);
        }

        /// <summary>
        /// Copies the unmanaged array to the specified managed array.
        /// 
        /// It is important that <paramref name="length" /> be less than or equal to the length of
        /// the array wrapped by this object. If it is not, at best garbage will be read and at worst
        /// an exception of type <see cref="AccessViolationException" /> will be thrown.
        /// </summary>
        /// <param name="array">The managed array to copy the unmanaged values to.</param>
        /// <param name="index">The index to start at when copying to <paramref name="array" />.</param>
        /// <param name="length">The number of items to copy to <paramref name="array" /></param>
        /// <exception cref="ObjectDisposedException">The unmanaged array wrapped by this object has been
        /// freed.</exception>
        /// <exception cref="InvalidOperationException">The pointer to the unmanaged array wrapped by this object
        /// is invalid.</exception>
        /// <exception cref="ArgumentNullException"><paramref name="array"/> is null.</exception>
        /// <exception cref="ArgumentOutOfRangeException"><paramref name="index"/> is less than zero.-or- 
        /// <paramref name="index" /> is greater than the length of <paramref name="array"/>.-or-
        /// <paramref name="length"/> is less than zero.</exception>
        /// <exception cref="ArgumentException">The sum of <paramref name="index" /> and <paramref name="length" />
        /// is greater than the length of <paramref name="array" />.</exception>
        public void CopyTo(T[] array, int index, int length)
        {
            if (this.IsClosed)
            {
                throw new ObjectDisposedException(this.ToString());
            }
            if (this.IsInvalid)
            {
                throw new InvalidOperationException("This object's buffer is invalid.");
            }
            if (array == null)
            {
                throw new ArgumentNullException("array");
            }
            if (index < 0 || array.Length < index)
            {
                throw new ArgumentOutOfRangeException("index", "index must be a nonnegative integer that is less than array's length.");
            }
            if (length < 0)
            {
                throw new ArgumentOutOfRangeException("length", "length must be a nonnegative integer.");
            }
            if (array.Length < index + length)
            {
                throw new ArgumentException("length", "length is greater than the number of elements from index to the end of array.");
            }

            for (int i = 0; i < length; ++i)
            {
                array[index + i] = this.GetArrayValue(i);
            }
        }
    }

    /// <summary>
    /// The type of logon operation to perform.
    /// </summary>
    internal enum LogonType : uint
    {
        LOGON32_LOGON_BATCH = 1,
        LOGON32_LOGON_INTERACTIVE = 2,
        LOGON32_LOGON_NETWORK = 3,
        LOGON32_LOGON_NETWORK_CLEARTEXT = 4,
        LOGON32_LOGON_NEW_CREDENTIALS = 5,
        LOGON32_LOGON_SERVICE = 6,
        LOGON32_LOGON_UNLOCK = 7
    }

    /// <summary>
    /// The logon provider to use.
    /// </summary>
    internal enum LogonProvider : uint
    {
        LOGON32_PROVIDER_DEFAULT = 0,
        LOGON32_PROVIDER_WINNT50 = 1,
        LOGON32_PROVIDER_WINNT40 = 2
    }
}

Answer 3

我认为MSDN 的定义很明确：

SafeHandle 类提供关键的 最终确定句柄资源， 防止手柄被 过早地被垃圾回收 收集和从被回收 意外引用的 Windows 非托管对象。 .NET 之前 框架2.0版本，全部运行 系统句柄只能是 封装在 IntPtr 托管 包装对象。

SafeHandle 类包含一个 终结器，确保句柄 已关闭并保证运行， 即使在意外的 AppDomain 期间 当主机可能不信任 状态的一致性 应用程序域。

有关更多信息 使用 SafeHandle 的好处，请参阅 安全手柄和关键 完成。

这个类是抽象的，因为你 无法创建通用句柄。至 实施 SafeHandle，您必须创建 一个派生类。创建 SafeHandle 派生类，你必须知道如何 创建和释放操作系统 处理。这个过程是不同的 不同的句柄类型，因为有些 使用 CloseHandle，而其他人使用更多 具体方法如 UnmapViewOfFile 或 FindClose。为了这 原因，你必须创建一个派生的 每个操作的 SafeHandle 类 系统句柄类型；如 MySafeRegistryHandle, MySafeFileHandle 和 MySpecialSafeFileHandle。其中一些 派生类是预先编写好的，并且 为您提供 Microsoft.Win32.SafeHandles 命名空间。