什么是SafeHandle？它与IntPtr有何不同？我应该什么时候使用？它有什么优势？
答案 0 :(得分：11)
另一种看待它的方式：使用SafeHandle，你应该几乎永远不需要写另一个终结器。
答案 1 :(得分：8)
我认为MSDN在定义中非常明确：
SafeHandle类提供关键功能 句柄资源的最终确定， 阻止手柄 被垃圾过早地收回 收集和回收 Windows无意中引用 非托管对象。在.NET之前 所有操作的框架版本2.0 系统句柄只能是 封装在IntPtr托管中 包装对象。
SafeHandle类包含一个 确保手柄的终结器 关闭并保证运行， 即使在意外的AppDomain期间 当主持人不信任时卸载 状态的一致性 AppDomain中。
有关的更多信息 使用SafeHandle的好处，请参阅 安全处理和关键 定稿。
这个类是抽象的，因为你 无法创建通用句柄。至 实现SafeHandle，你必须创建 派生类。创建SafeHandle 派生类，你必须知道如何 创建并释放操作系统 处理。这个过程是不同的 一些不同的句柄类型 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 每个操作的SafeHandle类 系统句柄类型;如 MySafeRegistryHandle， MySafeFileHandle，和 MySpecialSafeFileHandle。其中一些 派生类是预先编写的 为你提供的 Microsoft.Win32.SafeHandles名称空间。
答案 2 :(得分：5)
在托管代码从非托管代码接收IntPtr的情况下，您应尽可能使用SafeHandle的衍生版本。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，但是一些内部.NET框架类，如Microsoft.Win32.SafeHandles.SafeLocalAllocHandle和派生的那些来自公开的抽象类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，只要IntPtr从非托管代码返回到托管代码，即使它不需要清理，也可以创建此类的派生。
SafeHandle的既定目的是保证即使世界正在结束（例如正在卸载AppDomain或发生StackOverflowException），.NET框架也应该绝对确保调用SafeHandle的终结器关闭或释放被包装的IntPtr引用的非托管实体。 SafeHandle类通过继承CriticalFinalizerObject类来实现此目的。但是，继承这个类确实会使继承者有义务在调用终结器时不完全搞砸进程的状态，这可能是为什么它不常用于Windows操作系统句柄以外的实体。 .NET框架还提供了一些弱的终结顺序，因此可以安全地与任何不从CriticalFinalizerObject继承的类的终结器中的SafeHandle对象进行交互，但是必要的情况应该是很少的。
理想情况下，还应使用SafeHandle派生类，通过在派生类中封装预期的功能，更安全地与非托管实体引用进行交互。从SafeHandle继承的一个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使用它的开发人员需要直接与它包含的IntPtr交互的方法。添加此类方法还可以使其他开发人员清楚地了解在托管上下文中使用非托管方法调用的结果。即使在非托管方法通过在类的构造函数中调用base（false）而返回的指针上不需要清理，也可以使用继承自SafeHandle的类。
下面是两个使用从SafeHandle派生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的示例。第一个例子是一种更传统的方案，其中通过返回的用户令牌LogonUser是由SafeTokenHandle类的实例缠绕。在处理或完成对象时，此类将在令牌上调用CloseHandle。它还包括一个名为GetWindowsIdentity的方法，该方法为用户令牌表示的用户返回WindowsIdentity对象。第二个示例使用Windows内置函数CommandLineToArgvW来解析命令行。此函数返回一个指向包含连续内存块的数组的指针，该内存块可以通过单次调用LocalFree释放。 SafeLocalAllocWStrArray类（继承自SafeLocalAllocArray类（在此示例中也定义））将在处理或完成对象时调用阵列上的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
}
}