Marshal.ZeroFreeGlobalAllocUnicode(IntPtr) Method

public:
 static void ZeroFreeGlobalAllocUnicode(IntPtr s);

[System.Security.SecurityCritical]
public static void ZeroFreeGlobalAllocUnicode(IntPtr s);

public static void ZeroFreeGlobalAllocUnicode(IntPtr s);

[<System.Security.SecurityCritical>]
static member ZeroFreeGlobalAllocUnicode : nativeint -> unit

static member ZeroFreeGlobalAllocUnicode : nativeint -> unit

Public Shared Sub ZeroFreeGlobalAllocUnicode (s As IntPtr)

Parameters

Attributes

SecurityCriticalAttribute

Examples

The following example demonstrates how to use the SecureStringToGlobalAllocUnicode method with the unmanaged LogonUser function to perform impersonation with the SecureString class. The example then uses the ZeroFreeGlobalAllocUnicode method to zero out and free the unmanaged string reference.

using System;
using System.ComponentModel;
using System.Runtime.InteropServices;
using System.Security;
using System.Security.Principal;

class Example
{
 // Define the Windows LogonUser and CloseHandle functions.
 [DllImport("advapi32.dll", SetLastError = true, CharSet = CharSet.Unicode)]
 internal static extern bool LogonUser(String username, String domain, IntPtr password,
 int logonType, int logonProvider, ref IntPtr token);

 [DllImport("kernel32.dll", CharSet = CharSet.Auto)]
 public extern static bool CloseHandle(IntPtr handle);

 // Define the required LogonUser enumerations.
 const int LOGON32_PROVIDER_DEFAULT = 0;
 const int LOGON32_LOGON_INTERACTIVE = 2;

 static void Main()
 {
 // Display the current user before impersonation.
 Console.WriteLine("Before impersonation: {0}",
 WindowsIdentity.GetCurrent().Name);

 // Ask the user for a network domain.
 Console.Write("Please enter your domain: ");
 string domain = Console.ReadLine();

 // Ask the user for a user name.
 Console.Write("Please enter your user name: ");
 string username = Console.ReadLine();

 // Ask the user for a password.
 Console.Write("Please enter your password: ");
 SecureString passWord = GetPassword();

 // Impersonate the account provided by the user.
 try {
 WindowsImpersonationContext userContext =
 ImpersonateUser(passWord, username, domain);
 // Display the current user after impersonation.
 Console.WriteLine("After impersonation: {0}",
 WindowsIdentity.GetCurrent().Name);
 }
 catch (ArgumentException e) {
 Console.WriteLine("{0}: {1}", e.GetType().Name, e.Message);
 }
 catch (Win32Exception e) {
 Console.WriteLine("{0}: {1}", e.GetType().Name, e.Message);
 }
 finally {
 passWord.Dispose();
 }
 }

 public static SecureString GetPassword()
 {
 SecureString password = new SecureString();

 // get the first character of the password
 ConsoleKeyInfo nextKey = Console.ReadKey(true);

 while (nextKey.Key != ConsoleKey.Enter) {
 if (nextKey.Key == ConsoleKey.Backspace) {
 if (password.Length > 0) {
 password.RemoveAt(password.Length - 1);

 // erase the last * as well
 Console.Write(nextKey.KeyChar);
 Console.Write(" ");
 Console.Write(nextKey.KeyChar);
 }
 }
 else {
 password.AppendChar(nextKey.KeyChar);
 Console.Write("*");
 }

 nextKey = Console.ReadKey(true);
 }

 Console.WriteLine();

 // lock the password down
 password.MakeReadOnly();
 return password;
 }

 public static WindowsImpersonationContext ImpersonateUser(SecureString password, string userName, string domainName)
 {
 IntPtr tokenHandle = IntPtr.Zero;
 IntPtr passwordPtr = IntPtr.Zero;
 bool returnValue = false;
 int error = 0;

 // Marshal the SecureString to unmanaged memory.
 passwordPtr = Marshal.SecureStringToGlobalAllocUnicode(password);

 // Pass LogonUser the unmanaged (and decrypted) copy of the password.
 returnValue = LogonUser(userName, domainName, passwordPtr,
 LOGON32_LOGON_INTERACTIVE, LOGON32_PROVIDER_DEFAULT,
 ref tokenHandle);
 if (!returnValue && tokenHandle == IntPtr.Zero)
 error = Marshal.GetLastWin32Error();

 // Perform cleanup whether or not the call succeeded.
 // Zero-out and free the unmanaged string reference.
 Marshal.ZeroFreeGlobalAllocUnicode(passwordPtr);
 // Close the token handle.
 CloseHandle(tokenHandle);

 // Throw an exception if an error occurred.
 if (error != 0) {
 throw new System.ComponentModel.Win32Exception(error);
 }
 // The token that is passed to the following constructor must
 // be a primary token in order to use it for impersonation.
 WindowsIdentity newId = new WindowsIdentity(tokenHandle);

 return newId.Impersonate();
 }
}

Imports System.ComponentModel
Imports System.Runtime.InteropServices
Imports System.Security
Imports System.Security.Principal

Module Example
 ' PInvoke into the Win32 API to provide access to the
 ' LogonUser and CloseHandle functions.
 <DllImport("advapi32.dll", SetLastError:=True, CharSet:=CharSet.Unicode)> _
 Function LogonUser(ByVal username As String, ByVal domain As String, _
 ByVal password As IntPtr, ByVal logonType As Integer, _
 ByVal logonProvider As Integer, ByRef token As IntPtr) _
 As Boolean
 End Function

 <DllImport("kernel32.dll", CharSet:=CharSet.Auto)> _
 Function CloseHandle(ByVal handle As IntPtr) As Boolean
 End Function

 ' Define the required LogonUser enumerations.
 Const LOGON32_PROVIDER_DEFAULT As Integer = 0
 Const LOGON32_LOGON_INTERACTIVE As Integer = 2

 Sub Main(ByVal args() As String)
 ' Display the current user before impersonation.
 Console.WriteLine("Before impersonation: {0}",
 WindowsIdentity.GetCurrent().Name)

 ' Ask the user for a network domain.
 Console.Write("Please enter your domain:")
 Dim domain As String = Console.ReadLine()

 ' Ask the user for a user name.
 Console.Write("Please enter your user name:")
 Dim username As String = Console.ReadLine()

 ' Ask the user for a password.
 Console.Write("Please enter your password:")
 Dim passWord As SecureString = GetPassword()

 ' Impersonate the account provided by the user.
 Try
 Dim userContext As WindowsImpersonationContext =
 ImpersonateUser(passWord, username, domain)
 ' Display the current user after impersonation.
 Console.WriteLine("After impersonation: {0}",
 WindowsIdentity.GetCurrent().Name)
 Catch e As ArgumentException
 Console.WriteLine(e.Message)
 Console.WriteLine("{0}: {1}", e.GetType().Name, e.Message)
 Catch e As Win32Exception
 Console.WriteLine("{0}: {1}", e.GetType().Name, e.Message)
 Finally
 passWord.Dispose()
 End Try
 End Sub

 Function GetPassword() As SecureString
 Dim password As New SecureString()

 ' get the first character of the password
 Dim nextKey As ConsoleKeyInfo = Console.ReadKey(True)

 While nextKey.Key <> ConsoleKey.Enter
 If nextKey.Key = ConsoleKey.BackSpace Then
 If password.Length > 0 Then
 password.RemoveAt(password.Length - 1)

 ' erase the last * as well
 Console.Write(nextKey.KeyChar)
 Console.Write(" ")
 Console.Write(nextKey.KeyChar)
 End If
 Else
 password.AppendChar(nextKey.KeyChar)
 Console.Write("*")
 End If

 nextKey = Console.ReadKey(True)
 End While

 Console.WriteLine()

 ' lock the password down
 password.MakeReadOnly()
 Return password
 End Function

 Function ImpersonateUser(ByVal password As SecureString,
 ByVal userName As String,
 ByVal domainName As String) As WindowsImpersonationContext
 Dim tokenHandle As IntPtr = IntPtr.Zero
 Dim passwordPtr As IntPtr = IntPtr.Zero
 Dim returnValue As Boolean = False
 Dim err As Integer = 0
 
 ' Marshal the SecureString to unmanaged memory.
 passwordPtr = Marshal.SecureStringToGlobalAllocUnicode(password)

 ' Pass LogonUser the unmanaged (and decrypted) copy of the password.
 returnValue = LogonUser(userName, domainName, passwordPtr,
 LOGON32_LOGON_INTERACTIVE, LOGON32_PROVIDER_DEFAULT,
 tokenHandle)
 If Not returnValue AndAlso tokenHandle = IntPtr.Zero Then
 err = Marshal.GetLastWin32Error()
 End If
 
 ' Perform cleanup whether or not the call succeeded.
 ' Zero-out and free the unmanaged string reference.
 Marshal.ZeroFreeGlobalAllocUnicode(passwordPtr)
 ' Close the token handle.
 CloseHandle(tokenHandle)

 ' Throw an exception if an error occurred.
 If err <> 0 Then
 Throw New System.ComponentModel.Win32Exception(err)
 End If
 ' The token that is passed to the following constructor must 
 ' be a primary token in order to use it for impersonation.
 Dim newId As New WindowsIdentity(tokenHandle)

 Return newId.Impersonate()
 End Function
End Module

Remarks

The ZeroFreeGlobalAllocUnicode method first zeros out and then frees unmanaged memory that was allocated using the SecureStringToGlobalAllocUnicode method.