Marshal.ZeroFreeGlobalAllocUnicode Method (IntPtr)


Frees an unmanaged string pointer that was allocated using the SecureStringToGlobalAllocUnicode method.

Namespace:   System.Runtime.InteropServices
Assembly:  mscorlib (in mscorlib.dll)

Public Shared Sub ZeroFreeGlobalAllocUnicode (
	s As IntPtr


Type: System.IntPtr

The address of the unmanaged string to free.

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

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.

Imports System
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}",

        ' 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.
            Dim userContext As WindowsImpersonationContext =
                            ImpersonateUser(passWord, username, domain)
            ' Display the current user after impersonation.
            Console.WriteLine("After impersonation: {0}",
         Catch e As ArgumentException
             Console.WriteLine("{0}: {1}", e.GetType().Name, e.Message)
         Catch e As Win32Exception
             Console.WriteLine("{0}: {1}", e.GetType().Name, e.Message)
         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(" ")
                End If
            End If

            nextKey = Console.ReadKey(True)
        End While


        ' lock the password down
        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,
        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.
        ' Close the token handle.

        ' 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


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

Universal Windows Platform
Available since 8
.NET Framework
Available since 2.0
Portable Class Library
Supported in: portable .NET platforms
Windows Phone
Available since 8.1
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