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WindowsImpersonationContext Class

Represents the Windows user prior to an impersonation operation.

Namespace: System.Security.Principal
Assembly: mscorlib (in mscorlib.dll)

[ComVisibleAttribute(true)] 
public class WindowsImpersonationContext : IDisposable
/** @attribute ComVisibleAttribute(true) */ 
public class WindowsImpersonationContext implements IDisposable
ComVisibleAttribute(true) 
public class WindowsImpersonationContext implements IDisposable
Not applicable.

This class reverts a user's identity after the user impersonates another user.

Notes to Implementers: Because Microsoft Windows 98 and Windows Millennium Edition platforms do not have users or user tokens, impersonation cannot take place on those platforms.

The following example demonstrates how to impersonate a user and then revert to the original identity.

// This sample demonstrates the use of the WindowsIdentity class to impersonate a user.
// IMPORTANT NOTES: 
// This sample can be run only on Windows XP.  The default Windows 2000 security policy 
// prevents this sample from executing properly, and changing the policy to allow
// proper execution presents a security risk. 
// This sample requests the user to enter a password on the console screen.
// Because the console window does not support methods allowing the password to be masked, 
// it will be visible to anyone viewing the screen.
// The sample is intended to be executed in a .NET Framework 1.1 environment.  To execute
// this code in a 1.0 environment you will need to use a duplicate token in the call to the
// WindowsIdentity constructor. See KB article Q319615 for more information.

using System;
using System.Runtime.InteropServices;
using System.Security.Principal;
using System.Security.Permissions;
using System.Windows.Forms;

[assembly:SecurityPermissionAttribute(SecurityAction.RequestMinimum, UnmanagedCode=true)]
[assembly:PermissionSetAttribute(SecurityAction.RequestMinimum, Name = "FullTrust")]
public class ImpersonationDemo
{
    [DllImport("advapi32.dll", SetLastError=true)]
    public static extern bool LogonUser(String lpszUsername, String lpszDomain, String lpszPassword, 
        int dwLogonType, int dwLogonProvider, ref IntPtr phToken);

    [DllImport("kernel32.dll", CharSet=System.Runtime.InteropServices.CharSet.Auto)]
    private unsafe static extern int FormatMessage(int dwFlags, ref IntPtr lpSource, 
        int dwMessageId, int dwLanguageId, ref String lpBuffer, int nSize, IntPtr *Arguments);

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

    [DllImport("advapi32.dll", CharSet=CharSet.Auto, SetLastError=true)]
    public extern static bool DuplicateToken(IntPtr ExistingTokenHandle, 
        int SECURITY_IMPERSONATION_LEVEL, ref IntPtr DuplicateTokenHandle);

    // Test harness.
    // If you incorporate this code into a DLL, be sure to demand FullTrust.
    [PermissionSetAttribute(SecurityAction.Demand, Name = "FullTrust")]
    public static void Main(string[] args)
    {	
        IntPtr tokenHandle = new IntPtr(0);
        IntPtr dupeTokenHandle = new IntPtr(0);
        try
        {
            string userName, domainName;
            // Get the user token for the specified user, domain, and password using the 
            // unmanaged LogonUser method.  
            // The local machine name can be used for the domain name to impersonate a user on this machine.
            Console.Write("Enter the name of the domain on which to log on: ");
            domainName = Console.ReadLine();

            Console.Write("Enter the login of a user on {0} that you wish to impersonate: ", domainName);
            userName = Console.ReadLine();

            Console.Write("Enter the password for {0}: ", userName);
			
            const int LOGON32_PROVIDER_DEFAULT = 0;
            //This parameter causes LogonUser to create a primary token.
            const int LOGON32_LOGON_INTERACTIVE = 2;

            tokenHandle = IntPtr.Zero;
 
            // Call LogonUser to obtain a handle to an access token.
            bool returnValue = LogonUser(userName, domainName, Console.ReadLine(), 
                LOGON32_LOGON_INTERACTIVE, LOGON32_PROVIDER_DEFAULT,
                ref tokenHandle);
					
            Console.WriteLine("LogonUser called.");
	            
            if (false == returnValue)
            {
                int ret = Marshal.GetLastWin32Error();
                Console.WriteLine("LogonUser failed with error code : {0}", ret);
                throw new System.ComponentModel.Win32Exception(ret);
            }

            Console.WriteLine("Did LogonUser Succeed? " + (returnValue? "Yes" : "No"));
            Console.WriteLine("Value of Windows NT token: " + tokenHandle);

            // Check the identity.
            Console.WriteLine("Before impersonation: "
                + WindowsIdentity.GetCurrent().Name);
            // Use the token handle returned by LogonUser.
            WindowsIdentity newId = new WindowsIdentity(tokenHandle);
            WindowsImpersonationContext impersonatedUser = newId.Impersonate();

            // Check the identity.
            Console.WriteLine("After impersonation: "
                + WindowsIdentity.GetCurrent().Name);
	    
            // Stop impersonating the user.
            impersonatedUser.Undo();

            // Check the identity.
            Console.WriteLine("After Undo: " + WindowsIdentity.GetCurrent().Name);
	        
            // Free the tokens.
            if (tokenHandle != IntPtr.Zero)
                CloseHandle(tokenHandle);

        }
        catch(Exception ex)
        {
            Console.WriteLine("Exception occurred. " + ex.Message);
        }

    }
}


System.Object
  System.Security.Principal.WindowsImpersonationContext

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

Windows Server 2000 SP4, Windows Millennium Edition, Windows Server 2003, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP SP2, Windows XP Starter Edition

The Microsoft .NET Framework 3.0 is supported on Windows Vista, Microsoft Windows XP SP2, and Windows Server 2003 SP1.

.NET Framework

Supported in: 3.0, 2.0, 1.1, 1.0

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