WindowsImpersonationContext Class

// This sample demonstrates the use of the WindowsIdentity class to impersonate a user. 
// IMPORTANT NOTES: 
// 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. 
// On Windows Vista and later this sample must be run as an administrator.  


using System;
using System.Runtime.InteropServices;
using System.Security.Principal;
using System.Security.Permissions;
using Microsoft.Win32.SafeHandles;
using System.Runtime.ConstrainedExecution;
using System.Security;


public class ImpersonationDemo
{
    [DllImport("advapi32.dll", SetLastError = true, CharSet = CharSet.Unicode)]
    public static extern bool LogonUser(String lpszUsername, String lpszDomain, String lpszPassword,
        int dwLogonType, int dwLogonProvider, out SafeTokenHandle phToken);

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

    // 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)
    {
        SafeTokenHandle safeTokenHandle;
        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;

            // Call LogonUser to obtain a handle to an access token. 
            bool returnValue = LogonUser(userName, domainName, Console.ReadLine(),
                LOGON32_LOGON_INTERACTIVE, LOGON32_PROVIDER_DEFAULT,
                out safeTokenHandle);

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

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

                        // Check the identity.
                        Console.WriteLine("After impersonation: "
                            + WindowsIdentity.GetCurrent().Name);
                    }
                }
                // Releasing the context object stops the impersonation 
                // Check the identity.
                Console.WriteLine("After closing the context: " + WindowsIdentity.GetCurrent().Name);
            }
        }
        catch (Exception ex)
        {
            Console.WriteLine("Exception occurred. " + ex.Message);
        }

    }
}
public sealed class SafeTokenHandle : SafeHandleZeroOrMinusOneIsInvalid
{
    private SafeTokenHandle()
        : base(true)
    {
    }

    [DllImport("kernel32.dll")]
    [ReliabilityContract(Consistency.WillNotCorruptState, Cer.Success)]
    [SuppressUnmanagedCodeSecurity]
    [return: MarshalAs(UnmanagedType.Bool)]
    private static extern bool CloseHandle(IntPtr handle);

    protected override bool ReleaseHandle()
    {
        return CloseHandle(handle);
    }
}