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using System.Runtime.InteropServices;  // DllImport
using System.Security.Principal; // WindowsImpersonationContext
using System.Security.Permissions; // PermissionSetAttribute      

        // obtains user token
        [DllImport("advapi32.dll", SetLastError = true)]
        public static extern bool LogonUser(string pszUsername, string pszDomain, string pszPassword,
            int dwLogonType, int dwLogonProvider, ref IntPtr phToken);

        // closes open handes returned by LogonUser
        [DllImport("kernel32.dll", CharSet = CharSet.Auto)]
        public extern static bool CloseHandle(IntPtr handle);

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

        private System.Security.Principal.WindowsImpersonationContext someToken;

        [PermissionSetAttribute(SecurityAction.Demand, Name = "FullTrust")]
        public WindowsImpersonationContext Impersonate()
        {
            IntPtr tokenHandle = new IntPtr(0);
            IntPtr dupeTokenHandle = new IntPtr(0);
            try
            {
                string domainName;
                domainName = "local";             

                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(NewWinUser, domainName, NewWinPWD,
                    LOGON32_LOGON_INTERACTIVE, LOGON32_PROVIDER_DEFAULT,
                    ref tokenHandle);

                if (false == returnValue)
                {
                    int ret = Marshal.GetLastWin32Error();
                    throw new System.ComponentModel.Win32Exception(ret);
                }

                // Use the token handle returned by LogonUser.
                WindowsIdentity newId = new WindowsIdentity(tokenHandle);
                WindowsImpersonationContext impersonatedUser = newId.Impersonate();
                //MessageBox.Show(WindowsIdentity.GetCurrent().Name);
                return impersonatedUser;
            }

            catch (Exception ex)
            {
                MessageBox.Show(ex.Message);
                Application.Exit();
                return null;
            }
        }