Impersonation

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

    public class ImpersonateUser
    {
        [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 = CharSet.Auto)]
        private extern static bool CloseHandle(IntPtr handle);

        private static IntPtr tokenHandle = new IntPtr(0);
        private static WindowsImpersonationContext impersonatedUser;

        // If you incorporate this code into a DLL, be sure to demand that it
        // runs with FullTrust.
        [PermissionSetAttribute(SecurityAction.Demand, Name = "FullTrust")]
        public void Impersonate(string domainName, string userName, string password)
        {
            try
            {

                // Use the unmanaged LogonUser function to get the user token for
                // the specified user, domain, and password.
                const int LOGON32_PROVIDER_DEFAULT = 0;

                // Passing this parameter causes LogonUser to create a primary token.
                const int LOGON32_LOGON_INTERACTIVE = 2;
                tokenHandle = IntPtr.Zero;

                // Step -1 Call LogonUser to obtain a handle to an access token.
                bool returnValue = LogonUser(
                userName,
                domainName,
                password,
                LOGON32_LOGON_INTERACTIVE,
                LOGON32_PROVIDER_DEFAULT,
                ref tokenHandle); // tokenHandle - new security token

                if (false == returnValue)
                {
                    int ret = Marshal.GetLastWin32Error();
                    Console.WriteLine("LogonUser call failed with error code : " +
                    ret);
                    throw new System.ComponentModel.Win32Exception(ret);
                }

                // Step - 2
                WindowsIdentity newId = new WindowsIdentity(tokenHandle);
                // Step -3
                impersonatedUser = newId.Impersonate();

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


        /// <summary>
        /// Stops impersonation
        /// </summary>
        public void Undo()
        {
            impersonatedUser.Undo();
            // Free the tokens.
            if (tokenHandle != IntPtr.Zero)
                CloseHandle(tokenHandle);
        }
    }