ASP.NET - Forms authentication user impersonation

using System;
using System.Xml;
using System.Collections.Generic;
using System.Collections.Specialized;
using System.Configuration.Provider;
using System.Web.Security;
using System.Web.Hosting;
using System.Web.Management;
using System.Security.Permissions;
using System.Web;

public class ReadOnlyXmlMembershipProvider : MembershipProvider
{
    private Dictionary<string, MembershipUser> _Users;
    private string _XmlFileName;

    // MembershipProvider Properties
    public override string ApplicationName
    {
        get { throw new NotSupportedException(); }
        set { throw new NotSupportedException(); }
    }

    public override bool EnablePasswordRetrieval
    {
        get { return false; }
    }

    public override bool EnablePasswordReset
    {
        get { return false; }
    }

    public override int MaxInvalidPasswordAttempts
    {
        get { throw new NotSupportedException(); }
    }

    public override int MinRequiredNonAlphanumericCharacters
    {
        get { throw new NotSupportedException(); }
    }

    public override int MinRequiredPasswordLength
    {
        get { throw new NotSupportedException(); }
    }

    public override int PasswordAttemptWindow
    {
        get { throw new NotSupportedException(); }
    }

    public override MembershipPasswordFormat PasswordFormat
    {
        get { throw new NotSupportedException(); }
    }

    public override string PasswordStrengthRegularExpression
    {
        get { throw new NotSupportedException(); }
    }

    public override bool RequiresQuestionAndAnswer
    {
        get { throw new NotSupportedException(); }
    }

    public override bool RequiresUniqueEmail
    {
        get { throw new NotSupportedException(); }
    }

    // MembershipProvider Methods
    public override void Initialize(string name,
        NameValueCollection config)
    {
        // Verify that config isn't null
        if (config == null)
            throw new ArgumentNullException("config");

        // Assign the provider a default name if it doesn't have one
        if (String.IsNullOrEmpty(name))
            name = "ReadOnlyXmlMembershipProvider";

        // Add a default "description" attribute to config if the
        // attribute doesn't exist or is empty
        if (string.IsNullOrEmpty(config["description"]))
        {
            config.Remove("description");
            config.Add("description",
                "Read-only XML membership provider");
        }

        // Call the base class's Initialize method
        base.Initialize(name, config);

        // Initialize _XmlFileName and make sure the path
        // is app-relative
        string path = config["xmlFileName"];

        if (String.IsNullOrEmpty(path))
            path = "~/App_Data/Users.xml";

        if (!VirtualPathUtility.IsAppRelative(path))
            throw new ArgumentException
                ("xmlFileName must be app-relative");

        string fullyQualifiedPath = VirtualPathUtility.Combine
            (VirtualPathUtility.AppendTrailingSlash
            (HttpRuntime.AppDomainAppVirtualPath), path);

        _XmlFileName = HostingEnvironment.MapPath(fullyQualifiedPath);
        config.Remove("xmlFileName");

        // Make sure we have permission to read the XML data source and
        // throw an exception if we don't
        FileIOPermission permission =
            new FileIOPermission(FileIOPermissionAccess.Read,
            _XmlFileName);
        permission.Demand();

        // Throw an exception if unrecognized attributes remain
        if (config.Count > 0)
        {
            string attr = config.GetKey(0);
            if (!String.IsNullOrEmpty(attr))
                throw new ProviderException
                    ("Unrecognized attribute: " + attr);
        }
    }

    public override bool ValidateUser(string username, string password)
    {
        // Validate input parameters
        if (String.IsNullOrEmpty(username) ||
            String.IsNullOrEmpty(password))
            return false;

        try
        {
            // Make sure the data source has been loaded
            ReadMembershipDataStore();

            // Validate the user name and password
            MembershipUser user;
            if (_Users.TryGetValue(username, out user))
            {
                if (user.Comment == password) // Case-sensitive
                {
                    // NOTE: A read/write membership provider
                    // would update the user's LastLoginDate here.
                    // A fully featured provider would also fire
                    // an AuditMembershipAuthenticationSuccess
                    // Web event
                    return true;
                }
            }

            // NOTE: A fully featured membership provider would
            // fire an AuditMembershipAuthenticationFailure
            // Web event here
            return false;
        }
        catch (Exception)
        {
            return false;
        }
    }

    public override MembershipUser GetUser(string username,
        bool userIsOnline)
    {
        // Note: This implementation ignores userIsOnline

        // Validate input parameters
        if (String.IsNullOrEmpty(username))
            return null;

        // Make sure the data source has been loaded
        ReadMembershipDataStore();

        // Retrieve the user from the data source
        MembershipUser user;
        if (_Users.TryGetValue(username, out user))
            return user;

        return null;
    }

    public override MembershipUserCollection GetAllUsers(int pageIndex,
        int pageSize, out int totalRecords)
    {
        // Note: This implementation ignores pageIndex and pageSize,
        // and it doesn't sort the MembershipUser objects returned

        // Make sure the data source has been loaded
        ReadMembershipDataStore();

        MembershipUserCollection users =
            new MembershipUserCollection();

        foreach (KeyValuePair<string, MembershipUser> pair in _Users)
            users.Add(pair.Value);

        totalRecords = users.Count;
        return users;
    }

    public override int GetNumberOfUsersOnline()
    {
        throw new NotSupportedException();
    }

    public override bool ChangePassword(string username,
        string oldPassword, string newPassword)
    {
        throw new NotSupportedException();
    }

    public override bool
        ChangePasswordQuestionAndAnswer(string username,
        string password, string newPasswordQuestion,
        string newPasswordAnswer)
    {
        throw new NotSupportedException();
    }

    public override MembershipUser CreateUser(string username,
        string password, string email, string passwordQuestion,
        string passwordAnswer, bool isApproved, object providerUserKey,
        out MembershipCreateStatus status)
    {
        throw new NotSupportedException();
    }

    public override bool DeleteUser(string username,
        bool deleteAllRelatedData)
    {
        throw new NotSupportedException();
    }

    public override MembershipUserCollection
        FindUsersByEmail(string emailToMatch, int pageIndex,
        int pageSize, out int totalRecords)
    {
        throw new NotSupportedException();
    }

    public override MembershipUserCollection
        FindUsersByName(string usernameToMatch, int pageIndex,
        int pageSize, out int totalRecords)
    {
        throw new NotSupportedException();
    }

    public override string GetPassword(string username, string answer)
    {
        throw new NotSupportedException();
    }

    public override MembershipUser GetUser(object providerUserKey,
        bool userIsOnline)
    {
        throw new NotSupportedException();
    }

    public override string GetUserNameByEmail(string email)
    {
        throw new NotSupportedException();
    }

    public override string ResetPassword(string username,
        string answer)
    {
        throw new NotSupportedException();
    }

    public override bool UnlockUser(string userName)
    {
        throw new NotSupportedException();
    }

    public override void UpdateUser(MembershipUser user)
    {
        throw new NotSupportedException();
    }

    // Helper method
    private void ReadMembershipDataStore()
    {
        lock (this)
        {
            if (_Users == null)
            {
                _Users = new Dictionary<string, MembershipUser>
                   (16, StringComparer.InvariantCultureIgnoreCase);
                XmlDocument doc = new XmlDocument();
                doc.Load(_XmlFileName);
                XmlNodeList nodes = doc.GetElementsByTagName("User");

                foreach (XmlNode node in nodes)
                {
                    MembershipUser user = new MembershipUser(
                        Name,                       // Provider name
                        node["UserName"].InnerText, // Username
                        null,                       // providerUserKey
                        node["EMail"].InnerText,    // Email
                        String.Empty,               // passwordQuestion
                        node["Password"].InnerText, // Comment
                        true,                       // isApproved
                        false,                      // isLockedOut
                        DateTime.Now,               // creationDate
                        DateTime.Now,               // lastLoginDate
                        DateTime.Now,               // lastActivityDate
                        DateTime.Now, // lastPasswordChangedDate
                        new DateTime(1980, 1, 1)    // lastLockoutDate
                    );

                    _Users.Add(user.UserName, user);
                }
            }
        }
    }
}