Как вернуться из класса Membership?

См.: Как: пример реализации поставщика членства

Проверьте этот фрагмент кода

public override MembershipUser CreateUser(string username,
             string password,
             string email,
             string passwordQuestion,
             string passwordAnswer,
             bool isApproved,
             object providerUserKey,
             out MembershipCreateStatus status)
    {
      ValidatePasswordEventArgs args = 
        new ValidatePasswordEventArgs(username, password, true);

      OnValidatingPassword(args);

      if (args.Cancel)
      {
        status = MembershipCreateStatus.InvalidPassword;
        return null;
      }



      if (RequiresUniqueEmail && GetUserNameByEmail(email) != "")
      {
        status = MembershipCreateStatus.DuplicateEmail;
        return null;
      }

      MembershipUser u = GetUser(username, false);

      if (u == null)
      {
        DateTime createDate = DateTime.Now;

        if (providerUserKey == null)
        {
          providerUserKey = Guid.NewGuid();
        }
        else
        {
          if ( !(providerUserKey is Guid) )
          {
            status = MembershipCreateStatus.InvalidProviderUserKey;
            return null;
          }
        }

        OdbcConnection conn = new OdbcConnection(connectionString);
        OdbcCommand cmd = new OdbcCommand("INSERT INTO Users " +
              " (PKID, Username, Password, Email, PasswordQuestion, " +
              " PasswordAnswer, IsApproved," +
              " Comment, CreationDate, LastPasswordChangedDate, LastActivityDate," +
              " ApplicationName, IsLockedOut, LastLockedOutDate," +
              " FailedPasswordAttemptCount, FailedPasswordAttemptWindowStart, " +
              " FailedPasswordAnswerAttemptCount, FailedPasswordAnswerAttemptWindowStart)" +
              " Values(?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?)", conn);

        cmd.Parameters.Add("@PKID", OdbcType.UniqueIdentifier).Value = providerUserKey;
        cmd.Parameters.Add("@Username", OdbcType.VarChar, 255).Value = username;
        cmd.Parameters.Add("@Password", OdbcType.VarChar, 255).Value = EncodePassword(password);
        cmd.Parameters.Add("@Email", OdbcType.VarChar, 128).Value = email;
        cmd.Parameters.Add("@PasswordQuestion", OdbcType.VarChar, 255).Value = passwordQuestion;
        cmd.Parameters.Add("@PasswordAnswer", OdbcType.VarChar, 255).Value = EncodePassword(passwordAnswer);
        cmd.Parameters.Add("@IsApproved", OdbcType.Bit).Value = isApproved;
        cmd.Parameters.Add("@Comment", OdbcType.VarChar, 255).Value = "";
        cmd.Parameters.Add("@CreationDate", OdbcType.DateTime).Value = createDate;
        cmd.Parameters.Add("@LastPasswordChangedDate", OdbcType.DateTime).Value = createDate;
        cmd.Parameters.Add("@LastActivityDate", OdbcType.DateTime).Value = createDate;
        cmd.Parameters.Add("@ApplicationName", OdbcType.VarChar, 255).Value = pApplicationName;
        cmd.Parameters.Add("@IsLockedOut", OdbcType.Bit).Value = false;
        cmd.Parameters.Add("@LastLockedOutDate", OdbcType.DateTime).Value = createDate;
        cmd.Parameters.Add("@FailedPasswordAttemptCount", OdbcType.Int).Value = 0;
        cmd.Parameters.Add("@FailedPasswordAttemptWindowStart", OdbcType.DateTime).Value = createDate;
        cmd.Parameters.Add("@FailedPasswordAnswerAttemptCount", OdbcType.Int).Value = 0;
        cmd.Parameters.Add("@FailedPasswordAnswerAttemptWindowStart", OdbcType.DateTime).Value = createDate;

        try
        {
          conn.Open();

          int recAdded = cmd.ExecuteNonQuery();

          if (recAdded > 0)
          {
            status = MembershipCreateStatus.Success;
          }
          else
          {
            status = MembershipCreateStatus.UserRejected;
          }
        }
        catch (OdbcException e)
        {
          if (WriteExceptionsToEventLog)
          {
            WriteToEventLog(e, "CreateUser");
          }

          status = MembershipCreateStatus.ProviderError;
        }
        finally
        {
          conn.Close();
        }


        return GetUser(username, false);      
      }        
      else
      {
        status = MembershipCreateStatus.DuplicateUserName;
      }


      return null;
    }

/// //
// MembershipProvider.GetUser(string, bool) //

public override MembershipUser GetUser(string username, bool userIsOnline)
{
   OdbcConnection conn = new OdbcConnection(connectionString);
   OdbcCommand cmd = new OdbcCommand("SELECT PKID, Username, Email, PasswordQuestion," +
        " Comment, IsApproved, IsLockedOut, CreationDate, LastLoginDate," +
        " LastActivityDate, LastPasswordChangedDate, LastLockedOutDate" +
        " FROM Users WHERE Username = ? AND ApplicationName = ?", conn);

  cmd.Parameters.Add("@Username", OdbcType.VarChar, 255).Value = username;
  cmd.Parameters.Add("@ApplicationName", OdbcType.VarChar, 255).Value = pApplicationName;

  MembershipUser u = null;
  OdbcDataReader reader = null;

  try
  {
    conn.Open();

    reader = cmd.ExecuteReader();

    if (reader.HasRows)
    {
      reader.Read();
      u = GetUserFromReader(reader);

      if (userIsOnline)
      {
        OdbcCommand updateCmd = new OdbcCommand("UPDATE Users " +
                  "SET LastActivityDate = ? " +
                  "WHERE Username = ? AND Applicationname = ?", conn);

        updateCmd.Parameters.Add("@LastActivityDate", OdbcType.DateTime).Value = DateTime.Now;
        updateCmd.Parameters.Add("@Username", OdbcType.VarChar, 255).Value = username;
        updateCmd.Parameters.Add("@ApplicationName", OdbcType.VarChar, 255).Value = pApplicationName;

        updateCmd.ExecuteNonQuery();
      }
    }

  }
  catch (OdbcException e)
  {
    if (WriteExceptionsToEventLog)
    {
      WriteToEventLog(e, "GetUser(String, Boolean)");

      throw new ProviderException(exceptionMessage);
    }
    else
    {
      throw e;
    }
  }
  finally
  {
    if (reader != null) { reader.Close(); }

    conn.Close();
  }

  return u;      
}

Подробную информацию см. в документации: Реализация членства Провайдер

Пример кода для создания пользовательского поставщика членства