Entity Framework Update Entity Props - not whole Entity, just some props

UpdateEntityProps.cs

            using (SECEntities dbCtx = new SECEntities())
            {
                dbCtx.Configuration.LazyLoadingEnabled = false;
                dbCtx.Configuration.ProxyCreationEnabled = false; //NO dynamic PROXY, want strongly typed entities !

                //select * from TB_ADMIN_UserSession where accessToken = '12345678-...' and getDate() <= expireAt 
                var qry = dbCtx.TB_ADMIN_UserSession.Where(x => x.accessToken == token && DateTime.Now <= x.expireAt);
                var TB_SessionFound = qry
                                        .AsNoTracking() //readonly ! (faster)
                                        .SingleOrDefault();

                if (TB_SessionFound != null)
                {
                    //Extend expiry date from NOW + 20 mins
                    DateTime newExpiry = DateTime.Now.AddMinutes(20);
                    //remove annoying millisecs etc.
                    newExpiry = new DateTime(newExpiry.Year, newExpiry.Month, newExpiry.Day, newExpiry.Hour, newExpiry.Minute, newExpiry.Second);

                    //then re-SAVE the extended time... //BUT NOTE that TB_SessionFound is DISCONNECTED ( .AsNoTracking used above); 
                    var dbEntityEntry = dbCtx.Entry(TB_SessionFound); //SO YOU NEED TO RE-ATTACH
                    dbEntityEntry.State = EntityState.Unchanged; //change from .Detached (not tracked) to .Unchanged (tracked)
                    var TB_newSession = dbEntityEntry.Entity;  
                    TB_newSession.expireAt = newExpiry; //.. AND THEN UPDATE the prop or props to be updated (AFTER ATTACHING)

                    dbCtx.SaveChanges(); //.. only then you can Save !!! 
                    /* OR in EF6, you can log the statements generated as follows:
                            using (var sqlLogFile = new StreamWriter("C:\\temp\\EFlog.txt"))
                            {
                                dbCtx.Database.Log = sqlLogFile.Write;
                                dbCtx.SaveChanges();
                            }
                    --> file contents as per below:
                    Opened connection at 28/10/2019 3:43:29 PM +11:00
                    Started transaction at 28/10/2019 3:43:29 PM +11:00
                    UPDATE [dbo].[TB_ADMIN_UserSession]
                    SET [expireAt] = @0
                    WHERE ([accessToken] = @1)
                    -- @0: '28/10/2019 4:03:02 PM' (Type = DateTime2)
                    -- @1: '51c58a3b-5402-4d50-a8b8-e69abd45a880' (Type = Guid)
                    -- Executing at 28/10/2019 3:43:29 PM +11:00
                    -- Completed in 11 ms with result: 1

                    Committed transaction at 28/10/2019 3:43:29 PM +11:00
                    Closed connection at 28/10/2019 3:43:29 PM +11:00
                    */

                    return newExpiry;

                    /* TODO.
                    var rtnObj = new UserInfo()
                    {
                         NewExpiry = newExpiry,
                          UserID = TB_SessionFound.TB_ADMIN_USER_MASTER.userID,
                           TenantID = TB_SessionFound.TB_ADMIN_USER_MASTER.ten
                    }*/
                    
                }
                else
                {
                    return new DateTime(1970, 1, 1); //Not valid (Token Not Found, or expired).
                }

            } //end-Using




//--- ANOTHER EXAMPLE
        public static string ChangePwd(int userID, string oldPwd, string newPwd)
        {

            //-- step#1. verify that oldPwd IS VALID FOR current User
            string errMsgOut = "";
            if (!isPwdValid(userID, oldPwd))
            {
                return "Password change failed; current (old) password is incorrect.";
            }



            //-- step#2. Validate New Pwd rules:
            errMsgOut = "";
            if (!Utils.IsPasswordValid(newPwd, out errMsgOut))
            {
                return errMsgOut;
            }


            //================ YAY !! Proceed to persist changes to DB  =================
            // Hash / Salt, etc and INSERT INTO DB *** Actually leave Salt out, ran out of time, do the quick and dirty + reuse Cix-V1 code.
            //var hashStruct = Crypto.CreateHash(newPwd); ****

            var hashedPwd = RandomPasswordGenerator.encode(newPwd); //newPwd is clear-text  at this stage, but hashedPwd is the (unclear) text !
                                                                    //.encode() is a misnomer, it's actually doing a (one-way) Hash, rather than Encoding 
            //==============================

            //-- step#3. UPDATE row with new PWD ...
            int rowsAffected = -1;
            DateTime currDt = DateTime.Now;
            using (SECEntities dbCtx = new SECEntities())
            {
                dbCtx.Configuration.LazyLoadingEnabled = false;
                dbCtx.Configuration.ProxyCreationEnabled = false; //NO dynamic PROXY, want strongly typed entities !
                dbCtx.Configuration.ValidateOnSaveEnabled = false; //SO the Entity to Update can be a "PATCH", i.e. dont provide all mandatory fields, only the ones you want.

                TB_ADMIN_USER_MASTER Tb_userEntity = new TB_ADMIN_USER_MASTER()
                {
                    userID = userID, //IDENTITY / PK.
                    pwd = hashedPwd, //Save Hashed-PWD to DB.

                    lastPwdChange = currDt,
                    updatedAt = currDt,
                    updatedBy = userID //updated by the User itself which is logged in.
                };

                using (var sqlLogSW = new StreamWriter(@"C:\TMP\EFSqlLog.txt"))
                {
                    dbCtx.Database.Log = sqlLogSW.Write;
                    //Above sets up the loggin mechanism...
                    dbCtx.TB_ADMIN_USER_MASTER.Attach(Tb_userEntity); //State= Unchanged
                    var myState = dbCtx.Entry(Tb_userEntity).State;

                    //dbCtx.Entry(Tb_userEntity).Property("pwd").IsModified = true; //only this prop is dirty, so Generated SQL UPDATE STATEMENT only updates this field.
                    dbCtx.Entry(Tb_userEntity).Property(x => x.pwd).IsModified = true; //prefer to above line, strongly typed !
                    dbCtx.Entry(Tb_userEntity).Property(x => x.lastPwdChange).IsModified = true; //prefer to above line, strongly typed !
                    dbCtx.Entry(Tb_userEntity).Property(x => x.updatedAt).IsModified = true; //prefer to above line, strongly typed !
                    dbCtx.Entry(Tb_userEntity).Property(x => x.updatedBy).IsModified = true; //prefer to above line, strongly typed !
                    //**** ARRRRHHHHHHH FARKKKK - ONLY THE PROPS explicitly modified will appear in SQL STATEMENT ****.. wild goose chase ***
                    var myState2 = dbCtx.Entry(Tb_userEntity).State;
                    var myState3 = dbCtx.Entry(Tb_userEntity).Property(x => x.email).IsModified; //verify
                    
                    rowsAffected = dbCtx.SaveChanges();
                    /* ERROR (1)
                        Member 'IsModified' cannot be called for property 'pwd' because the entity of type 'TB_ADMIN_USER_MASTER' does not exist in the context. To add an entity to the context call the Add or Attach method of DbSet<TB_ADMIN_USER_MASTER>.
                   
                ERROR (2)
                    Entity of type TB_ADMIN_USER_MASTER in state Modified has the following validation errors:
                        - Property: firstName, Error: The firstName field is required.
                        - Property: lastName, Error: The lastName field is required.
                        - Property: loginName, Error: The loginName field is required.
                        - Property: email, Error: The email field is required.
                    , *** unless you set dbCtx.Configuration.ValidateOnSaveEnabled = FALSE ****
                 * 
                 * YAY! log confirms the UPDATE sql statement is the "partial/patch" one that we want few fields only;
                            UPDATE [dbo].[TB_ADMIN_USER_MASTER]
                            SET [pwd]       = @0
                            WHERE ([userID] = @1)
                            -- @0: 'cjhweXBKaFgrNlZHTnIwOS9maDB1dGErYTBZPQ==' (Type = AnsiString, Size = 400)
                            -- @1: '18' (Type = Int32)

                    OR: (after adding additional fields 2,3,4)
                        UPDATE [dbo].[TB_ADMIN_USER_MASTER]
                        SET [pwd] = @0, [lastPwdChange] = @1, [updatedAt] = @2, [updatedBy] = @3
                        WHERE ([userID] = @4)
                        -- @0: 'YlJiVVNHaXNUVzNudjNvL3d6R2lrcDZRbFI0PQ==' (Type = AnsiString, Size = 400)
                        -- @1: '8/03/2020 6:16:37 AM' (Type = DateTime2)
                        -- @2: '8/03/2020 6:16:37 AM' (Type = DateTime2)
                        -- @3: '18' (Type = Int32)
                        -- @4: '18' (Type = Int32)

                 *  FURTHERMORE, stackoverflow says:
                 *  I think is worth mention that if you are using db.Configuration.ValidateOnSaveEnabled = false, 
                 *  ; you might want to keep validating the field you are updating,
                 *  ; if (db.Entry(user).Property(x => x.pwd).GetValidationErrors().Count == 0) for all fields.
                 
                 
                 */
                }
            }

            if (rowsAffected > 0)
            {
                return "success";
            }
            else
            {
                return "Failed to change password.";
            }
            //...OR OTHER ERROR is handled by the caller.
        }