Microsoft Interview Question for SDE-2s


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in C#, a quick console application:

class Program
    {
        
        class ObjStatus
        {
            public status Status { get; set; }
            public int ReadCount { get; set; }
        }
        enum status
        {
            none,
            reading,
            writing
        }

        static ObjStatus objStatus = new ObjStatus();

        static void Main(string[] args)
        {
            Thread t1 = new Thread(Read);
            Thread t2 = new Thread(Write);
            Thread t3 = new Thread(Read);
            Thread t4 = new Thread(Write);
            Thread t5 = new Thread(Read);
            Thread t6 = new Thread(Write);
            t1.Start();
            t2.Start();
            t3.Start();
            t4.Start();
            t5.Start();
            t6.Start();
        }

        static void Read()
        {
            //Console.WriteLine("Read starting");
            WaitForAccess(status.reading);
            Console.WriteLine("Read access confirmed");

            //do stuff
            Thread.Sleep(GetSleepTime());

            Console.WriteLine("Read ended - pulsing");

            UpdateAccess(status.reading);

            Console.WriteLine("Read ended - finished");

        }

        static void Write()
        {
            //this is only for writing
            //Console.WriteLine("Write started");

            WaitForAccess(status.writing);

            Console.WriteLine("Write access confirmed");

            //write
            Thread.Sleep(GetSleepTime());

            UpdateAccess(status.writing);

            Console.WriteLine("Write ended");
        }

        private static void UpdateAccess(status accessFinished)
        {
            lock (objStatus)
            {
                if (accessFinished == status.writing)
                {
                    objStatus.Status = status.none;
                    objStatus.ReadCount = 0;
                }
                else
                {
                    objStatus.ReadCount--;

                    if (objStatus.ReadCount == 0)
                    {
                        //we waited, we got the lock, now update
                        objStatus.Status = status.none;
                    }
                }
                Monitor.PulseAll(objStatus);
            }
        }

        private static void WaitForAccess(status accessWanted)
        {
            lock (objStatus)
            {
                if(accessWanted == status.writing)
                {
                    //wants to write, needs to be "none"
                    while(objStatus.Status != status.none)
                    {
                        Monitor.Wait(objStatus);
                    }

                    //we waited, we got the lock, now update
                    objStatus.Status = status.writing;
                    objStatus.ReadCount = 0;
                }
                else
                {
                    //wants to read.
                    //needs to be either read or none
                    while (objStatus.Status == status.writing)
                    {
                        Monitor.Wait(objStatus);
                    }

                    //we waited, we got the lock, now update
                    objStatus.Status = status.reading;
                    objStatus.ReadCount++;
                }
            }
        }

        private static int GetSleepTime()
        {
            return new Random().Next(1500)+500;
        }

    }

- Diego August 01, 2019 | Flag Reply


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