Sorting of Array Object ?

Sorry, Keith, but you've lost me there.

I just tried the DisplayPrevious() method which I put in and it worked fine, displaying what the Bus fields were before the current update.

Thanks Alan, there's no bug.

But why can't I display the previous updates ?
The purpose of storing previous updates is actually to display current and previous updates. I tried modifying your codes but itdoesn't seem to work, can you help, thanks.

You'll need to introduce some more fields into the Bus class to store the previous values. Here's some code (written very quickly so watch out for possible bugs!):

using System;
using System.Collections; // namespace for IComparer interface
using System.Collections.Generic;
using System.Text;

namespace ConsoleApplication1
{
    class Bus
    {
        int busNo;
        static List busNos = new List(); // to hold existing bus numbers
        string startDepot, endDepot;
        float departTime;
        // fields to hold values for previous updates
        int prevBusNo = -1;
        string prevStartDepot, prevEndDepot;
        float prevDepartTime;
        // flag to record whether info updated more than once
        bool updatedTwice = false;

        public void Update()
        {
            // save current values to 'prev' fields
            prevBusNo = busNo;
            prevStartDepot = startDepot;
            prevEndDepot = endDepot;
            prevDepartTime = departTime;
            if (!updatedTwice) updatedTwice = true;
            // now get new values
            bool uniqueBusNumber = false; // flag to check whether bus number unique
            while(!uniqueBusNumber)
            {
                Console.Write("Enter Bus number: ");
                busNo = int.Parse(Console.ReadLine());
                // it's not a problem if busNo is same as prevBusNo
                // IndexOf returns -1 if busNo not in list
                if (busNos.IndexOf(busNo) > -1 && busNo != prevBusNo)
                {
                   Console.WriteLine("Bus number already used, enter another one");
                }
                else
                {
                   uniqueBusNumber = true;
                   // only add bus number to list if different to prevBusNo
                   if (busNo != prevBusNo)
                   {
                       busNos.Add(busNo); // add new bus number to list
                       busNos.Remove(prevBusNo); // remove previous one
                   }
                }
            }    
            Console.Write("Enter Start depot: ");
            startDepot = Console.ReadLine();
            Console.Write("Enter End depot: ");
            endDepot = Console.ReadLine();
            Console.Write("Enter depart time: ");
            departTime = float.Parse(Console.ReadLine());
        }

        public void Display()
        {
            Console.WriteLine("Bus No. is " + busNo);
            Console.WriteLine("Start depot is " + startDepot);
            Console.WriteLine("End depot is " + endDepot);
            Console.WriteLine("Depart time is " + departTime);
        }

        // insert a new method to display prev values
        public void DisplayPrevious()
        {
            if (updatedTwice)
            {
               Console.WriteLine("Previous Bus No. is " + prevBusNo);
               Console.WriteLine("Previous Start depot is " + prevStartDepot);
               Console.WriteLine("Previous End depot is " + prevEndDepot);
               Console.WriteLine("Previous Depart time is " + prevDepartTime);
            }
            else
            {
               Console.WriteLine("Bus info has only been updated once");
            }
        }
  
        // insert a nested class which implements IComparer
        internal class BusComparer : IComparer
        {
            // this is only method that needs implementing
            // and is used to sort buses by their numbers (ascending)
            public int Compare(object obj1, object obj2)
            {
               Bus bus1 = (Bus)obj1;
               Bus bus2 = (Bus)obj2;
               return bus1.busNo.CompareTo(bus2.busNo);
            }
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            char choice;
            Bus[] group1 = null;
            int num = 0;

            do
            {
               
                Console.WriteLine("a) Update   b) Display  e) Exit");
                Console.Write("Enter choice: ");
                choice = char.Parse(Console.ReadLine());
               
                switch (choice)
                {
                    case 'a':
                        Console.WriteLine("Enter Number of Buses"); // changed text here
                        num = int.Parse(Console.ReadLine());
                        group1 = new Bus[num]; //dynamic sizing

                        for (int row = 0; row < num; row++)
                        {
                            group1[row] = new Bus();
                            group1[row].Update();
                        }
                        break;

                    case 'b':
                        Console.WriteLine("Display all Bus(es)");
                        // sort group1 based on our nested class  
                        Array.Sort(group1, new Bus.BusComparer());
                        for (int row = 0; row < num; row++)
                        {
                            //group1[row] = new Bus();
                            group1[row].Display();
                        }
                        break;

                    // insert new case here to stop it saying no such case
                    case 'e':                                                
                        break;

                    default: Console.WriteLine("No such choice"); break;
                }

            } while (choice != 'e');

            // update values in the group1[0] bus object
            Console.WriteLine("\nUpdate values for first bus :");
            group1[0].Update();
            // now display them
            group1[0].Display();
            // now display previous values
            group1[0].DisplayPrevious();

        }
    }
}

Thanks Alan, your code is great.

Based on the above code, I realised that whenever I do a 2nd update, the information in 1st update will be erased. Is there any way to keep the 1st update ? I want the program to keep the previous update whenever there's a new update. Can you help ? Thanks in advance.

What I'd do is to give the Bus class a static field of type List to record the bus numbers which have been used for Bus objects created so far. You can then make sure that a new bus number is unique by checking it against this List. The input of the bus number needs to be placed in a loop so that you can ask the user to re-enter if in fact a number has been used before. Something like the following:

using System;
using System.Collections; // namespace for IComparer interface
using System.Collections.Generic;
using System.Text;

namespace ConsoleApplication1
{
    class Bus
    {
        int busNo;
        static List busNos = new List(); // to hold existing bus numbers
        string startDepot, endDepot;
        float departTime;

        public void Update()
        {
            bool uniqueBusNumber = false; // flag to check whether bus number unique
            while(!uniqueBusNumber)
            {
                Console.Write("Enter Bus number: ");
                busNo = int.Parse(Console.ReadLine());
                if (busNos.IndexOf(busNo) > -1 ) // IndexOf returns -1 if busNo not in list
                {
                   Console.WriteLine("Bus number already used, enter another one");
                }
                else
                {
                   uniqueBusNumber = true; // stops loop from interating
                   busNos.Add(busNo); // add new bus number to list
                }
            }    
            Console.Write("Enter Start depot: ");
            startDepot = Console.ReadLine();
            Console.Write("Enter End depot: ");
            endDepot = Console.ReadLine();
            Console.Write("Enter depart time: ");
            departTime = float.Parse(Console.ReadLine());
        }

        public void Display()
        {
            Console.WriteLine("Bus No. is " + busNo);
            Console.WriteLine("Start depot is " + startDepot);
            Console.WriteLine("End depot is " + endDepot);
            Console.WriteLine("Depart time is " + departTime);
        }
 
        // insert a nested class which implements IComparer
        internal class BusComparer : IComparer
        {
            // this is only method that needs implementing
            // and is used to sort buses by their numbers (ascending)
            public int Compare(object obj1, object obj2)
            {
               Bus bus1 = (Bus)obj1;
               Bus bus2 = (Bus)obj2;
               return bus1.busNo.CompareTo(bus2.busNo);
            }
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            char choice;
            Bus[] group1 = null;
            int num = 0;

            do
            {
               
                Console.WriteLine("a) Update   b) Display  e) Exit");
                Console.Write("Enter choice: ");
                choice = char.Parse(Console.ReadLine());
               
                switch (choice)
                {
                    case 'a':
                        Console.WriteLine("Enter Number of Buses"); // changed text here
                        num = int.Parse(Console.ReadLine());
                        group1 = new Bus[num]; //dynamic sizing

                        for (int row = 0; row < num; row++)
                        {
                            group1[row] = new Bus();
                            group1[row].Update();
                        }
                        break;

                    case 'b':
                        Console.WriteLine("Display all Bus(es)");
                        // sort group1 based on our nested class  
                        Array.Sort(group1, new Bus.BusComparer());
                        for (int row = 0; row < num; row++)
                        {
                            //group1[row] = new Bus();
                            group1[row].Display();
                        }
                        break;

                    // insert new case here to stop it saying no such case
                    case 'e':                                                
                        break;

                    default: Console.WriteLine("No such choice"); break;
                }

            } while (choice != 'e');

        }
    }
}

To sort the Bus array by bus number, you need to add some functionality to the Bus class, namely a nested class BusComparer which implements the IComparer interface.

Here's the revised program. I've taken the opportunity to correct or clarify  a couple of small points I noticed when I tried to run it myself:

using System;
using System.Collections; // add namespace for IComparer interface
using System.Collections.Generic;
using System.Text;

namespace ConsoleApplication1
{
    class Bus
    {
        int busNo;
        string startDepot, endDepot;
        float departTime;

        public void Update()
        {
            Console.Write("Enter Bus number: "); // changed text for this line
            busNo = int.Parse(Console.ReadLine());
            Console.Write("Enter Start depot: ");
            startDepot = Console.ReadLine();
            Console.Write("Enter End depot: ");
            endDepot = Console.ReadLine();
            Console.Write("Enter depart time: ");
            departTime = float.Parse(Console.ReadLine());
        }

        public void Display()
        {
            Console.WriteLine("Bus No. is " + busNo);
            Console.WriteLine("Start depot is " + startDepot);
            Console.WriteLine("End depot is " + endDepot);
            Console.WriteLine("Depart time is " + departTime);
        }
 
        // insert a nested class which implements IComparer
        internal class BusComparer : IComparer
        {
            // this is the only method that needs implementing
            // and is used to sort buses by their numbers (ascending)
            public int Compare(object obj1, object obj2)
            {
               Bus bus1 = (Bus)obj1;
               Bus bus2 = (Bus)obj2;
               return bus1.busNo.CompareTo(bus2.busNo);
            }
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            char choice;
            Bus[] group1 = null;
            int num = 0;

            do
            {
               
                Console.WriteLine("a) Update   b) Display  e) Exit");
                Console.Write("Enter choice: ");
                choice = char.Parse(Console.ReadLine());
               
                switch (choice)
                {
                    case 'a':
                        Console.WriteLine("Enter Number of Buses"); // changed text here
                        num = int.Parse(Console.ReadLine());
                        group1 = new Bus[num]; //dynamic sizing

                        for (int row = 0; row < num; row++)
                        {
                            group1[row] = new Bus();
                            group1[row].Update();
                        }
                        break;

                    case 'b':
                        Console.WriteLine("Display all Bus(es)");
                        // sort group1 based on our nested class  
                        Array.Sort(group1, new Bus.BusComparer());               

                        for (int row = 0; row < num; row++)
                        {
                            //group1[row] = new Bus();
                            group1[row].Display();
                        }
                        break;

                    // insert new case here to stop it saying no such choice when exit pressed
                    case 'e':                                                
                        break;

                    default: Console.WriteLine("No such choice"); break;
                }

            } while (choice != 'e');

        }
    }
}