Can't figure out why Object reference is null

Question

Working on a program for class, and am about 95% complete, but have run into a roadblock. I've got a Flight class that holds information about the flight, as well as a seating chart. Using a windows form listbox to select from the flight objects I created by reading from a text file. I can get values for every property from the class object, except for one, SeatChart.

Here's the pertinent code in the main program:

    private void lstFlights_SelectedIndexChanged(object sender, EventArgs e)
    {
        curFlight = (Flight)lstFlights.SelectedItem;
        DisplayNewFlightChart();
    }

    private void DisplayNewFlightChart()
    {

        int seats = curFlight.Rows * curFlight.Seats;
        lstSeatingChart.Items.Clear();
        string[] seatChart = curFlight.SeatChart;
        for (int x = 0; x <= seats; x++)
        {
            lstSeatingChart.Items.Add("Seat " + (x + 1) + " " + seatChart[x]);
        }
    }

And here is the code from the class:

class Flight
{
    private string mPlane;
    private string mDepartureTime;
    private string mDestination;
    private int mRows;
    private int mSeats;
    private string[] mSeatChart;

    public Flight()
    {
    }

    // Create the overloaded Constructor
    public Flight(string planeType, string departureTime,
                  string destination, int numRows,
                  int numSeatsPerRow)
    {
        this.Plane = planeType;
        this.DepartureTime = departureTime;
        this.Destination = destination;
        this.Rows = numRows;
        this.Seats = numSeatsPerRow;
        this.SeatChart = mSeatChart;


        mSeatChart = new string[Rows * Seats];

        for (int seat = 0; seat <= mSeatChart.GetUpperBound(0); seat++)
        {
            mSeatChart[seat] = "Open";
        }



    }

    public string Plane
    {
        get { return mPlane; }
        set { mPlane = value; }
    }

    public string DepartureTime
    {
        get { return mDepartureTime; }
        set { mDepartureTime = value; }
    }

    public string Destination
    {
        get { return mDestination; }
        set { mDestination = value; }
    }

    public int Rows
    {
        get { return mRows; }
        set { mRows = value; }
    }

    public int Seats
    {
        get { return mSeats; }
        set { mSeats = value; }
    }

    public string[] SeatChart
    {
        get { return mSeatChart; }
        set { mSeatChart = value; }
    }

    public void MakeReservation(string passName, int seat)
    {
        bool seatTaken = false;
        if (mSeatChart[seat] != "Open") seatTaken = true;

        if (passName != "" && seatTaken == false)
        {
            mSeatChart[seat] = passName;
        }
        else
        {
            MessageBox.Show("Please Enter a Passenger Name, in an unreserved seat");
        }

    }

It's telling me the curFlight.SeatChart is null, even though I can pull .Rows and .Seats from the curFlight just fine. I have no clue why .SeatChart is messing up. lstFlights is the list of flight objects pulled from the text file, and lstSeatingChart is where I want to display a list of seats.

Answer 1

You are setting SeatChart to mSeatChart, which is null at that time. So no reference to an object is made for this.SeatChart.

After that you initialize mSeatChart and fill it.

You should move setting this.SeatChart after initializing mSeatChart.

mSeatChart = new string[Rows * Seats];
this.SeatChart = mSeatChart;

Edit: In addition, SeatChart is the property and mSeatChart is the member variable. SeatChart will be used to expose mSeatChart, so it's really weird to set SeatChart with mSeatChart. So weird that I didn't even think you were doing that. So in your case leave the following out in the constructor:

this.SeatChart = mSeatChart;

I think the actual cause of your issue is somewhere else in the code, where you initiate Flight and fill the list. If I understand correctly you get a null reference error on the concatenation in the for loop?

    string[] seatChart = curFlight.SeatChart;
    for (int x = 0; x <= seats; x++)
    {
        lstSeatingChart.Items.Add("Seat " + (x + 1) + " " + seatChart[x]);
    }

Check where you initate each Flight object. I bet you are using the empty constructor: new Flight() Remove the empty constructor, because you don't expect empty values apparently. And if you really need the empty constructor then either initiate all member variables as expected or perform a null check wherever you want to use them.

And once you found the cause, make sure you change the for loop to

    for (int x = 0; x < seats; x++)

since you are checking for the number of seats and do a zero-based loop. If x = seats you would have performed the loop seats + 1 times (rows*seats + 1).

Answer 2

If your code relies on a particular property never being null, you need to make sure it is initialized in all constructors.

Based on the logic of your class, I would suggest you shouldn't have a parameter less constructor. It doesn't make sense to have a flight that didn't have a known number of seats (in your implementation at least).

Also some style things.

You don't need to declare your private instance variables. Just use

    public string destination {get; set;}

Declare "open" as a class constant and use that constant rather than the hard coded string value.