C# NullReferenceException was unhandled - Object reference not set to an instance of an object

问题

I'm trying to create a application using Windows Forms in C#, however I keep getting an error stating:

NullReferenceException was unhandled - Object reference not set to an instance of an object

It's pointing to the line which has this code on:

carBootSaleList.AddCarBootSale(newCarBootSale);

The method I have on the forms interface:

  CarBootSaleList carBootSaleList;

  public void AddCarBootSale()
    {
        AddNewCarBootSale addForm = new AddNewCarBootSale();
        if (addForm.ShowDialog() == DialogResult.OK)
        {
            CarBootSale newCarBootSale = addForm.GetCarBootSaleData();

            carBootSaleList.AddCarBootSale(newCarBootSale);

            txtCarBootSale.Clear();
            txtCarBootSale.Text = newCarBootSale.Display();
        }
    }

The CarBootSaleList class:

public class CarBootSaleList : IDisplay
{

    private List<CarBootSale> carbootsales;

    public CarBootSaleList()
    {
        carbootsales = new List<CarBootSale>();
    }

    public bool AddCarBootSale(CarBootSale carbootsale)
    {
        bool success = true;
        foreach (CarBootSale cbs in carbootsales)
        {
            if (cbs.ID == carbootsale.ID)
            {
                success = false;
            }
        }
        if (success)
        {
            carbootsales.Add(carbootsale);
        }
        return success;
    }

    public int GetListSize()
    {
        return carbootsales.Count();
    }

    //public CarBootSale FindCarBootSale(string cbsID)
    //{
    //    CarBootSale carbootsale = null;
    //    foreach (CarBootSale cbs in carbootsale)
    //    {
    //        if (cbs.ID == cbsID)
    //        {
    //            carbootsale = cbs;
    //        }
    //    }
    //    return carbootsale;
    //}

    public List<CarBootSale> ReturnList()
    {
        return carbootsales;
    }

    public string Display()
    {
        string msg = "";

        foreach (CarBootSale cbs in carbootsales)
        {
            msg += String.Format("{0}  {1}", cbs.ID, cbs.Location, cbs.Date);
            msg += Environment.NewLine;
        }
        return msg;
    }
}

The CarBootSale class:

public class CarBootSale : IDisplay
{

    public string ID { get; set; }
    public string Date { get; set; }
    public string Location { get; set; }
    public double PitchCost { get; set; }
    public int Capacity { get; set; }
    public string Charity { get; set; }
    public string CharityName { get; set; }
    public string Catering { get; set; }

    public CarBootSale(string id, string date, string location, double pitchcost, int capacity, string charity, string charityname, string catering)
    {
        ID = id;
        Date = date;
        Location = location;
        PitchCost = pitchcost;
        Capacity = capacity;
        Charity = charity;
        CharityName = charityname;
        Catering = catering;
    }

    public CarBootSale() { }

    public override string ToString()
    {
        return String.Format("{0}  {1}", ID, Date, Location);
    }

    public string Display()
    {
        string msg;
        string CR = Environment.NewLine;

        msg = String.Format("ID: {0} {1}", ID, CR);
        msg += String.Format(" Date: {0} {1}", Date, CR);
        msg += String.Format(" Location: {0} {1}", Location, CR);
        msg += String.Format(" Pitch Cost: {0} {1}", PitchCost, CR);
        msg += String.Format(" Capacity: {0} {1}", Capacity, CR);
        msg += String.Format(" Charity: {0} {1}", Charity, CR);
        msg += String.Format(" CharityName: {0} {1}", CharityName, CR);
        msg += String.Format(" Catering: {0} {1}", Catering, CR);

        return msg;
    }
}

回答1:

The variable carBootSaleList wasn't initialized, and it is therefore null.

Try replacing

CarBootSaleList carBootSaleList;

with

CarBootSaleList carBootSaleList = new CarBootSaleList();

回答2:

You haven't initialized carBootSaleList in your code anywhere.

You can either instantiate it in the method or with the declaration.

CarBootSaleList carBootSaleList = new CarBootSaleList();

回答3:

You have not initialized carBootSaleList.

CarBootSaleList carBootSaleList = new carBootSaleList();

回答4:

carBootSaleList is null.

somewhere, type :

 carBootSaleList = new CarBootSaleList();

来源：https://stackoverflow.com/questions/15112456/c-sharp-nullreferenceexception-was-unhandled-object-reference-not-set-to-an-in