NullReferenceException when trying to populate an empty instance of a custom class with a struct

Question

I have created a custom struct and a class. The structs are points in 3D Space:

public struct Point3D
{
    //fields
    private static Point3D center = new Point3D(0,0,0);

    //properties
    public int X { get; set; }
    public int Y { get; set; }
    public int Z { get; set; }
    public static Point3D Center { get { return center; } }

    //constructors
    public Point3D(int x, int y, int z) : this()
    {
        this.X = x;
        this.Y = y;
        this.Z = z;
    }

    public override string ToString() { return string.Format("({0}; {1}; {2})", this.X, this.Y, this.Z); }
}

and the custom class is path which is supposed to store the points:

public class Path
{
    private List<Point3D> storedPoints = new List<Point3D>();

    public List<Point3D> StoredPoints { get; set; }

    public void AddPoint(Point3D point) { this.StoredPoints.Add(point); }

    public void DeletePointAt(int index) { this.StoredPoints.RemoveAt(index); }

    public void ClearPath() { this.StoredPoints.Clear(); }

    public override string ToString()
    {
        StringBuilder sb = new StringBuilder();
        foreach (var item in this.StoredPoints)
        {
            sb.Append(item);
            sb.Append(System.Environment.NewLine);
        }
        return sb.ToString();
    }
}

I have not created a constructor for the path class as I will always want to have an instance with an empty list List\\ in it. However, when I run the program, I get NullReferenceException. This is the code of the main method:

    static void Main(string[] args)
    {
        Point3D point1 = new Point3D(-2, -4, -10);
        Point3D point2 = new Point3D(6, 7, 8);
        Path path1 = new Path();
        path1.AddPoint(point1);
        path1.AddPoint(point2);
        path1.AddPoint(new Point3D(2, 4, 6));
        path1.AddPoint(new Point3D(-9, 12, 6));
        Console.WriteLine(path1);
    }

I get the error when I try to add the first point. In the debugger I see that the Path object has the value null before adding the first point, but how do I overcome this problem without having to write a constructor, taking as a parameter at least one point, that is, create an empty path.

Answer 1

You have two members storedPoints and StoredPoints that are not related!

You should write out the getter of StoredPoints explicitly and let it return storedPoints .

(Also, the struct you created is a mutable struct. That is considered dangerous by many.)

Answer 2

Your StoredPoints property is alone and not initialized. Your intention is probably that it will get/set your private storedPoints field. Modify the get/set functions for StoredPoints to get/set your private field and you'll solve your problem.

public List<Point3D> StoredPoints 
{ 
  get
  {
    return storedPoints;
  }
}

Edit:

If you eliminate the storedPoints field, and still don't want a constructor, you can do the following:

public void AddPoint(Point3D point) 
{ 
  if (this.StoredPoints == null)
    this.StoredPoints = new List<Point3D>();
  this.StoredPoints.Add(point); 
}

This is called lazy initialization. However, the above implementation is not thread-safe. If you're guaranteed to be single-threaded, then it should be fine. You may want to put similar initialization in other modifiers of StoredPoints . You'll also need to check StoredPoints for null whenever accessing it directly from outside the class.

Edit:

I have not created a constructor for the path class as I will always want to have an instance with an empty list List\\ in it.

One has nothing to do with the other. You can have a constructor and not initialize your list.

Something else to keep in mind: does having a null StoredPoints property actually mean anything different than an empty StoredPoints property? If not, then be safe and initialize StoredPoints to an empty list.

Answer 3

public List<Point3D> StoredPoints { get; set; }

When you are calling AddPoints , you are trying to access a property that has not been initialized. Before you use the StoredPoints property, you must do

StoredPoints = new List<Point3D>();