所以我每次运行此命令时都会出错 System.NullReferenceException Ball.boxContent.get返回null
我在此行收到此错误 box1.Content.Add（Myballs [5]）;
我正在做的是制作不同颜色，重量和材料的球 将它们添加到名为MyBalls的列表中 然后 将它们从该列表添加到一个框（该框具有一个名为“内容”的列表的属性）
完成此操作后，我应该调用一些box函数并计算重量并打印里面的内容。我还没达到这个阶段
任何帮助将不胜感激
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Collections;
namespace Ball
{
class Ball
{
private string color;
private string material;
private double weight;
public string Color { get; set; }
public string Material { get; set; }
public double Weight { get; set; }
public Ball() { }
public Ball(string color, double weight, string material)
{
this.Color = color;
this.Material = material;
this.Weight = weight;
}
public bool changecolorbymorethanweight(string c, double w)
{
if (Weight >= w)
{
Color = c;
return true;
}
else return false;
}
public void printall()
{
Console.WriteLine("Η μπάλα είναι χρώματος {0}\nυλικού {1}\nκαι βάρους {2}", Color, Material, Weight);
}
}
-
class Box
{
private double height;
private double width;
private double length;
private string material;
public List<Ball> content = new List<Ball>();
private double weight;
public double Height { get; set; }
public double Width { get; set; }
public double Length { get; set; }
public string Material { get; set; }
public List<Ball> Content { get; set; }
public double Weight
{
get
{
double varos = 0;
for (int i = 0; i < content.Count; i++)
{
varos = varos + content[i].Weight;
}
weight = varos;
return weight;
}
set
{
Weight = weight;
}
}
public void removefirst3()
{
Content.RemoveAt(0);
Content.RemoveAt(1);
Content.RemoveAt(2);
}
public void removeall()
{
Content.Clear();
}
public void removeallbycolor(string rbc)
{
for (int i = 0; i < Content.Count; i++)
if (Content[i].Color == rbc)
Content.RemoveAt(i);
}
public int getnumberbycolor(string gnbc)
{
int counter = 0;
for (int i = 0; i < Content.Count; i++)
if (Content[i].Color == gnbc)
counter++;
return counter;
}
public bool removeallmorethanbyweight(double rbwb)
{
bool result = false;
for (int i = 0; i < Content.Count; i++)
if (Content[i].Weight >= rbwb)
{
Content.RemoveAt(i);
result = true;
}
return result;
}
public bool removealllessthanbyweight(double rbws)
{
bool result = false;
for (int i = 0; i < Content.Count; i++)
if (Content[i].Weight >= rbws)
{
Content.RemoveAt(i);
result = true;
}
return result;
}
public bool removeallbymaterial(string rabm)
{
bool result = false;
for (int i = 0; i < Content.Count; i++)
if (Content[i].Material == rabm)
{
Content.RemoveAt(i);
result = true;
}
return result;
}
public void changecolorbyposition(int p, string c)
{
for (int i = 0; i < Content.Count; i++)
if (i == p)
Content[i].Color = c;
}
public void printall()
{
Console.WriteLine("Tο κουτί μας έχει \n{0} ύψος\n{1} πλάτος\n{2} μήκος\nείναι φτιαγμένο από {3}\nτο βάρος του είναι {4}\n και μέσα του έχει:\n{5}", Height, Width, Length, Material, Weight, Content);
}
public Box() { }
public Box(double length, double width, double height, string material, double weight, List<Ball> content)
{
this.Length = length;
this.Width = width;
this.Height = height;
this.Material = material;
this.Weight = weight;
this.Content = content;
}
public Box(double length, double width, double height, string material)
{
this.Length = length;
this.Width = width;
this.Height = height;
this.Material = material;
}
}
-
class Program
{
static void Main(string[] args)
{
Box box1 = new Box(1.2, 1.3, 1.8, "χάρτινο");
Box box2 = new Box(1.2, 2.3, 2.5, "ξύλινο");
Ball b1 = new Ball("Red", 2.5, "μεταλλική");
Ball b2 = new Ball("Red", 2.5, "μεταλλική");
Ball b3 = new Ball("Red", 2.5, "μεταλλική");
Ball b4 = new Ball("Red", 1.5, "μεταλλική");
Ball b5 = new Ball("Red", 1.5, "μεταλλική");
Ball b6 = new Ball("Black", 0.5, "πλαστική");
Ball b7 = new Ball("Black", 0.5, "πλαστική");
Ball b8 = new Ball("Black", 0.5, "πλαστική");
Ball b9 = new Ball("Black", 0.5, "πλαστική");
Ball b10 = new Ball("Black", 0.5, "πλαστική");
Ball b11 = new Ball("Άσπρο", 1.1, "λαστιχένια");
Ball b12 = new Ball("Άσπρο", 1.1, "λαστιχένια");
Ball b13 = new Ball("Άσπρο", 1.1, "λαστιχένια");
Ball b14 = new Ball("Άσπρο", 1.1, "λαστιχένια");
Ball b15 = new Ball("Άσπρο", 1.1, "λαστιχένια");
List<Ball> Myballs = new List<Ball>() { b1, b2 };
Myballs.Capacity = 15;
Myballs.Add(b1);
Myballs.Add(b2);
Myballs.Add(b3);
Myballs.Add(b4);
Myballs.Add(b5);
Myballs.Add(b6);
Myballs.Add(b7);
Myballs.Add(b8);
Myballs.Add(b9);
Myballs.Add(b10);
Myballs.Add(b11);
Myballs.Add(b12);
Myballs.Add(b13);
Myballs.Add(b14);
Myballs.Add(b15);
box1.Content.Add(Myballs[5]);
box1.Content.Add(Myballs[6]);
box1.Content.Add(Myballs[7]);
box1.Content.Add(Myballs[8]);
box1.Content.Add(Myballs[9]);
box1.Content.Add(Myballs[10]);
box1.Content.Add(Myballs[11]);
box1.Content.Add(Myballs[12]);
box1.Content.Add(Myballs[13]);
box1.Content.Add(Myballs[14]);
box2.Content.Add(Myballs[0]);
box2.Content.Add(Myballs[1]);
box2.Content.Add(Myballs[2]);
box2.Content.Add(Myballs[3]);
box2.Content.Add(Myballs[4]);
Console.ReadKey();
}
}
}
答案 0 :(得分：1)
在Content类中初始化Box属性：
public List<Ball> Content { get; set; } = new List<Ball>();
...或在添加任何球之前使用Main方法：
box1.Content = new List<Ball>();
box1.Content.Add(Myballs[5]);
...