图片框始终抛出未设置为对象错误实例的对象引用

Question

我有一堂课，里面有一个画框。 我已经创建了我的类的对象数组，但是当我尝试创建新的图片框时，出现错误提示，对象引用未设置为对象的实例。

这是我创建图片框的方式

 GreyHound[] dog;

    public Form1()
    {
       InitializeComponent();
       dog[1].MyPictureBox = new PictureBox();
       dog[1].MyPictureBox = pictureBox1;

       dog[2].MyPictureBox = new PictureBox();
       dog[2].MyPictureBox = pictureBox2;

       dog[3].MyPictureBox = new PictureBox();
       dog[3].MyPictureBox = pictureBox3;

       dog[4].MyPictureBox = new PictureBox();
       dog[4].MyPictureBox = pictureBox4;

编辑代码：

GreyHound []狗；

    public Form1()
    {

       InitializeComponent();
       dog = new GreyHound[4];
       dog[0].MyPictureBox = new PictureBox();
       dog[0].MyPictureBox = pictureBox1;

       dog[1].MyPictureBox = new PictureBox();
       dog[1].MyPictureBox = pictureBox2;

       dog[2].MyPictureBox = new PictureBox();
       dog[2].MyPictureBox = pictureBox3;

       dog[3].MyPictureBox = new PictureBox();
       dog[3].MyPictureBox = pictureBox4;



    }

Answer 1

您的代码有几个严重的问题：

1. NullReferenceException的原因有两个：
   1. 您声明了GreyHound数组，但从未对其进行初始化。 这意味着该dog为null并且尝试访问dog[1]导致NullReferenceException。
   2. 如果要解决此问题，您仍然会遇到另一个问题：
      假设GreyHound是引用类型，则意味着您将必须为数组中的每个项目创建一个实例。 使用您当前的代码，数组中的所有项目均为null ，这会导致在尝试访问数组中任何项目的属性MyPictureBox时导致NullReferenceException ，因为dog[1]为null 。
2. 对于数组中的每个项目，首先要为属性MyPictureBox分配一个PictureBox的新实例，但是在下一行使用pictureBoxX覆盖该值。
3. 就像我们在C＃中一样，数组的索引为零，因此第一项的索引为0，而不是1。

像这样修复您的代码：

GreyHound[] dog = new GreyHound[4]; // solves problem 1.1

dog[0] = new GreyHound(); // solves problem 1.2
dog[0].MyPictureBox = pictureBox1;

dog[1] = new GreyHound(); // solves problem 1.2
dog[1].MyPictureBox = pictureBox2;

dog[2] = new GreyHound(); // solves problem 1.2
dog[2].MyPictureBox = pictureBox3;

dog[3] = new GreyHound(); // solves problem 1.2
dog[3].MyPictureBox = pictureBox4;

Answer 2

您不会在任何地方初始化数组：

dog = new GreyHound[4];

数组索引也从0开始，所以：

dog[0].MyPictureBox = new PictureBox();
dog[0].MyPictureBox = pictureBox1;

dog[1].MyPictureBox = new PictureBox();
dog[1].MyPictureBox = pictureBox2;

dog[2].MyPictureBox = new PictureBox();
dog[2].MyPictureBox = pictureBox3;

dog[3].MyPictureBox = new PictureBox();
dog[3].MyPictureBox = pictureBox4;