圖片框始終拋出未設置為對象錯誤實例的對象引用

Question

我有一堂課，里面有一個畫框。 我已經創建了我的類的對象數組，但是當我嘗試創建新的圖片框時，出現錯誤提示，對象引用未設置為對象的實例。

這是我創建圖片框的方式

 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;

編輯代碼：

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

您的代碼有幾個嚴重的問題：

1. NullReferenceException的原因有兩個：
   1. 您聲明了GreyHound數組，但從未對其進行初始化。 這意味着該dog為null並且嘗試訪問dog[1]導致NullReferenceException。
   2. 如果要解決此問題，您仍然會遇到另一個問題：
      假設GreyHound是引用類型，則意味着您將必須為數組中的每個項目創建一個實例。 使用您當前的代碼，數組中的所有項目均為null ，這會導致在嘗試訪問數組中任何項目的屬性MyPictureBox時導致NullReferenceException ，因為dog[1]為null 。
2. 對於數組中的每個項目，首先要為屬性MyPictureBox分配一個PictureBox的新實例，但是在下一行使用pictureBoxX覆蓋該值。
3. 就像我們在C＃中一樣，數組的索引為零，因此第一項的索引為0，而不是1。

像這樣修復您的代碼：

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

您不會在任何地方初始化數組：

dog = new GreyHound[4];

數組索引也從0開始，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;