C＃：switch語句有問題嗎？

Question

這是我的項目的代碼，您可以在其中選擇列表框中的項目，然后會彈出圖片和說明。

        fruitBox = new ListBox();
        fruitImage = new PictureBox();

        fruitBox.Items.Add("Mangosteen");
        fruitBox.Items.Add("Bael");
        fruitBox.Items.Add("Coffee Berries");
        fruitBox.Items.Add("Jujube");
        fruitBox.Items.Add("Durian");

        string selectedFruit;


    }

    private void openButton_Click(object sender, EventArgs e)
    {
        string selectedFruit;
        selectedFruit = fruitBox.SelectedItem.ToString();

        if (fruitBox.SelectedIndex == -1)

        {
            selectedFruit = fruitBox.SelectedItem.ToString();

            switch (selectedFruit)
            {

                case "Mangosteen":
                    fruitImage.Image = imageList1.Images[0];
                    fruitDescription.Text = "Mangosteen description";
                    break;

                case "Bael":
                    fruitImage.Image = imageList1.Images[1];
                    fruitDescription.Text = "Bael description";
                    break;

                case "Durian":
                    fruitImage.Image = imageList1.Images[2];
                    fruitDescription.Text = "Durian description";
                    break;

                case "Coffee Berries":
                    fruitImage.Image = imageList1.Images[3];
                    fruitDescription.Text = "Coffee Berries description";
                    break;

                case "Jujube":
                    fruitImage.Image = imageList1.Images[4];
                    fruitDescription.Text = "Jujube description";
                    break;
            }
        }
        else
        {
            MessageBox.Show("Select a fruit");

但是，當我嘗試運行它時，會彈出此消息：

“ Exotic Fruits.exe中發生了'System.NullReferenceException類型的未處理的異常。

附加信息：對象引用未設置為對象的實例。”

Answer 1

您應該在if語句之前刪除此行：

selectedFruit = fruitBox.SelectedItem.ToString();

如果沒有SelectedItem fruitBox.SelectedItem可能為null 。您正在檢查SelectedIndex但是在嘗試訪問SelectedItem ，這會使if語句毫無意義 。您還可以像下面這樣更改if語句：

if(fruitBox.SelectedItem != null)