Question

您好我有一个代码可以读取文本文件并将内容复制到列表框中。一切都运行正常，但当我在没有项目的地方点击列表框内部时，会出现一条新的错误消息，并指出我对此行有错误：

switch (listBox3.SelectedItem.ToString()) {
    case "Accessories":
        label4.Text = "None Required"; //Approval
        label13.Text = " "; //Approval
        label5.Text = "TTS";  //sent by
        label6.Text = "IT Co.";   //sender
        label7.Text = "2";  //urgent
        label8.Text = "3";  //normal
        label9.Text = "PC Name";   // required filed 1
        label10.Text = "PC Brand && Model";  // required filed 2
        label11.Text = "B.C";  // required filed 3
        label12.Text = "Location";  // required filed 4
        label14.Text = "User Name";  // required filed 5
    break;

这只是一段代码和编译器指出的行是这样的：

switch (listBox3.SelectedItem.ToString())

如何解决此问题？

Answer 1

您需要检查所选项目是否为空

if (listBox3.SelectedItem!=null)
{

  // write code for it 

}

Answer 2

试图总结每个人所说的话：

if(listBox3.SelectedItem != null) {
    switch (listBox3.SelectedItem.ToString()) {
        case "Accessories":
            label4.Text = "None Required"; //Approval
            label13.Text = " "; //Approval
            label5.Text = "TTS";  //sent by
            label6.Text = "IT Co.";   //sender
            label7.Text = "2";  //urgent
            label8.Text = "3";  //normal
            label9.Text = "PC Name";   // required filed 1
            label10.Text = "PC Brand && Model";  // required filed 2
            label11.Text = "B.C";  // required filed 3
            label12.Text = "Location";  // required filed 4
            label14.Text = "User Name";  // required filed 5
        break;
    }
}

Answer 3

尝试从ToString()值调用null（或任何方法）将产生NRE。如果没有选择任何内容，SelectedItem将最终成为null。您必须事先检查null或使用Convert.ToString()执行此操作，因为它在给定null值时不会抛出，它只返回字符串"null"。

switch (Convert.ToString(listBox3.SelectedItem))
{
    // etc...
}

Answer 4

尝试此修复：

if(listBox3.SelectedItem != null) {
    switch (listBox3.SelectedItem.ToString()) {
        case "Accessories":
            label4.Text = "None Required"; //Approval
            label13.Text = " "; //Approval
            label5.Text = "TTS";  //sent by
            label6.Text = "IT Co.";   //sender
            label7.Text = "2";  //urgent
            label8.Text = "3";  //normal
            label9.Text = "PC Name";   // required filed 1
            label10.Text = "PC Brand && Model";  // required filed 2
            label11.Text = "B.C";  // required filed 3
            label12.Text = "Location";  // required filed 4
            label14.Text = "User Name";  // required filed 5
        break;
    }
}

else {
    //nothing selected
}

在列表框中选择任何内容时出错

4 个答案: