每当我运行代码时，我都会收到此错误消息：
Picture-Resize.exe中出现未处理的“System.NullReferenceException”类型异常附加信息：对象引用未设置为对象的实例。
我的代码如下，这是为抓取网站而编写的：
public partial class Form1 : Form
{
List<string> mainitems = new List<string>();
List<string> mainitems2 = new List<string>();
List<string> address2 = new List<string>();
public Form1()
{
InitializeComponent();
}
private void Form1_Load(object sender, EventArgs e)
{
HtmlAgilityPack.HtmlDocument doc = new HtmlAgilityPack.HtmlDocument();
HtmlWeb hw = new HtmlWeb();
doc = hw.Load("http://rahnama.com");
HtmlNodeCollection nodes = doc.DocumentNode.SelectNodes("//div[@class=\"hierarchylist-title\"]");
listBox1.Items.Clear();
int j=0;
foreach (var item in nodes)
{
mainitems.Add( item.InnerText);
}
for (int i = 0; i < mainitems.Count; i++)
{
listBox1.Items.Add(mainitems[i]);
HtmlNodeCollection nodes2 = doc.DocumentNode.SelectNodes(string.Format("(//div[@class='hierarchylist-content'])[{0:D}]//div[@class='ch-link']", i+1));
foreach (var item in nodes2)
{
mainitems2.Add(item.InnerText);
}
foreach (HtmlNode link in doc.DocumentNode.SelectNodes(string.Format("(//div[@class='hierarchylist-content'])[{0:D}]//div[@class='ch-link']//a[@href]", i + 1)))
{
// Get the value of the HREF attribute
string hrefValue = link.GetAttributeValue("href", string.Empty);
address2.Add(hrefValue);
}
}
for (int i = 0; i < mainitems2.Count; i++)
{
listBox1.Items.Add(mainitems2[i]);
listBox1.Items.Add(address2[i]);
}
for (int i = 0; i < 51; i++)
{
doc = hw.Load(address2[i]);
nodes = doc.DocumentNode.SelectNodes("//div[@class='hierarchylist-title2']");
foreach (var item in nodes)
{
listBox2.Items.Add(item.InnerText);
}
}
}
}
为什么我会收到此错误，我该如何避免？ 这一行正在抛出异常：
foreach (var item in nodes)
{
listBox2.Items.Add(item.InnerText);
}