documentcompleted事件没有被执行？

c# 写一个简单的爬虫，使用webbrowser爬网页的时候，发现wb_DocumentComplete没有被执行，程序就结束了。。。。怎么破啊，大神们

void wb_DocumentCompleted(object sender ,WebBrowserDocumentCompletedEventArgs e) 
{
     WebBrowser wb = (WebBrowser)sender;
     if ( wb.ReadyState == WebBrowserReadyState.Complete)
     {
            dosomething();
      }
}

private void GetHtmlWithBrowser(object url)
{
     WebBrowser wb = new WebBrowser();
     wb.Navigate(new Uri("https://www.baidu.com"));
     wb.DocumentCompleted += new WebBrowserDocumentCompletedEventHandler(wb_DocumentCompleted);

        }

我试了一下，执行了。
能把运行的结果贴出来么

public partial class Form1 : Form
{
public Form1()
{
InitializeComponent();
}

    private void Form1_Load(object sender, EventArgs e)
    {
    }

    private void GetHtmlWithBrowser(Uri uri)
    {
        WebBrowser wb = new WebBrowser() { Dock = DockStyle.Fill };
        wb.Navigate(uri);
        wb.DocumentCompleted += new WebBrowserDocumentCompletedEventHandler(wb_DocumentCompleted);
        this.Controls.Add(wb);
    }

    void wb_DocumentCompleted(object sender, WebBrowserDocumentCompletedEventArgs e)
    {
        WebBrowser wb = (WebBrowser)sender;
        if (wb.ReadyState == WebBrowserReadyState.Complete)
        {
            MessageBox.Show("finish.");
        }
    }

    private void button1_Click(object sender, EventArgs e)
    {
        GetHtmlWithBrowser(new Uri("https://www.baidu.com"));
    }
}

我试了一下，执行了。