private void button1_Click(object sender, EventArgs e)
        {
            try
            {
                SHDocVw.InternetExplorer IE = new InternetExplorer();
                IE.Visible = true;
                string URL = "http://mail.
                object nullArg = null;
                IE.Navigate(URL, ref nullArg, ref nullArg, ref nullArg, ref nullArg);
                System.Threading.Thread.Sleep(1000);
                //得到IE的文档对象模型
                mshtml.IHTMLDocument2 DOM = (mshtml.IHTMLDocument2)IE.Document;
                //声明用户名
                mshtml.IHTMLInputTextElement txtUserName = (mshtml.IHTMLInputTextElement)DOM.all.item("username", null);
                txtUserName.value = "weijiezhang";
                //声明密码
                mshtml.IHTMLInputTextElement txtPwd = (mshtml.IHTMLInputTextElement)DOM.all.item("Password", null);
                txtPwd.value = "wj310791";
                //声明登录               
                mshtml.HTMLInputButtonElement btnLogin = (mshtml.HTMLInputButtonElement)DOM.all.item("null",0);
                System.Threading.Thread.Sleep(1000);               
                btnLogin.click();
               
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message);
            }

        }

其中
               //声明登录               
                mshtml.HTMLInputButtonElement btnLogin = (mshtml.HTMLInputButtonElement)DOM.all.item("null",0);
                System.Threading.Thread.Sleep(1000);               
                btnLogin.click();
,在btnlogin.click()处出现异常，“没有实例化对象”，请问该如何解决？

mshtml.HTMLInputButtonElement btnLogin = (mshtml.HTMLInputButtonElement)DOM.all.item("null",0);
 你要分析网站源码，那个按钮名字肯定不是null!