webclient异步调用出现错误

webclient异步调用出现异常

C# code

WebClient client = new WebClient();            client.DownloadStringAsync(new Uri("http://finance.sina.com.cn/chanjing/cyxw/20110408/01589654545.shtml"));            client.DownloadStringCompleted += (s, ea) => {                this.textBox1.Text = ea.Result;            };