private void button1_Click(object sender, EventArgs e)
        {
            //判断txtURI字符是否为空
            if (string.IsNullOrWhiteSpace(txtURI.Text))
            {
                MessageBox.Show("请输入图像URI", "提示", MessageBoxButtons.OK, MessageBoxIcon.Warning);
                return;
            }
            //关闭加载按钮
            button1.Enabled = false;
            //异步加载图像,加载txtURI的图像
            pictureBox1.LoadAsync(txtURI.Text);
        }

 private void pictureBox1_LoadProgressChanged(object sender, ProgressChangedEventArgs e)
        {
            //显示进度
            this.lblMsg.Text = string.Format("当前进度：{0}.", e.ProgressPercentage);
        }

        private void pictureBox1_LoadCompleted(object sender, AsyncCompletedEventArgs e)
        {
            //加载错误处理事件
            if (e.Error != null)
            {
                lblMsg.Text = "错误信息：" + e.Error.Message;
                return;
            }
            //如果取消
            if (e.Cancelled)
            {
                lblMsg.Text = "操作被取消。";
            }
            else
            {
                lblMsg.Text = "加载完成.";
            }
            button1.Enabled = true;
        }