private async void button1_Click(object sender, EventArgs e)
        {
            #region 单个执行的异步，效率慢
            HttpClient wc = new HttpClient();
            string s1 = await wc.GetStringAsync(textBox1.Text);
            label1.Text = s1.Length.ToString();
            string s2 = await wc.GetStringAsync(textBox2.Text);
            label2.Text = s2.Length.ToString();
            string s3 = await wc.GetStringAsync(textBox3.Text);
            label3.Text = s3.Length.ToString();
            #endregion

            #region 并发全部完成的异步更快
            HttpClient hc = new HttpClient();
            var task1 = hc.GetStringAsync(textBox1.Text);
            var task2 = hc.GetStringAsync(textBox2.Text);
            var task3 = hc.GetStringAsync(textBox3.Text);
            Task.WaitAll(task1, task2, task3);
            label1.Text = task1.Result.Length.ToString();
            label2.Text = task2.Result.Length.ToString();
            label3.Text = task3.Result.Length.ToString();
            #endregion

        }