区分异步和多线程

使用多线程获取网页内容的代码如下：

using System;
using System.IO;
using System.Net;
using System.Threading;
using System.Windows.Forms;

namespace CS_71
{
    public partial class Form1 : Form
    {
        // This delegate enables asynchronous calls for setting
        // the text property on a TextBox control.
        delegate void SetTextCallback(string text);

        public Form1()
        {
            InitializeComponent();
        }

        private void btnGetPageThread_Click(object sender, EventArgs e)
        {
            Thread t = new Thread(new ThreadStart(() =>
            {
                var request = HttpWebRequest.Create("http://www.cnblogs.com/ryukaka");
                var response = request.GetResponse();
                var stream = response.GetResponseStream();

                using (StreamReader reader = new StreamReader(stream))
                {
                    var content = reader.ReadToEnd();
                    //MessageBox.Show(content);
                    //txtPage.Text = content;
                    SetText(content);
                }
            }));
            t.Start();
        }

        private void SetText(string text)
        {
            // InvokeRequired required compares the thread ID of the
            // calling thread to the thread ID of the creating thread.
            // If these threads are different, it returns true.
            if (txtPage.InvokeRequired)
            {
                SetTextCallback d = new SetTextCallback(SetText);
                Invoke(d, new object[] { text });
            }
            else
            {
                txtPage.Text = text;
            }
        }
    }
}

为了获取网页，CLR新起了一个工作线程，然后在读取网页的整个过程中，该工作线程始终被阻滞，直到获取网页完毕为止。

在整个过程中工作线程被占用着，这意味着系统的资源始终被消耗着、等待着。

修改为如下异步模型：

using System;
using System.IO;
using System.Net;
using System.Threading;
using System.Windows.Forms;

namespace CS_71
{
    public partial class Form1 : Form
    {
        // This delegate enables asynchronous calls for setting
        // the text property on a TextBox control.
        delegate void SetTextCallback(string text);

        public Form1()
        {
            InitializeComponent();
        }

        private void btnGetPageAsync_Click(object sender, EventArgs e)
        {
            var request = HttpWebRequest.Create("http://www.cnblogs.com/ryukaka");
            request.BeginGetRequestStream(this.AsyncCallbackImpl, request);
        }

        public void AsyncCallbackImpl(IAsyncResult ar)
        {
            WebRequest request = ar.AsyncState as WebRequest;
            var response = request.EndGetResponse(ar);
            var stream = response.GetResponseStream();
            using (StreamReader reader = new StreamReader(stream))
            {
                var content = reader.ReadToEnd();
                txtPage.Text = content;
            }
        }
    }
}

异步模式使用线程池进行管理。新起异步操作后，CLR会将工作丢给线程池中的某个工作线程来完成。

当I/O操作开始的时候，异步会将工作线程还给线程池，这时候就相当于获取网页的这个过程不会再占用CPU的资源了。

应用场景：

计算密集型工作，采用多线程。

IO密集型工作，采用异步机制。