Does WebClient use KeepAlive?

问题:

I need to issue around 50 HTTP requests to a single host (API calls). Performance is important, so I'd like to use HTTP KeepAlive's. Does WebClient support this?

回答1:

It does on my machine, but I can't see that it's documented to. I'd certainly expect it to by default. The simplest way to tell is to run Wireshark (or Fiddler) and look at exactly what's going down the wire.

For example, this program:

using System;
using System.Net;

class Test
{    
    static void Main()
    {
        WebClient client = new WebClient();
        for (int i = 0; i < 50; i++)
        {
            string text = client.DownloadString("http://www.microsoft.com");
            Console.WriteLine(text.Length);
        }
    }
}

Generates a first request of:

GET / HTTP/1.1   
Host: www.microsoft.com    
Connection: Keep-Alive

Subsequence requests are just:

GET / HTTP/1.1
Host: www.microsoft.com

... presumably because once a connection is in KeepAlive mode, it's assumed it will stay that way.

回答2:

As documented here, WebClient makes use of WebRequest in its private implementation, http://msdn.microsoft.com/en-us/library/system.net.webclient.aspx. Microsoft does not expose that as a public property for you to control.

Therefore, reviewing its implementation using Reflector you can see how KeepAlive is set for the WebRequest object in use. Like @Jon pointed out, an experiment shows that KeepAlive is set to true. This matches other scenarios too, such as .NET remoting's private implementation.

In rare cases you may find KeepAlive = true can lead to SocketException, and then you have to use reflection or other tricks to set it to false which is very annoying.