System.Net.Http.dll

//使用 HttpClient创建Get请求
HttpClient client = new HttpClient();
Task<HttpResponseMessage> resp = client.GetAsync("http://www.gongjuji.net");
resp.ContinueWith(q =>
{
    //获取相应状态
    HttpResponseMessage respMsg = q.Result;
    Console.WriteLine(respMsg.StatusCode);

    //获取相应内容
    HttpContent respClient = respMsg.Content;
    respClient.ReadAsStringAsync().ContinueWith(str =>
    {
        string result = str.Result;

        Console.WriteLine(result);
    });
});

//使用HttpClient 创建Post请求
HttpClient client = new HttpClient();
//指定提交表单数据
List<KeyValuePair<String, String>> paramList = new List<KeyValuePair<String, String>>();
paramList.Add(new KeyValuePair<string, string>("Content", "e"));
FormUrlEncodedContent data = new FormUrlEncodedContent(paramList);

client.PostAsync("http://md5.gongjuji.net/common/md5encrypt", data)
    .ContinueWith(q =>
    {
        HttpResponseMessage respMsg = q.Result;
        //读取请求结果
        respMsg.Content.ReadAsStringAsync().ContinueWith(str =>
        {
            string result = str.Result;
            Console.WriteLine(result);
        });
    });