Microsoft 在.Net Framework 4.5中引入了HttpClient，並且是在.NET伺服器端程式碼中使用Web API的最常用方法。但它有一些嚴重的問題，如釋放HttpClient物件不立即關閉套接字，太多例項影響效能和單個的HttpClient或共享HttpClient例項不尊重DNS生存時間（TTL）設定。當並沒有大批量或者併發呼叫時，選擇 HttpClient 還行：

 private static readonly HttpClient HttpClient = new HttpClient();//定義




 var res = HttpClient.GetStringAsync(url).Result;//呼叫

　.net core 2.1 引入了HttpClientFactory的一些問題，當高併發時可以使用HttpClientFactory，例子連結裡有，我只介紹一個在類庫中使用方法：

1.類庫中建封裝類：

using System;
using System.Collections.Generic;
using System.Linq;
using System.Net.Http;
using System.Threading.Tasks;

namespace Oss.Utils
{
    public class HttpClientFactory
    {
        public HttpClient Client { get; private set; }

        public HttpClientFactory(HttpClient httpClient)
        {
            httpClient.BaseAddress = new Uri("https://api.github.com/");
            httpClient.DefaultRequestHeaders.Add("Accept", "application/vnd.github.v3+json");
            httpClient.DefaultRequestHeaders.Add("User-Agent", "HttpClientFactory-Sample");
            Client = httpClient;
        }
    }
}

2.其他類中呼叫就好了:

  private static readonly HttpClient HttpClient = new HttpClient();
  private static readonly HttpClientFactory HttpClientFactory = new HttpClientFactory(HttpClient);
...
...
...


var res = HttpClientFactory.Client.GetStringAsync(url).Result;

okay，呼叫完成。

說明一下:在.net core 專案中使用HttpClientFactory的時候，要是在Startup.cs註冊時，要安裝:

Microsoft.Extensions.Http   nuget包，

直接搜尋即可。

具體使用方法和參考連結如下:https://www.cjavapy.com/article/150/