.NET Framework 4.8中HttpClientFactory.Create()失效问题求助

我开发了基于.NET Framework 4.8的Web服务，原本web.config中httpRuntime的targetFramework指向4.0时运行正常。将其修改为4.8后，通过HttpClientFactory.Create()发起的Http请求要么超时，要么返回403 Forbidden错误，但直接实例化HttpClient对象发起请求则正常，且targetFramework为4.0时两种方式均无问题。不清楚直接创建HttpClient对象与调用HttpClientFactory.Create()的核心差异，求解决经验。

配置（Web.Config）：失效配置 #

<httpRuntime targetFramework="4.8"/>

调用API的简化代码 #

public async Task<string> TestMethodAsync()
{     
    HttpClientWrapper clientWrapper = new HttpClientWrapper();
    HttpResponseMessage response = clientWrapper.GetAsync("https://www.google.com").Result;
    return response.Content.ReadAsStringAsync().Result;   
}

public class HttpClientWrapper
{
    private readonly CustomDelegatingHandler _delegatingHandler = new CustomDelegatingHandler();
    public async Task<HttpResponseMessage> GetAsync(string requestUri)
    {
        using (var client = HttpClientFactory.Create(_delegatingHandler))
        {
            return await client.GetAsync(requestUri);
        }
    }   
    public class CustomDelegatingHandler : DelegatingHandler
    {         
              
    }
}

问题分析与解决方案 #

核心差异点 #

直接实例化HttpClient时，默认使用系统原生的HttpClientHandler作为底层处理逻辑；而HttpClientFactory.Create()传入自定义DelegatingHandler时，若未显式设置InnerHandler，.NET Framework 4.8会使用与4.0不同的默认处理链逻辑，导致请求配置（如安全协议、代理、请求头）不匹配目标服务要求。

具体解决步骤 #

1. 给自定义DelegatingHandler显式设置InnerHandler
   确保处理链完整，使用标准的HttpClientHandler作为底层实现：

   public async Task<HttpResponseMessage> GetAsync(string requestUri)
   {
       var innerHandler = new HttpClientHandler();
       var delegatingHandler = new CustomDelegatingHandler { InnerHandler = innerHandler };
       using (var client = HttpClientFactory.Create(delegatingHandler))
       {
           return await client.GetAsync(requestUri);
       }
   }
   

   也可在自定义Handler的构造函数中直接初始化：

   public class CustomDelegatingHandler : DelegatingHandler
   {         
       public CustomDelegatingHandler()
       {
           InnerHandler = new HttpClientHandler();
       }
   }
   

2. 显式指定安全协议版本
   .NET Framework 4.8默认的安全协议可能与目标服务不兼容，需主动指定：

   var innerHandler = new HttpClientHandler();
   innerHandler.SslProtocols = SslProtocols.Tls12 | SslProtocols.Tls13;
   

3. 修复同步阻塞异步方法的问题
   代码中使用.Result阻塞异步方法可能引发死锁或请求异常，改为全异步调用：

   public async Task<string> TestMethodAsync()
   {     
       HttpClientWrapper clientWrapper = new HttpClientWrapper();
       HttpResponseMessage response = await clientWrapper.GetAsync("https://www.google.com");
       return await response.Content.ReadAsStringAsync();   
   }
   

内容的提问来源于stack exchange，提问作者Mathiyazhagan