public void ConfigureServices(IServiceCollection services)
{
 //普通注入
 serviceCollection.AddHttpClient();
 //命名注入
 serviceCollection.AddHttpClient(Constants.SERVICE_USERACCOUNT, (serviceProvider, c) =>
 {
  var configuration = serviceProvider.GetRequiredService<IConfiguration>();
 c.BaseAddress = new Uri(configuration.GetValue<string>("ServiceApiBaseAddress:UserAccountService"));
 });
 //类型化客户端
 services.AddHttpClient<TypedClientService>();
}

public class AccreditationService
{
 private IHttpClientFactory _httpClientFactory;
 private const string _officialAccreName = "manage/CommitAgencyOfficialOrder";
 private const string _abandonAccUserName = "info/AbandonUserAccreditationInfo";

 public AccreditationService(IHttpClientFactory clientFactory)
 {
  _httpClientFactory = clientFactory;
 }

 public async Task<string> CommitAgentOfficial(CommitAgencyOfficialOrderRequest request)
 {
    //使用factory 创建httpclient
   var httpClient = _httpClientFactory.CreateClient(Constants.SERVICE_ACCREDITATION);
   var response = await httpClient.PostAsJsonAsync(_officialAccreName, request);
   if (!response.IsSuccessStatusCode) return string.Empty;
   var result = await response.Content.ReadAsAsync<AccreditationApiResponse<CommitAgencyOfficialOrderResult();
   if (result.ReturnCode != "0") return string.Empty;
    return result.Data.OrderNo;
 }
}

public class TypedClientService
{
 private HttpClient _httpClient;

 public TypedClientService(HttpClient httpClient)
 {
  _httpClient = httpClient;
 }
}

public class ValidateHeaderHandler : DelegatingHandler
{
 protected override async Task<HttpResponseMessage> SendAsync(
  HttpRequestMessage request,
  CancellationToken cancellationToken)
 {
  if (!request.Headers.Contains("X-API-KEY"))
  {
   return new HttpResponseMessage(HttpStatusCode.BadRequest)
   {
    Content = new StringContent(
     "You must supply an API key header called X-API-KEY")
   };
  }

  return await base.SendAsync(request, cancellationToken);
 }
}

public void ConfigureServices(IServiceCollection services)
{
 services.AddTransient<ValidateHeaderHandler>();

 services.AddHttpClient("externalservice", c =>
 {
  // Assume this is an "external" service which requires an API KEY
  c.BaseAddress = new Uri("https://localhost:5001/");
 })
 .AddHttpMessageHandler<ValidateHeaderHandler>();
}

public void ConfigureServices(IServiceCollection services)
{   
 services.AddHttpClient("extendedhandlerlifetime")
  .SetHandlerLifetime(TimeSpan.FromMinutes(5));
}

public void ConfigureServices(IServiceCollection services)
{   
 services.AddHttpClient<UnreliableEndpointCallerService>()
  .AddTransientHttpErrorPolicy(p => 
   p.WaitAndRetryAsync(3, _ => TimeSpan.FromMilliseconds(600)));

}