在 HttpClient 上使用带有自定义数据的 DelegatingHandler

Question

鉴于众所周知的使用 HttpClient 的困境和问题 - 即套接字耗尽和不尊重 DNS 更新，它被认为是使用 IHttpClientFactory 并让容器决定何时以及如何利用 http 池连接效率的最佳实践。这一切都很好，但现在我无法在每个请求上使用自定义数据实例化自定义 DelegatingHandler。

下面的示例说明了我在使用工厂方法之前是如何做到的：

public class HttpClientInterceptor : DelegatingHandler
{
    private readonly int _id;
    public HttpClientInterceptor(int id)
    {
        _id = id;
    }

    protected override async Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
    {
        // associate the id with this request
        Database.InsertEnquiry(_id, request);
        return await base.SendAsync(request, cancellationToken);
    }
}

每次我实例化一个 HttpClient 时，都可以传递一个 Id：

public void DoEnquiry()
{
    // Insert a new enquiry hypothetically 
    int id = Database.InsertNewEnquiry();
    using (var http = new HttpClient(new HttpClientInterceptor(id)))
    {
        // and do some operations on the http client
        // which will be logged in the database associated with id
        http.GetStringAsync("http://url.com");
    }
}

但现在我无法实例化 HttpClient 或 Handlers。

public void DoEnquiry(IHttpClientFactory factory)
{
    int id = Database.InsertNewEnquiry();
    var http = factory.CreateClient();
    // and now??

    http.GetStringAsync("http://url.com");
}

我怎样才能使用工厂实现类似的功能？

Answer 1

I cannot instantiate a custom DelegatingHandler with custom data on each request.

这是正确的。但是您可以使用可重用（且无状态）的自定义 DelegatingHandler，并将数据作为请求的一部分传递。这就是 HttpRequestMessage.Properties 的用途。在进行这些类型的“自定义上下文”操作时，我更喜欢将我的上下文“属性”定义为扩展方法：

public static class HttpRequestExtensions
{
  public static HttpRequestMessage WithId(this HttpRequestMessage request, int id)
  {
    request.Properties[IdKey] = id;
    return request;
  }

  public static int? TryGetId(this HttpRequestMessage request)
  {
    if (request.Properties.TryGetValue(IdKey, out var value))
      return value as int?;
    return null;
  }

  private static readonly string IdKey = Guid.NewGuid().ToString("N");
}

然后你可以在（可重复使用的）DelegatingHandler:

public class HttpClientInterceptor : DelegatingHandler
{
  protected override async Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
  {
    var id = request.TryGetId();
    if (id == null)
      throw new InvalidOperationException("This request must have an id set.");

    // associate the id with this request
    Database.InsertEnquiry(id.Value, request);
    return await base.SendAsync(request, cancellationToken);
  }
}

这种方法的缺点是每个调用点都必须指定 id。这意味着您不能使用像 GetStringAsync 这样的内置便捷方法；如果你这样做，上面的异常将被抛出。相反，您的代码将不得不使用较低级别的 SendAsync 方法：

var request = new HttpRequestMessage(HttpMethod.Get, "http://url.com");
request.SetId(id);
var response = await client.SendAsync(request);

调用样板相当难看。您可以将其包装到您的 own GetStringAsync 便捷方法中；这样的事情应该有效：

public static class HttpClientExtensions
{
  public static async Task<string> GetStringAsync(int id, string url)
  {
    var request = new HttpRequestMessage(HttpMethod.Get, url);
    request.SetId(id);
    var response = await client.SendAsync(request);
    return await response.Content.ReadAsStringAsync();
  }
}

现在您的呼叫站点再次看起来更干净了：

public async Task DoEnquiry(IHttpClientFactory factory)
{
  int id = Database.InsertNewEnquiry();
  var http = factory.CreateClient();
  var result = await http.GetStringAsync(id, "http://url.com");
}

在 HttpClient 上使用带有自定义数据的 DelegatingHandler

Using DelegatingHandler with custom data on HttpClient

c#

dotnet-httpclient

.net-core