Intercept HttpClient with third party extensions using state

Question

Injecting state into your HttpRequest when using IHttpClientFactory is achievable by populating HttpRequestMessage.Properties see Using DelegatingHandler with custom data on HttpClient

Now if I have third party extensions on HttpClient (such as IdentityModel), how would I intercept these http requests using custom state?

public async Task DoEnquiry(IHttpClientFactory factory)
{
    var id = Database.InsertEnquiry();
    var httpClient = factory.CreateClient();
    // GetDiscoveryDocumentAsync is a third party extension method on HttpClient
    // I therefore cannot inject or alter the request message to be handled by the InterceptorHandler
    var discovery = await httpClient.GetDiscoveryDocumentAsync();
    // I want id to be associated with any request / response GetDiscoveryDocumentAsync is making
}

The only plausible solution I currently have is to override HttpClient.

public class InspectorHttpClient: HttpClient
{
    
    private readonly HttpClient _internal;
    private readonly int _id;

    public const string Key = "insepctor";

    public InspectorHttpClient(HttpClient @internal, int id)
    {
        _internal = @internal;
        _id = id;
    }
    
    public override Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
    {
        // attach data into HttpRequestMessage for the delegate handler
        request.Properties.Add(Key, _id);
        return _internal.SendAsync(request, cancellationToken);
    }

    // override all methods forwarding to _internal
}

A then I'm able to intercept these requests.

public async Task DoEnquiry(IHttpClientFactory factory)
{
    var id = Database.InsertEnquiry();
    var httpClient = new InspectorHttpClient(factory.CreateClient(), id);
    var discovery = await httpClient.GetDiscoveryDocumentAsync();
}

Is that a plausible solution? Something tell me now not to override HttpClient. Quoting from https://docs.microsoft.com/en-us/dotnet/api/system.net.http.httpclient?view=net-5.0

The HttpClient also acts as a base class for more specific HTTP clients. An example would be a FacebookHttpClient providing additional methods specific to a Facebook web service (a GetFriends method, for instance). Derived classes should not override the virtual methods on the class. Instead, use a constructor overload that accepts HttpMessageHandler to configure any pre- or post-request processing instead.

Stephen Cleary Stephen Cleary · Accepted Answer · 2021-02-19T13:35:20

I almost included this in my other answer as an alternative solution, but I figured it was too long already. :)

The technique is practically the same, but instead of HttpRequestMessage.Properties, use AsyncLocal<T>. "Async local" is kind of like thread-local storage but for a specific asynchronous code block.

There are a few caveats to using AsyncLocal<T> that aren't particularly well-documented:

Use an immutable nullable type for T.
When setting the async local value, return an IDisposable that resets it.
- If you don't do this, then only set the async local value from an async method.

You don't have to follow these guidelines, but they will make your life much easier.

With that out of the way, the solution is similar to the last one, except it just uses AsyncLocal<T> instead. Starting with the helper methods:

public static class AmbientContext
{
  public static IDisposable SetId(int id)
  {
    var oldValue = AmbientId.Value;
    AmbientId.Value = id;
    // The following line uses Nito.Disposables; feel free to write your own.
    return Disposable.Create(() => AmbientId.Value = oldValue);
  }

  public static int? TryGetId() => AmbientId.Value;

  private static readonly AsyncLocal<int?> AmbientId = new AsyncLocal<int?>();
}

Then the calling code is updated to set the ambient value:

public async Task DoEnquiry(IHttpClientFactory factory)
{
  var id = Database.InsertEnquiry();
  using (AmbientContext.SetId(id))
  {
    var httpClient = factory.CreateClient();
    var discovery = await httpClient.GetDiscoveryDocumentAsync();
  }
}

Note that there is an explicit scope for that ambient id value. Any code within that scope can get the id by calling AmbientContext.TryGetId. Using this pattern ensures that this is true for any code: synchronous, async, ConfigureAwait(false), whatever - all code within that scope can get the id value. Including your custom handler:

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

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

Followup readings:

Blog post on "async local" - written before AsyncLocal<T> existed, but has details on how it works. This answers the questions "why should T be immutable?" and "if I don't use IDisposable, why do I have to set the value from an async method?".

Intercept HttpClient with third party extensions using state

1 Answers