Fix: Missing Content Headers After Automatic Decompression

Hey guys! Today, we're diving deep into a fascinating issue in the .NET runtime related to how Content-Encoding and Content-Length headers are handled in HTTP responses. Specifically, we're going to break down the discussions around why these headers sometimes disappear when automatic decompression kicks in. This is super important if you're building applications that rely on accurate header information, so let's get started!

This article provides a thorough triage of issue #118746 ("Content-Encoding and Content-Length headers are removed from response upon automatic decompression"), offering insights and recommendations.

Summary of Issue

The core problem? When you're using HttpClientHandler.AutomaticDecompression, the HttpResponseMessage.Content.Headers.ContentLength and ContentEncoding headers can go missing in a DelegatingHandler, even if the server diligently sent them. This is a head-scratcher because you'd expect to see the size of the compressed response and the encoding method used. But, after automatic decompression, poof! They're gone.

Think of it like this: you order a package online, and the shipping label tells you the weight and dimensions of the box. But when it arrives, the label has been mysteriously wiped clean. Annoying, right? That's the same kind of frustration developers face when these headers vanish.

This behavior can lead to issues in several scenarios:

• Monitoring and Metrics: If you're trying to track the size of data transferred over the network, missing Content-Length headers throw a wrench in your calculations.
• Debugging: Understanding the original encoding helps diagnose issues, especially when dealing with different compression algorithms.
• Custom Logic: Some applications might rely on these headers for specific processing steps, and their absence can break things.

So, why does this happen? Is it a bug? A feature? Let's dig into the related issues and discussions to find out.

Highly Related Issues & Discussions

To really understand what's going on, let's look at some key discussions and issues on the .NET runtime repository. These conversations shed light on the rationale behind this behavior and potential workarounds.

1. Issue #95708 (December 2023) - "HttpResponseMessage does not contain Content-Type and Content-Length headers in DelegatingHandler"

This issue is like the smoking gun in our investigation. The author (the same one who raised the initial issue #118746) reported missing Content-Length and Content-Type headers within a DelegatingHandler.

• The Discussion: The .NET team, including the ever-helpful MihaZupan, jumped into the conversation. The consensus? When AutomaticDecompression is enabled, the original headers (Content-Length, Content-Encoding) are deliberately removed. Why? Because they no longer accurately represent the decompressed content. Imagine telling someone a rope is 10 meters long, but after stretching it, it's actually 15 meters. The original measurement is now misleading.
• Key Comments:
  • [MihaZupan]: This explanation is gold: "The missing headers in your case appear to be 'content-encoding' and 'content-length'. This is consistent with the behavior you would see for compressed responses when you have AutomaticDecompression set. ... The Content-Length that the server sends indicates how many bytes long the content will be. But if we're decompressing the content, that content length value is now misleading as it doesn't match the amount of decompressed data you can read from the HttpContent. Content-Encoding would likewise be misleading as the content is no longer encoded, and so the header is removed."
  • [MihaZupan] also pointed out the current workaround: "For that the only current workaround is not to use automatic decompression - see #42789."
• The Takeaway: This is by design. The .NET team made a conscious decision to remove the original headers to prevent confusion. Tracking the original size of the compressed data becomes impossible when automatic decompression is enabled. It's a trade-off between convenience and transparency.

2. Issue #42789 (September 2020) - "Accessing original Content-Encoding header in HttpResponseMessage"

This issue is like a historical precedent. Users were already clamoring for access to the original Content-Encoding header for metrics and debugging purposes, only to find it gone with automatic decompression.

• The Discussion: The team doubled down on the