feat: Add support for HTTP response trailers

[HosseinDinarvand]

PR Classification

New feature and API enhancement for preserving HTTP trailer headers.

PR Summary

RestSharp now exposes HTTP response trailers separately from regular and content headers. Integration coverage verifies trailer preservation on .NET 8+.

• RestResponse: Maps HttpResponseMessage.TrailingHeaders under NET targets.
• RestResponseBase: Adds nullable TrailingHeaders with documentation.

Related to #2413

[qodo-free-for-open-source-projects]

PR Summary by Qodo

Expose HTTP response trailers separately from other headers

✨ Enhancement 🧪 Tests 🕐 10-20 Minutes

AI Description

• Expose response trailers separately so callers can distinguish them from regular and content
 headers.
• Add .NET 8+ integration coverage for trailer preservation and header separation.

Diagram

graph TD
  H["Test handler"] --> M["HTTP response"] --> B["Body read"] --> R["RestResponse"] --> C["Caller"]
  M --> X["Header conversion"] --> R

Loading

High-Level Assessment

Keep the separate response property and reuse the existing header converter. Merging trailers into Headers would lose their distinction, while a new conversion path would duplicate existing behavior.

Files changed (3) +37 / -0

Enhancement (2) +8 / -0

RestResponse.csMap HTTP response trailers into RestResponse +3/-0

Map HTTP response trailers into RestResponse

• On NET targets, the default response mapping converts HttpResponseMessage.TrailingHeaders into HeaderParameter entries. Trailers remain separate from response and content headers.

src/RestSharp/Response/RestResponse.cs

RestResponseBase.csAdd a public TrailingHeaders response property +5/-0

Add a public TrailingHeaders response property

• Adds a documented, nullable read-only collection of HeaderParameter entries for response trailers. The property is available to RestResponse and its generic subclass.

src/RestSharp/Response/RestResponseBase.cs

Tests (1) +29 / -0

ResponseTrailerTests.csVerify trailer preservation and separation +29/-0

Verify trailer preservation and separation

• Adds a .NET 8+ integration test using a custom HTTP handler to return a Checksum trailer. It verifies that the trailer appears in TrailingHeaders but not in Headers or ContentHeaders.

test/RestSharp.Tests.Integrated/ResponseTrailerTests.cs

[sonarqubecloud]

Quality Gate passed

Issues
1 New issue
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[qodo-free-for-open-source-projects]

Code Review by Qodo

🐞 Bugs (1) 📘 Rule violations (0) 📎 Requirement gaps (0) 📜 Skill insights (0)

1. Older targets return null for trailers 🐞 Bug ≡ Correctness

Description

TrailingHeaders has no empty default, and its only assignment in GetDefaultResponse is excluded
from non-NET builds. Successful responses on .NET Framework and .NET Standard therefore leave the
property null, unlike modern targets, where responses without trailers expose an empty collection.

Code

src/RestSharp/Response/RestResponseBase.cs[117]

+    public IReadOnlyCollection<HeaderParameter>? TrailingHeaders { get; set; }

Evidence

The project targets netstandard2.0, net471, and net48 alongside modern .NET. The new property has no
initializer, and the only code that assigns it is guarded by #if NET, so successful responses on
the older targets retain its default null value.

src/Directory.Build.props[1-6]
src/RestSharp/Response/RestResponseBase.cs[114-117]
src/RestSharp/Response/RestResponse.cs[72-80]

Agent prompt

The issue below was found during a code review. Follow the provided context and guidance below and implement a solution

## Issue description
Successful responses on older targets leave `TrailingHeaders` null because their builds exclude its only assignment.
## Fix Focus Areas
- src/RestSharp/Response/RestResponseBase.cs[114-117]
- src/RestSharp/Response/RestResponse.cs[76-78]
## Recommended Fix
Initialize `TrailingHeaders` to an empty collection for targets without trailer support, while preserving the populated collection on supported targets.

ⓘ Copy this prompt and use it to remediate the issue with your preferred AI generation tools

---

Context sources

✅ Tickets: 🎫 Expose HTTP response trailers on RestResponse

Review mode: Auto: ⚖️ Balanced: Public API and HTTP trailer lifecycle changes require careful review.

Tip of the day

💡 Did you know, you can pick a display mode instead of tweaking every Display option by hand

More tips ↗ | Customize Qodo ↗ | Qodo docs ↗

[qodo-free-for-open-source-projects]

1. Older targets return null for trailers 🐞 Bug ≡ Correctness

TrailingHeaders has no empty default, and its only assignment in GetDefaultResponse is excluded
from non-NET builds. Successful responses on .NET Framework and .NET Standard therefore leave the
property null, unlike modern targets, where responses without trailers expose an empty collection.

Agent Prompt

## Issue description
Successful responses on older targets leave `TrailingHeaders` null because their builds exclude its only assignment.
## Fix Focus Areas
- src/RestSharp/Response/RestResponseBase.cs[114-117]
- src/RestSharp/Response/RestResponse.cs[76-78]
## Recommended Fix
Initialize `TrailingHeaders` to an empty collection for targets without trailer support, while preserving the populated collection on supported targets.

ⓘ Copy this prompt and use it to remediate the issue with your preferred AI generation tools

[HosseinDinarvand]

@dotnet-policy-service agree