[csharp] Fix oneOf/anyOf response deserialization in httpclient library

[cguldner]

Fixes #25079

Problem

In the csharp httpclient library, ApiClient.ExecAsync has a special case for oneOf/anyOf response types (types derived from AbstractOpenAPISchema): it reflectively invokes the model's static FromJson(string jsonString) method.

The call passed response.Content — an HttpContent — rather than the JSON string that FromJson expects. Because the invocation goes through reflection, it compiled fine but threw at runtime on every oneOf/anyOf response:

System.ArgumentException: Object of type 'HttpConnectionResponseContent' cannot be converted to type 'System.String'.

This affects any operation whose response schema uses oneOf/anyOf, and has been present since the httpclient library gained AbstractOpenAPISchema support.

Fix

Read the response body as a string first and pass that to FromJson. The content is already buffered by the preceding Deserialize<T> call, so re-reading it is safe:

string jsonString = await response.Content.ReadAsStringAsync().ConfigureAwait(false);
responseData = (T)typeof(T).GetMethod("FromJson").Invoke(null, new object[] { jsonString });

Note: the restsharp library is not affected — there response.Content is already a string.

Changes

• Updated template modules/openapi-generator/src/main/resources/csharp/libraries/httpclient/ApiClient.mustache.
• Regenerated the affected csharp httpclient samples (net9, net10, net10 non-public, net9 non-public, standard2.0).

PR checklist

• Read the contribution guidelines.
• Ran generation and committed all changed files. Only the httpclient template and its generated samples changed.
• Filed as targeting master.

cc C# technical committee: @mandrean @shibayan @Blackclaws @lucamazzanti

---

Summary by cubic

Fixes oneOf/anyOf response deserialization in the csharp httpclient library so these responses no longer throw at runtime.

ApiClient.ExecAsync was passing response.Content (an HttpContent) into the model's reflective FromJson(string) call, which threw ArgumentException on every such response. The fix reads the response body as a string first and passes that; the content is already buffered by the preceding Deserialize<T> call, so re-reading is safe. The restsharp library is unaffected because its response.Content is already a string.

• Adds a regression test to the net9, net10, non-public, and standard2.0 httpclient samples that exercises GetAsync<Pig> (a oneOf) through a stub handler, asserts the correct concrete type is deserialized, and disposes the injected HttpClient and ApiClient.
• Updates the httpclient template and its regenerated samples.

Fixes #25079.

^{Written for commit 9da6b3b. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 6 files

_{Re-trigger cubic}

[cguldner]

Added a regression test (OneOfResponseDeserializationTests) to the net10, net9, and standard2.0 httpclient Petstore samples. It drives ApiClient.GetAsync<Pig> (where Pig is a oneOf) through a stub HttpMessageHandler returning {"className":"BasquePig"} and asserts the response deserializes to the correct concrete type.

Verified with .NET SDK 10.0.401:

• With the fix: test passes.
• Reverting the fix (passing response.Content again): the test fails with the exact error from the issue — System.ArgumentException : Object of type 'System.Net.Http.StringContent' cannot be converted to type 'System.String'.

net9 and standard2.0 test projects build cleanly as well.

[cubic-dev-ai]

1 issue found across 3 files (changes from recent commits).

Prompt for AI agents (unresolved issues)

Check if these issues are valid — if so, understand the root cause of each and fix them. When an issue isn't valid or won't be fixed in this PR, reply in its thread with the reason and then resolve the thread. If appropriate, use sub-agents to investigate and fix each issue separately.


<file name="samples/client/petstore/csharp/httpclient/net9/Petstore/src/Org.OpenAPITools.Test/Client/OneOfResponseDeserializationTests.cs">

<violation number="1" location="samples/client/petstore/csharp/httpclient/net9/Petstore/src/Org.OpenAPITools.Test/Client/OneOfResponseDeserializationTests.cs:26">
P2: This regression test is hand-written into a generated sample directory with no mustache template backing it, so it is not reproducible from the generator. A clean sample regeneration (bin/generate-samples.sh rebuilds these outputs, and several csharp sample scripts remove the target directory first) will silently drop the coverage this PR adds; the net9/net10 non-public variants, which got the same ApiClient fix, also don't receive the test. Back the test with a C# template (e.g., a small mustache emitted into the Test source set) and regenerate all affected variants, or at minimum confirm the regen scripts preserve this file.</violation>
</file>

<sub>Reply with feedback, questions, or to request a fix.

Re-trigger cubic</sub>

[wing328]

thanks for the fix, which has been merged.