AIDL CSharpCodeGenerator: oneway methods generate synchronous Proxy code

[jonathanpeppers]

Summary

Xamarin.Android.Tools.Aidl.CSharpCodeGenerator does not honor AIDL method-level oneway semantics on the client (Proxy) side. For a method declared as:

oneway void fireAndForget(int value);

the generated Proxy still:

1. Allocates a reply Parcel (var __reply = Parcel.Obtain ();),
2. Passes it to Transact (...),
3. Calls __reply.ReadException () after the transact returns.

True AIDL oneway semantics require:

• No reply parcel (pass null),
• The FlagOneway transact flag,
• No ReadException call (oneway calls cannot return exceptions to the caller).

The current behavior turns oneway into an effectively synchronous call and can block the caller until the remote side returns, defeating the purpose of oneway.

Where

src/Xamarin.Android.Tools.Aidl/CSharpCodeGenerator.cs — Proxy method emission for methods with IsOneway == true.

Repro / evidence

See the golden output snapshotted in tests/Xamarin.Android.Tools.Aidl-Tests/TestData/OnewayMethods.txt (added in #11460), which captures the current Proxy output for a oneway method.

Notes

• Stub side already correctly skips reply.WriteNoException () for oneway methods.
• A fix here is a behavior change for anyone consuming AIDL-generated code, so it likely needs a brief compat note.
• Related: Xamarin.Android.Tools.Aidl.CSharpCodeGenerator needs unit tests. #5011 (needs unit tests — addressed by Add unit tests for Xamarin.Android.Tools.Aidl.CSharpCodeGenerator #11460), Xamarin AIDL CSharpCodeGenerator doesn't support specifying transaction IDs #8735 (transaction IDs).