[Mono.Android] Fix build warnings in hand-written code by simonrozsival · Pull Request #12003 · dotnet/android

simonrozsival · 2026-07-07T12:12:23Z

Supersedes #11999 (that PR was from a fork; this one is rebased on latest main and pushed to an origin branch so it runs the full CI). #11999 will be closed.

Summary

Reduces the C# build warnings in src/Mono.Android by fixing the warnings that originate in hand-written source, and completes the API-37 public API baseline so the public API analyzer passes.

It intentionally does not remove every NoWarn suppression, nor the Mono.Android.csproj entry in _AllowProjectWarnings (in Directory.Build.props). The warnings that remain all originate in generated code and require changes to the code generators — see "Why suppressions remain" below.

What is fixed (hand-written code)

CS8765 - override parameter nullability aligned to the base member (System.DrawingTypeConverter subclasses, SyncContext, Java stream adapters, Color/AndroidBitmapInfo equality, AdapterView.RawAdapter).
CS8764 - JavaObject.ToString() returns non-null to match Java.Lang.Object.ToString().
CS8767 - interface-implementation parameter nullability aligned to the interface (IComparer, ISpliterator, ISetCookie, IX509TrustManager/ISSLSession, IXmlPullParser).
CS0114 / CS0108 - new added to members that intentionally hide an inherited member (generic collection specializations, static helpers, IXmlResourceParser disambiguation, StringBuffer/StringBuilder.Append, AsyncTask.class_ref, JavaProxyThrowable.InnerException, SparseArray<E>, ArrayAdapter<T>, JavaList.Equals).

Public API baseline (RS0016 / RS0017)

Some of the nullability fixes change the annotations of public members, and the API-37 baseline was missing a large number of generated binding members. PublicAPI/API-37/PublicAPI.Unshipped.txt is regenerated (via the RS0016 code fix, plus *REMOVED* markers for stale signatures) so RS0016/RS0017 are satisfied and remain enforced (they are not in NoWarn).

Why suppressions remain

The remaining warnings are all in generated code and cannot be fixed here without changing the generators:

mcw/*.cs (binding generator): CS0108, CS0114, CS0618, CS0809, CS8613, CS8764, CS8765, CS8766, CS8767.
JNIEnv.g.cs (jnienv-gen, // Generated file; DO NOT EDIT!): RS0041 (public members using oblivious reference types).

These files are regenerated on every build, so the fix must happen in the generator. Until those follow-ups land, we keep the corresponding codes in <NoWarn> and keep <_AllowProjectWarnings Condition=" '$(MSBuildProjectFile)' == 'Mono.Android.csproj' ">true</_AllowProjectWarnings>. Removing either now would turn hundreds of generated-code warnings into build errors under TreatWarningsAsErrors.

Follow-ups

Binding generator - emit warning-free mcw/*.cs, then drop CS0108;CS0114;CS0618;CS0809;CS8613;CS8764;CS8765;CS8766;CS8767 from NoWarn.
jnienv-gen - annotate JNIEnv.g.cs output, then drop RS0041 from NoWarn.
CS0436 - handled in [Mono.Android] Compile RuntimeFeature into a single assembly to avoid conflict warnings #11669.

Once those land, the remaining NoWarn codes and the _AllowProjectWarnings opt-in for Mono.Android.csproj can be removed.

Match the nullability of overridden base-class parameters (TypeConverter, SynchronizationContext, Java stream adapters, Color/AndroidBitmapInfo equality, AdapterView.RawAdapter) so overrides no longer widen non-nullable base parameters, guarding newly-nullable arguments where the body dereferences them.

JavaObject.ToString() overrides Java.Lang.Object.ToString() (non-null), so return a non-null string instead of string?.

Match the parameter nullability of the implemented interface members (IComparer, ISpliterator, ISetCookie, IX509TrustManager/ISSLSession, IXmlPullParser). Guard TrustManager callbacks with ArgumentNullException.ThrowIfNull and return null early from GetNamespace for a null prefix.

Add the 'new' keyword to members that intentionally hide an inherited virtual member (SparseArray<E>.Get/ValueAt, ArrayAdapter<T>.GetItem/Sort, JavaList.Equals(Java.Lang.Object)).

Add the 'new' keyword to members that intentionally hide an inherited member: generic collection specializations (JavaCollection/JavaList/JavaSet/JavaDictionary/ArrayAdapter/BaseAdapter), static helpers (FromJniHandle/ToLocalJniHandle/CreateFromResource), IXmlResourceParser members disambiguating IAttributeSet/IXmlPullParser, StringBuffer/StringBuilder.Append, AsyncTask.class_ref, and JavaProxyThrowable.InnerException.

The API-37 public API baseline was missing ~4570 members (generated bindings plus a few hand-written types), and the nullability fixes in earlier commits changed the annotations of several public members. Populate PublicAPI.Unshipped.txt via the RS0016 code fix and add *REMOVED* markers for the stale signatures (RS0017) so the PublicApiAnalyzer passes with RS0016/RS0017 enforced.

Guard CheckClientTrusted params (consistency with CheckServerTrusted); keep the clr_typemap_managed_to_java P/Invoke param non-nullable and guard the call site when Type.FullName is null; drop the redundant null-forgiving operator in AdapterView.RawAdapter getter; replace null-forgiving operators in ValueAnimator/AnimatorSet marshaling with explicit null checks that throw a clear exception.

main removed JavaPeerProxy's invokerType constructor parameter and InvokerType property and added Java.Interop.BaseExportAttribute; the rebased constructor-visibility change also exposes the JavaPeerProxy constructors as protected. Update PublicAPI.Unshipped.txt (API 37) accordingly so RS0016/RS0017 stay clean.

Copilot

Pull request overview

This PR reduces build warnings in src/Mono.Android by aligning nullability annotations in hand-written code with base members/interfaces, tightening up a few marshaling paths, and updating the API-37 public API baselines so the public API analyzer passes.

Changes:

Updated a number of overrides/interface implementations to match expected nullable signatures and added targeted null checks where appropriate.
Adjusted dynamic-code vs AOT code paths (notably typemap/collection conversion) and replaced/added specific suppressions so IL3050 can be removed from project-wide NoWarn.
Regenerated/cleaned PublicAPI baseline files (API-36.1/API-37) to keep RS0016/RS0017 enforced.

Show a summary per file

File	Description
src/Mono.Android/Xamarin.Android.Net/ServerCertificateCustomValidator.cs	Align trust manager / SSL session method nullability with interfaces and add argument validation.
src/Mono.Android/Xamarin.Android.Net/AndroidMessageHandler.cs	Align helper method parameter nullability with `HttpRequestException`/`WebException` usage.
src/Mono.Android/System.Drawing/SizeFConverter.cs	Align `TypeConverter` override nullability and handle null `CultureInfo`.
src/Mono.Android/System.Drawing/SizeConverter.cs	Align `TypeConverter` override nullability and handle null `CultureInfo`.
src/Mono.Android/System.Drawing/RectangleConverter.cs	Align `TypeConverter` override nullability and handle null `CultureInfo`.
src/Mono.Android/System.Drawing/PointConverter.cs	Align `TypeConverter` override nullability.
src/Mono.Android/PublicAPI/API-37/PublicAPI.Shipped.txt	Remove duplicate baseline entries.
src/Mono.Android/PublicAPI/API-36.1/PublicAPI.Shipped.txt	Remove duplicate baseline entries.
src/Mono.Android/Org.Apache.Http.Impl.Cookie/BasicClientCookie.cs	Align cookie setter nullability with interface expectations.
src/Mono.Android/Mono.Android.csproj	Remove `IL3050` from `NoWarn` now that code is suppressed/guarded more precisely.
src/Mono.Android/Microsoft.Android.Runtime/TrimmableTypeMapTypeManager.cs	Route array-type generation based on dynamic-code support vs pre-generated typemap proxies; add `RequiresDynamicCode`.
src/Mono.Android/Microsoft.Android.Runtime/ManagedTypeManager.cs	Add `IL3050` suppression for reflection-based type manager base.
src/Mono.Android/Microsoft.Android.Runtime/JniRemappingLookup.cs	Suppress CS0649 for struct fields used as interop shapes.
src/Mono.Android/Microsoft.Android.Runtime/JavaMarshalRegisteredPeers.cs	Suppress CS0649 for interop struct fields.
src/Mono.Android/Java.Util/Spliterators.cs	Align `ISpliterator`-related nullability in `TryAdvance`.
src/Mono.Android/Java.Lang/StringBuilder.cs	Mark intentional member hiding with `new`.
src/Mono.Android/Java.Lang/StringBuffer.cs	Mark intentional member hiding with `new`.
src/Mono.Android/Java.Interop/TypeManager.cs	Improve nullability handling in typemap paths (e.g., pointer-to-string conversion, class-name lookup).
src/Mono.Android/Java.Interop/JavaPeerProxy.cs	Adjust proxy attribute ctor accessibility and validate ctor arguments.
src/Mono.Android/Java.Interop/JavaConvert.cs	Guard reflection-based generic collection conversion behind dynamic-code support and add `RequiresDynamicCode`.
src/Mono.Android/Android.Widget/BaseAdapter.cs	Mark intentional member hiding with `new`.
src/Mono.Android/Android.Widget/ArrayAdapter.cs	Mark intentional member hiding with `new` for members that shadow base APIs.
src/Mono.Android/Android.Widget/AdapterView.cs	Update `RawAdapter` nullability and remove null-forgiving in getter.
src/Mono.Android/Android.Util/SparseArray.cs	Mark intentional member hiding with `new`.
src/Mono.Android/Android.Runtime/XmlReaderPullParser.cs	Align nullability to `IXmlPullParser` expectations; add defensive null handling.
src/Mono.Android/Android.Runtime/XmlPullParserReader.cs	Mark intentional member hiding with `new`.
src/Mono.Android/Android.Runtime/OutputStreamAdapter.cs	Align override nullability and add argument validation.
src/Mono.Android/Android.Runtime/JNIEnv.cs	Refine dynamic-code/AOT array creation behavior and use targeted `IL3050` suppression only when required.
src/Mono.Android/Android.Runtime/JavaSet.cs	Mark intentional member hiding with `new`.
src/Mono.Android/Android.Runtime/JavaProxyThrowable.cs	Mark intentional member hiding with `new`.
src/Mono.Android/Android.Runtime/JavaObject.cs	Ensure `ToString()` returns non-null to match Java expectations.
src/Mono.Android/Android.Runtime/JavaList.cs	Mark intentional member hiding with `new`.
src/Mono.Android/Android.Runtime/JavaDictionary.cs	Mark intentional member hiding with `new`.
src/Mono.Android/Android.Runtime/JavaCollection.cs	Mark intentional member hiding with `new`.
src/Mono.Android/Android.Runtime/InputStreamAdapter.cs	Align override nullability and add argument validation.
src/Mono.Android/Android.OS/AsyncTask.cs	Mark intentional member hiding with `new`.
src/Mono.Android/Android.Graphics/Color.cs	Align override nullability for marshaler APIs.
src/Mono.Android/Android.Graphics/AndroidBitmapInfo.cs	Align `Equals(object)` nullability.
src/Mono.Android/Android.Content.Res/IXmlResourceParser.cs	Disambiguate interface conflicts with `new` members.
src/Mono.Android/Android.Bluetooth/BluetoothDevice.cs	Add OS-platform obsoletion metadata for API-37.
src/Mono.Android/Android.App/SyncContext.cs	Align `SynchronizationContext` override nullability.
src/Mono.Android/Android.App/AlertDialog.cs	Switch to `EventHelper` helper to align event wiring nullability.
src/Mono.Android/Android.Animation/ValueAnimator.cs	Make delegate field nullable and add null checks for marshaling results.
src/Mono.Android/Android.Animation/AnimatorSet.cs	Make delegate field nullable and add null checks for marshaling results.

Copilot's findings

Comments suppressed due to low confidence (1)

src/Mono.Android/Android.Graphics/Color.cs:451

Avoid the null-forgiving operator when reflecting for the Color(int) constructor. Even if it "should" exist, trimming/linking (or future refactors) can make this return null; an explicit check keeps the failure mode clear and avoids !.

 public override Expression CreateParameterToManagedExpression (JniValueMarshalerContext context, ParameterExpression sourceValue, ParameterAttributes synchronize, Type? targetType)
{
var c = typeof (Color).GetConstructor (new[]{typeof (int)})!;
var v = Expression.Variable (typeof (Color), sourceValue.Name + "_val");
context.LocalVariables.Add (v);
context.CreationStatements.Add (Expression.Assign (v, Expression.New (c, sourceValue)));

Files reviewed: 44/45 changed files
Comments generated: 4

+ protected override Java.Lang.Object? RawAdapter {
+ get { return JavaObjectExtensions.JavaCast<Java.Lang.Object>(JavaConvert.ToJavaObject (Adapter)); }
 set { Adapter = JavaConvert.FromJavaObject<T>(value)!; }
 }


simonrozsival · 2026-07-07T14:53:02Z

@copilot fix this warning:

Xamarin.Android.Build.Tests.BuildTest2.BuildHasNoWarnings(True,False,"apk",NativeAOT)
build.log should have exactly 2 MSBuild warnings for NativeAOT, but found 0.
Expected: 2
But was: 0

…ore expected ILC warnings Co-authored-by: simonrozsival <374616+simonrozsival@users.noreply.github.com>

Copilot · 2026-07-07T15:23:45Z

@copilot fix this warning:

...

Fixed in the latest commit. The class-level [UnconditionalSuppressMessage ("Trimming", "IL3050")] I added to ManagedTypeManager was silencing the ILC warnings at the AOT analysis step, so the test found 0 instead of the expected 2. Removing that attribute lets the ILC emit the 2 IL3050 warnings (from ManagedTypeManager inheriting [RequiresDynamicCode]ReflectionJniTypeManager) that the test expects. The build stays clean because _AllowProjectWarnings=true for Mono.Android.csproj.

simonrozsival · 2026-07-07T21:09:42Z

@copilot what's still left to do?

Copilot · 2026-07-07T21:34:57Z

@copilot what's still left to do?

One open reviewer thread remains: the ! in the RawAdapter setter in AdapterView.cs (JavaConvert.FromJavaObject<T>(value)!). That ! was already present before this PR — I only changed the property from set to override to fix CS8765. If you'd like me to address it I can, otherwise the PR is otherwise complete:

✅ CS8765/CS8764/CS8767/CS0114/CS0108 warnings fixed in hand-written code
✅ API-37 public API baseline regenerated (RS0016/RS0017 passing)
✅ IL3050 removed from NoWarn (replaced with targeted suppressions)
✅ BuildHasNoWarnings(NativeAOT) test fix (restored expected 2 IL3050 ILC warnings)
✅ All other reviewer threads resolved

Use the AOT warning category for the ManagedTypeManager IL3050 suppression so NativeAOT analysis recognizes it. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Keep the ManagedTypeManager AOT IL3050 suppression needed for the NativeAOT build. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

simonrozsival · 2026-07-07T22:21:59Z

/review

github-actions · 2026-07-07T22:22:41Z

✅ Android PR Reviewer completed successfully!

github-actions

🤖 Automated review — PR #12003

Verdict: ✅ Clean, well-scoped change. This is a mechanical build-warnings cleanup by a maintainer; I found no bugs — only 3 minor 💡 suggestions posted inline (non-blocking).

What this PR does (independent read)

It removes IL3050 from <NoWarn> in Mono.Android.csproj and instead eliminates or precisely suppresses the underlying warnings in hand-written code:

Replaces the old IsCoreClrRuntime / IsNativeAotRuntime branch logic in JNIEnv.ArrayCreateInstance, JavaConvert.GetJniHandleConverter, and TrimmableTypeMapTypeManager.GetArrayTypes with RuntimeFeature.IsDynamicCodeSupported as the primary guard (a BCL [FeatureGuard(typeof(RequiresDynamicCodeAttribute))], so [RequiresDynamicCode] calls inside those blocks no longer trip IL3050).
Adds targeted [UnconditionalSuppressMessage] / [RequiresDynamicCode] where a suppression is genuinely required (e.g. the ManagedTypeMap path).
Silences a batch of nullability (CS8764/CS8765/CS8767) and member-hiding (CS0108/CS0114) warnings via signature alignment and the new keyword.
Correspondingly removes the NativeAOT "expect 2 IL3050 warnings" special-case in BuildTest2.BuildHasNoWarnings, which now asserts zero warnings for every runtime — a strengthened regression guard.

Verification notes

Behavior-preserving restructures: I traced the three rewritten methods across CoreCLR-JIT (IsDynamicCodeSupported == true), Mono (true), and NativeAOT (false → TrimmableTypeMap proxy path). Each new fail-fast throw NotSupportedException is only reachable in genuinely-unsupported configurations. ✔️
EventHelper rename in AlertDialog.cs is correct — AndroidEventHelper is [Obsolete("Use EventHelper")], and this matches the established pattern (MediaController, ViewTreeObserver, AccountManager, and the generator's InterfaceListenerEvent). ✔️
GetClassNameFromInstance → string? (public API): every product caller uses it in string interpolation/concatenation (null-safe); the 3 test callers in JnienvArrayMarshaling.cs assign to string, but that file has no #nullable enable and the test csproj only sets WarningsAsErrors=IL2037, so no CS8600 is produced. No downstream break. ✔️
Stream adapters / converters / new-hidden members: all mechanical alignments to base/interface signatures; ArgumentNullException.ThrowIfNull on the stream adapters matches Java NPE semantics. ✔️

CI

At review time the full Azure DevOps (dnceng-public) build — which actually validates the zero-warnings claim behind removing IL3050 from <NoWarn> — was still in progress with no failures observed (Java.Interop Tests on Windows/Mac already green; license/CLA green). Mergeability depends on those build stages completing green.

Inline suggestions (all 💡, non-blocking)

JavaConvert.cs L109 — NotSupportedException message says "array element type" for a generic-collection branch.
JavaProxyThrowable.cs L14 — awareness note on the now-explicit InnerException shadow (behavior unchanged).
PointConverter.cs L67 — continuation-line indentation drifted out of alignment (recurs across the System.Drawing converters).

Nice, tidy cleanup overall. 👍

Generated by Android PR Reviewer for #12003 · 367.8 AIC · ⌖ 23.2 AIC · ⊞ 6.8K
_{Comment /review to run again}

The ManagedTypeManager AOT warning is now suppressed, so NativeAOT builds should be warning-free in these tests. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

simonrozsival added 16 commits July 7, 2026 13:05

Fix IL3050 warnings
be64df0

Fix CA1422 (obsoleted os platform)
82f22fb

Address nullability warnings
5faddfe

Avoid using obsolete APIs internally
a4844b6

Suppress CS0649 for structs which are marsahled from native to managed
d415d88

fixup! Address nullability warnings
c0fc721

Fix RS0025
d84f2c1

Fix constructor visibility
98b2968

Fix CS8764 nullable return warning
9d7271b
JavaObject.ToString() overrides Java.Lang.Object.ToString() (non-null), so return a non-null string instead of string?.

Fix CS0114 member-hiding warnings with 'new'
fdaa8a4
Add the 'new' keyword to members that intentionally hide an inherited virtual member (SparseArray<E>.Get/ValueAt, ArrayAdapter<T>.GetItem/Sort, JavaList.Equals(Java.Lang.Object)).

CopilotAI review requested due to automatic review settings July 7, 2026 12:12

simonrozsival requested a review from jonathanpeppers as a code owner July 7, 2026 12:12

simonrozsival mentioned this pull request Jul 7, 2026
[Mono.Android] Fix build warnings in hand-written code #11999
Closed

Copilotstarted reviewing on behalf of simonrozsival July 7, 2026 12:12 View session

CopilotAI reviewed Jul 7, 2026
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Copilotstarted work on behalf of simonrozsival July 7, 2026 14:55 View session

Remove class-level IL3050 suppression from ManagedTypeManager to rest…
1bc4c87
…ore expected ILC warnings Co-authored-by: simonrozsival <374616+simonrozsival@users.noreply.github.com>

Copilotfinished work on behalf of simonrozsival July 7, 2026 15:24

Copilotstarted work on behalf of simonrozsival July 7, 2026 21:11 View session

Copilotfinished work on behalf of simonrozsival July 7, 2026 21:35

simonrozsivaland others added 2 commits July 8, 2026 00:07

[Mono.Android] Suppress ManagedTypeManager AOT warning
7768fbe
Use the AOT warning category for the ManagedTypeManager IL3050 suppression so NativeAOT analysis recognizes it. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Merge remote warning suppression update
63316d1
Keep the ManagedTypeManager AOT IL3050 suppression needed for the NativeAOT build. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

github-actionsBot reviewed Jul 7, 2026
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Comment threadsrc/Mono.Android/Java.Interop/JavaConvert.cs
Comment threadsrc/Mono.Android/Android.Runtime/JavaProxyThrowable.cs
Comment threadsrc/Mono.Android/System.Drawing/PointConverter.cs

[tests] Update NativeAOT warning expectations
f0a2ede
The ManagedTypeManager AOT warning is now suppressed, so NativeAOT builds should be warning-free in these tests. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

simonrozsival added the ready-to-review This PR is ready to review/merge, I think any CI failures are just flaky (ignorable). label Jul 8, 2026

jonathanpeppers approved these changes Jul 13, 2026
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jonathanpeppers merged commit a1266b2 into mainJul 13, 2026
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jonathanpeppers deleted the dev/simonrozsival/fix-mono-android-build-warnings branch July 13, 2026 13:17

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[Mono.Android] Fix build warnings in hand-written code - #12003

[Mono.Android] Fix build warnings in hand-written code#12003
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simonrozsival commented Jul 7, 2026

Summary

What is fixed (hand-written code)

Public API baseline (RS0016 / RS0017)

Why suppressions remain

Follow-ups

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Pull request overview

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🤖 Automated review — PR #12003

What this PR does (independent read)

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