[bgen] Propagate nullability information for generic type arguments. Fixes #16860 by rolfbjarne · Pull Request #25541 · dotnet/macios

rolfbjarne · 2026-05-27T12:56:36Z

Summary

When properties have generic delegate types (like Action<NSObject?, NSError?>), bgen now correctly propagates the ? nullability annotations on the generic type arguments in the generated C# code.

Previously, bgen only read the first byte of the [NullableAttribute(byte[])] array (for the outer type's nullability), ignoring the remaining bytes that encode nullability for generic type arguments. This caused properties like Action<UIViewController?, NSError?> to be rendered as Action<UIViewController, NSError> — losing the nullability annotations.

Changes

src/bgen/AttributeManager.cs: Added GetNullabilityBytes() method that extracts the full byte array from [NullableAttribute].
src/bgen/TypeManager.cs: Added FormatType(Type?, Type?, byte[]?) overload and a recursive FormatTypeUsedIn helper that consumes nullability bytes in depth-first traversal order, correctly skipping value types (which don't have bytes in the array).
src/bgen/Generator.cs: Updated property rendering to pass nullability bytes. The "wrap" path applies nullability for all delegate types. The "non-wrap" path only applies for void-returning delegates (Action<>) to avoid Func<> covariance issues with trampoline signatures.
tests/bgen/BGenTests.cs: Added GenericTypeNullability test with 10 assertions.
tests/bgen/tests/generic-type-nullability.cs: Test input covering nullable args, non-nullable args, value types, many args, mixed patterns, and alternating nullability.

Key design decisions

Value types skip byte consumption: The C# compiler does NOT emit bytes for value types in the NullableAttribute array. The fix correctly identifies value types and skips them during traversal.
Func<> covariance guard: Trampolines use non-nullable generic args. This is fine for Action<in T> (contravariant) but breaks for Func<out T> (covariant). The non-wrap path only applies nullability for void-returning delegates.
Wrap path is unrestricted: The wrap path generates self-contained code without trampoline interaction, so it can safely apply nullability for any delegate type.

Fixes #16860

🤖 Pull request created by Copilot

…ixes #16860. When a property has a generic type like Action<NSObject?, NSError?>, the C# compiler emits a [NullableAttribute(byte[])] where each byte encodes the nullability for the outer type and each generic argument in depth-first order. Previously, bgen only looked at the first byte (the outer type) and ignored nullability for generic type arguments. This change: - Adds GetNullabilityBytes() to AttributeManager to extract the full byte array - Adds a FormatType overload in TypeManager that processes the byte array to annotate each generic type argument with ? where appropriate - Uses the new method when rendering property type declarations Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

…onsumption. - Use braces for all if/else/for blocks in new code. - Add complex test samples with many generic args, value types, and alternating nullability patterns. - Fix bug where value types incorrectly consumed nullability bytes. - Only apply nullability for void-returning delegates (Action<>) in the non-wrap path to avoid Func<> covariance issues with trampolines. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

…tead. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

rolfbjarne · 2026-06-03T18:56:41Z

/azp run

azure-pipelines · 2026-06-03T18:57:09Z

Azure Pipelines successfully started running 3 pipeline(s).

rolfbjarne · 2026-06-04T07:33:21Z

/azp run

azure-pipelines · 2026-06-04T07:33:51Z

Azure Pipelines successfully started running 3 pipeline(s).

The MarkdownFormatter had the old and new text swapped in two places: - DiffModification wrapped 'old' with addition markers and 'new' with removal markers (backwards compared to HtmlFormatter). - The Diff method assigned the wrong Clean() arguments to the - and + lines, causing the old line to show added content and vice versa. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

…-actions

vs-mobiletools-engineering-service2 · 2026-06-23T16:18:17Z

✅ [PR Build #60617e8] Build passed (Detect API changes) ✅

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vs-mobiletools-engineering-service2 · 2026-06-23T16:20:40Z

✅ [PR Build #60617e8] Build passed (Build packages) ✅

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vs-mobiletools-engineering-service2 · 2026-06-23T16:30:31Z

✅ API diff for current PR / commit

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iOS: vsdrops gist ( No breaking changes )
tvOS: vsdrops gist ( No breaking changes )
MacCatalyst: vsdrops gist ( No breaking changes )
macOS: vsdrops gist ( No breaking changes )

✅ API diff vs stable

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iOS: vsdrops gist ( No breaking changes )
tvOS: vsdrops gist ( No breaking changes )
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macOS: vsdrops gist ( No breaking changes )

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vs-mobiletools-engineering-service2 · 2026-06-23T16:39:40Z

✅ [PR Build #60617e8] Build passed (Build macOS tests) ✅

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Hash: 60617e89b28dde5f5685110fc8ee2e3054aed75f [PR build]

Copilot

Pull request overview

This PR updates bgen and related tooling to correctly propagate C# nullable-reference-type annotations (?) into generated signatures for generic type arguments (notably Action<T1,...> completion handlers), addressing #16860.

Changes:

bgen: extract full [Nullable] byte arrays and use them to render ? on generic type arguments in generated C# signatures (with a guard to avoid Func<> trampoline mismatches).
api-tools: render full nullability in mono-api-info outputs and improve markdown/html diff formatting for nullability-only changes.
tests: add a new bgen test input + assertions covering generic-argument nullability patterns (including value-type skipping).

Reviewed changes

Copilot reviewed 12 out of 12 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
tools/api-tools/mono-api-info/mono-api-info.cs	Formats member type names with full nullability (incl. generic args) and omits nullable metadata attributes from output.
tools/api-tools/mono-api-html/MultiplexedFormatter.cs	Ensures placeholder `<`/`>` tokens are replaced per-formatter before diff rendering.
tools/api-tools/mono-api-html/MarkdownFormatter.cs	Fixes add/remove handling in modification diffs and handles nullability-only diff markers.
tools/api-tools/mono-api-html/ApiChange.cs	Treats placeholder-escaped `>` as a valid separator when detecting `?` suffixes.
tests/bgen/tests/generic-type-nullability.cs	Adds binding input covering nullable/non-nullable generic args, value types, and mixed patterns.
tests/bgen/BGenTests.cs	Adds a new test validating that generated output includes `?` on the correct generic arguments.
src/Foundation/NSUrlSessionHandler.cs	Updates redirection completion handler to accept nullable `NSUrlRequest?` and passes `null` directly.
src/foundation.cs	Updates the delegate signature for redirection to `Action<NSUrlRequest?>` (scoped `#nullable enable`).
src/bgen/TypeManager.cs	Adds formatting that consumes nullability-byte arrays to render nullable generic arguments.
src/bgen/Models/AsyncMethodInfo.cs	Updates async-method analysis to infer nullable `NSError?` from the outer parameter’s nullability bytes.
src/bgen/Generator.cs	Threads nullability-byte info into parameter/property signature rendering (with `Func<>` covariance guard).
src/bgen/AttributeManager.cs	Adds `GetNullabilityBytes` to retrieve the full `[Nullable]` byte array.

+		if (nullabilityBytes is null || nullabilityBytes.Length <= 1) {
+			return FormatTypeUsedIn (usedIn?.Namespace, type);
+		}


+		// Reference types consume one byte from the array
+		byte currentByte = index < nullabilityBytes.Length ? nullabilityBytes [index] : (byte) 0;
+		index++;
+


+			if (nullabilityBytes is not null && nullabilityBytes.Length > 1) {
+				// Walk the type arguments depth-first to find the byte index for the last param.
+				// Value types don't consume bytes, reference types do.
+				// byte[0] is for the Action<> itself, then each reference type arg consumes one byte.
+				int byteIndex = 1; // start after the outer type byte
+				var genericArgs = lastType.GetGenericArguments ();
+				for (int i = 0; i < genericArgs.Length; i++) {
+					if (!genericArgs [i].IsValueType) {
+						if (i == genericArgs.Length - 1) {
+							// This is the last generic argument (NSError)
+							IsNSErrorNullable = byteIndex < nullabilityBytes.Length && nullabilityBytes [byteIndex] == 2;
+						}
+						byteIndex++;
+					}
+				}
+			} else {
+				IsNSErrorNullable = generator.AttributeManager.IsNullable (lastParam);
+			}


vs-mobiletools-engineering-service2 · 2026-06-23T17:54:36Z

🔥 [CI Build #60617e8] Test results 🔥

Test results

❌ Tests failed on VSTS: test results

0 tests crashed, 5 tests failed, 202 tests passed.

Failures

❌ fsharp tests

2 tests failed, 2 tests passed.

Failed tests

fsharp/iOS - simulator/Debug: LaunchTimedOut
fsharp/tvOS - simulator/Debug: LaunchTimedOut

Html Report (VSDrops) Download

❌ xcframework tests

2 tests failed, 2 tests passed.

Failed tests

xcframework-test/iOS - simulator/Debug: LaunchTimedOut
xcframework-test/tvOS - simulator/Debug: LaunchTimedOut

Html Report (VSDrops) Download

❌ Tests on macOS Sequoia (15) tests

1 tests failed, 4 tests passed.

Failed tests

monotouch-test: Failed (exit code 2)
- AcceptSslCertificatesServicePointManager(System.Net.Http.HttpClientHandler) : Assert.That(ex, Is.Null)

Html Report (VSDrops) Download

Successes

✅ assembly-processing: All 1 tests passed. Html Report (VSDrops) Download
✅ cecil: All 1 tests passed. Html Report (VSDrops) Download
✅ dotnettests (iOS): All 1 tests passed. Html Report (VSDrops) Download
✅ dotnettests (MacCatalyst): All 1 tests passed. Html Report (VSDrops) Download
✅ dotnettests (macOS): All 1 tests passed. Html Report (VSDrops) Download
✅ dotnettests (Multiple platforms): All 1 tests passed. Html Report (VSDrops) Download
✅ dotnettests (tvOS): All 1 tests passed. Html Report (VSDrops) Download
✅ framework: All 2 tests passed. Html Report (VSDrops) Download
✅ generator: All 5 tests passed. Html Report (VSDrops) Download
✅ interdependent-binding-projects: All 4 tests passed. Html Report (VSDrops) Download
✅ introspection: All 4 tests passed. Html Report (VSDrops) Download
✅ linker (iOS): All 15 tests passed. Html Report (VSDrops) Download
✅ linker (MacCatalyst): All 15 tests passed. Html Report (VSDrops) Download
✅ linker (macOS): All 21 tests passed. Html Report (VSDrops) Download
✅ linker (tvOS): All 15 tests passed. Html Report (VSDrops) Download
✅ monotouch (iOS): All 20 tests passed. Html Report (VSDrops) Download
✅ monotouch (MacCatalyst): All 19 tests passed. Html Report (VSDrops) Download
✅ monotouch (macOS): All 20 tests passed. Html Report (VSDrops) Download
✅ monotouch (tvOS): All 20 tests passed. Html Report (VSDrops) Download
✅ msbuild: All 2 tests passed. Html Report (VSDrops) Download
✅ sharpie: All 1 tests passed. Html Report (VSDrops) Download
✅ windows: All 3 tests passed. Html Report (VSDrops) Download
✅ xtro: All 1 tests passed. Html Report (VSDrops) Download

macOS tests

✅ Tests on macOS Monterey (12): All 5 tests passed. Html Report (VSDrops) Download
✅ Tests on macOS Ventura (13): All 5 tests passed. Html Report (VSDrops) Download
✅ Tests on macOS Sonoma (14): All 5 tests passed. Html Report (VSDrops) Download
✅ Tests on macOS Tahoe (26): All 5 tests passed. Html Report (VSDrops) Download

Linux Build Verification

✅ Linux build succeeded

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Hash: 60617e89b28dde5f5685110fc8ee2e3054aed75f [PR build]

rolfbjarne and others added 3 commits May 27, 2026 14:16

[bgen] Remove null-forgiving operator usage, use pattern matching ins…

bce4d13

…tead. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

rolfbjarne added the copilot label May 27, 2026