[dotnet] Fix support for startup hooks. Fixes #24492. #24664

rolfbjarne · 2026-02-10T16:16:46Z

Enable support by default, except when Optimize=true (aka Release builds).
Set the STARTUP_HOOKS runtime property if the DOTNET_STARTUP_HOOKS
environment variable is set - this is typically done at startup by CoreCLR
or MonoVM, but we're using both in an embedding mode, so we have to do it
ourselves.
Add a test.

Refs:

* Enable support by default, except when Optimize=true (aka Release builds). * Set the STARTUP_HOOKS runtime property if the DOTNET_STARTUP_HOOKS environment variable is set - this is typically done at startup by CoreCLR or MonoVM, but we're using both in an embedding mode, so we have to do it ourselves. * Add a test. Refs: * dotnet/android#10670 * https://github.com/dotnet/runtime/blob/242f7b23752599f22157268de41fee91cb97ef6c/docs/design/features/host-startup-hook.md

Copilot

Pull request overview

Fixes Startup Hook support for .NET-on-Apple platforms by enabling it by default for non-optimized builds, explicitly flowing DOTNET_STARTUP_HOOKS into the runtime property bag, and adding a regression test to validate behavior.

Changes:

Enable StartupHookSupport by default except when Optimize=true (typical Release builds).
Pass STARTUP_HOOKS host property to the embedded runtime when DOTNET_STARTUP_HOOKS is set.
Add a new StartupHookTest app + NUnit test coverage, and extend the test helper to allow asserting non-zero exit codes.

Reviewed changes

Copilot reviewed 16 out of 16 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
tests/dotnet/UnitTests/TestBaseClass.cs	Allows asserting an expected (non-zero) exit code when running test apps.
tests/dotnet/UnitTests/StartupHookTest.cs	Adds unit tests covering startup hook enabled/disabled behavior across configurations.
tests/dotnet/StartupHookTest/AppDelegate.cs	Adds a minimal app that reports whether the startup hook ran.
tests/dotnet/StartupHookTest/shared.csproj	Shared project definition for the new startup hook test app.
tests/dotnet/StartupHookTest/shared.mk	Shared make configuration to integrate the new test app into existing test infrastructure.
tests/dotnet/StartupHookTest/Makefile	Hooks the new test app into the dotnet test make targets.
tests/dotnet/StartupHookTest/iOS/StartupHookTest.csproj	iOS target project for the new test app.
tests/dotnet/StartupHookTest/iOS/Makefile	iOS make target for the new test app.
tests/dotnet/StartupHookTest/tvOS/StartupHookTest.csproj	tvOS target project for the new test app.
tests/dotnet/StartupHookTest/tvOS/Makefile	tvOS make target for the new test app.
tests/dotnet/StartupHookTest/macOS/StartupHookTest.csproj	macOS target project for the new test app.
tests/dotnet/StartupHookTest/macOS/Makefile	macOS make target for the new test app.
tests/dotnet/StartupHookTest/MacCatalyst/StartupHookTest.csproj	Mac Catalyst target project for the new test app.
tests/dotnet/StartupHookTest/MacCatalyst/Makefile	Mac Catalyst make target for the new test app.
runtime/runtime.m	Forwards startup hooks from `DOTNET_STARTUP_HOOKS` into the runtime host property `STARTUP_HOOKS`.
dotnet/targets/Xamarin.Shared.Sdk.targets	Adjusts default `StartupHookSupport` behavior and reserves the `STARTUP_HOOKS` runtimeconfig property.

tests/dotnet/UnitTests/StartupHookTest.cs

tests/dotnet/StartupHookTest/AppDelegate.cs

jonathanpeppers

The overall good looks good to me. 👍

But I think you should put the startup hook in a class library for testing.

jonathanpeppers · 2026-02-10T16:29:11Z

dotnet/targets/Xamarin.Shared.Sdk.targets

+		<!-- StartupHookSupport is used for Hot Reload, so we enable it unless the Optimize flag is set. -->
+		<StartupHookSupport Condition="'$(StartupHookSupport)' == '' And '$(Optimize)' != 'true'">true</StartupHookSupport>
+		<StartupHookSupport Condition="'$(StartupHookSupport)' == ''">false</StartupHookSupport>


On Android I also had this in a block for "applications" (that have OutputType=Exe or AndroidApplication=true).

It may not matter, though.

tests/dotnet/StartupHookTest/AppDelegate.cs

rolfbjarne · 2026-02-11T15:57:45Z

@jonathanpeppers the tests revealed a problem; there's a trimmer warning now for debug builds:

ILLink: System.StartupHookProvider.ProcessStartupHooks(String): Using member 'System.StartupHookProvider.CallStartupHook(StartupHookProvider.StartupHookNameOrPath)' which has 'RequiresUnreferencedCodeAttribute' can break functionality when trimming application code. The StartupHookSupport feature switch has been enabled for this app which is being trimmed. Startup hook code is not observable by the trimmer and so required assemblies, types and members may be removed.

Should we disable startup hooks by default when the trimmer is enabled? AFAIK hot restart isn't compatible with trimming anyway

jonathanpeppers · 2026-02-11T17:38:38Z

@rolfbjarne it will still work when trimmed if you do:

<TrimmerRootAssembly Include="StartupHook" RootMode="All" />

I did this to verify startup hooks work in NativeAOT, Release mode, etc., but it shows warnings.

rolfbjarne · 2026-02-11T17:54:29Z

@rolfbjarne it will still work when trimmed if you do:
<TrimmerRootAssembly Include="StartupHook" RootMode="All" />
I did this to verify startup hooks work in NativeAOT, Release mode, etc., but it shows warnings.

Since our device builds are trimmed by default (unless using the interpreter), many projects will trigger the warning now :/

Maybe we could just disable the warning if we enabled StartupHook ourselves.

…upHookSupport=true When we automatically enable StartupHookSupport for Hot Reload (because the developer didn't set it explicitly), suppress the resulting IL2026 trimmer warning from System.StartupHookProvider.ProcessStartupHooks. This is done by setting a feature switch (ObjCRuntime.SuppressStartupHookTrimWarning) that conditionally adds [UnconditionalSuppressMessage] to the method via the linker's link-attributes XML.

vs-mobiletools-engineering-service2 · 2026-02-11T19:10:34Z

🔥 [CI Build #883f1d7] Test results 🔥

Test results

❌ Tests failed on VSTS: test results

0 tests crashed, 5 tests failed, 125 tests passed.

Failures

❌ dotnettests tests (iOS)

1 tests failed, 0 tests passed.

Failed tests

DotNet tests: Failed (Execution failed with exit code 1)
- Xamarin.Tests.TemplateTest.CreateAndBuildItemTemplates(iOS,CShar...: Build warnings:
  System.StartupHookProvider.ProcessStartupHooks(String): Using member 'System.StartupHookProvider.CallStartupH...
- Xamarin.Tests.TemplateTest.CreateAndBuildProjectTemplate(ios-csh...: Build warnings:
  System.StartupHookProvider.ProcessStartupHooks(String): Using member 'System.StartupHookProvider.CallStartupH...
- Xamarin.Tests.TemplateTest.CreateAndBuildProjectTemplate(ios-not...: Build warnings:
  System.StartupHookProvider.ProcessStartupHooks(String): Using member 'System.StartupHookProvider.CallStartupH...
- ... and 6 more

Html Report (VSDrops) Download

❌ dotnettests tests (MacCatalyst)

1 tests failed, 0 tests passed.

Failed tests

DotNet tests: Failed (Execution failed with exit code 1)
- Xamarin.Tests.TemplateTest.CreateAndBuildItemTemplates(MacCataly...: Build warnings:
  System.StartupHookProvider.ProcessStartupHooks(String): Using member 'System.StartupHookProvider.CallStartupH...
- Xamarin.Tests.TemplateTest.CreateAndBuildProjectTemplate(maccata...: Build warnings:
  System.StartupHookProvider.ProcessStartupHooks(String): Using member 'System.StartupHookProvider.CallStartupH...
- Xamarin.Tests.TemplateTest.CreateAndBuildProjectTemplate(maccata...: Build warnings:
  System.StartupHookProvider.ProcessStartupHooks(String): Using member 'System.StartupHookProvider.CallStartupH...
- ... and 3 more

Html Report (VSDrops) Download

❌ dotnettests tests (macOS)

1 tests failed, 0 tests passed.

Failed tests

DotNet tests: Failed (Execution failed with exit code 1)
- Xamarin.Tests.TrimmerWarningsTest.TrimmerWarningsDynamicRegistra...: Unexpected warning: ILLink: System.StartupHookProvider.ProcessStartupHooks(String): Using member 'System.StartupHookProvider.Cal...
- Xamarin.Tests.TrimmerWarningsTest.TrimmerWarningsManagedStaticRe...: Unexpected warning: ILLink: System.StartupHookProvider.ProcessStartupHooks(String): Using member 'System.StartupHookProvider.Cal...
- Xamarin.Tests.TrimmerWarningsTest.TrimmerWarningsStaticRegistrar...: Unexpected warning: ILLink: System.StartupHookProvider.ProcessStartupHooks(String): Using member 'System.StartupHookProvider.Cal...

Html Report (VSDrops) Download

❌ dotnettests tests (tvOS)

1 tests failed, 0 tests passed.

Failed tests

DotNet tests: Failed (Execution failed with exit code 1)
- Xamarin.Tests.TemplateTest.CreateAndBuildItemTemplates(TVOS,CSha...: Build warnings:
  System.StartupHookProvider.ProcessStartupHooks(String): Using member 'System.StartupHookProvider.CallStartupH...
- Xamarin.Tests.TemplateTest.CreateAndBuildProjectTemplate(tvos-cs...: Build warnings:
  System.StartupHookProvider.ProcessStartupHooks(String): Using member 'System.StartupHookProvider.CallStartupH...
- Xamarin.Tests.TemplateTest.CreateAndBuildProjectTemplate(tvos-vi...: Build warnings:
  System.StartupHookProvider.ProcessStartupHooks(String): Using member 'System.StartupHookProvider.CallStartupH...
- ... and 3 more

Html Report (VSDrops) Download

❌ monotouch tests (iOS)

1 tests failed, 10 tests passed.

Failed tests

monotouch-test/iOS - simulator/Debug (managed static registrar): Crashed

Html Report (VSDrops) Download

Successes

✅ cecil: All 1 tests passed. Html Report (VSDrops) Download
✅ dotnettests (Multiple platforms): All 1 tests passed. Html Report (VSDrops) Download
✅ framework: All 2 tests passed. Html Report (VSDrops) Download
✅ fsharp: All 4 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 6 tests passed. Html Report (VSDrops) Download
✅ linker: All 44 tests passed. Html Report (VSDrops) Download
✅ monotouch (MacCatalyst): All 15 tests passed. Html Report (VSDrops) Download
✅ monotouch (macOS): All 12 tests passed. Html Report (VSDrops) Download
✅ monotouch (tvOS): All 11 tests passed. Html Report (VSDrops) Download
✅ msbuild: All 2 tests passed. Html Report (VSDrops) Download
✅ windows: All 3 tests passed. Html Report (VSDrops) Download
✅ xcframework: All 4 tests passed. Html Report (VSDrops) Download
✅ xtro: All 1 tests passed. Html Report (VSDrops) Download

Pipeline on Agent
Hash: 883f1d76c73fecf26aebbf6a8f346a5c9acb31e1 [PR build]

vs-mobiletools-engineering-service2 · 2026-02-12T08:54:49Z

✅ [PR Build #05bb490] Build passed (Detect API changes) ✅

Pipeline on Agent
Hash: 05bb49024d21cfcbc9e715ee990c7bd90a77159f [PR build]

vs-mobiletools-engineering-service2 · 2026-02-12T08:58:11Z

✅ [CI Build #05bb490] Build passed (Build packages) ✅

Pipeline on Agent
Hash: 05bb49024d21cfcbc9e715ee990c7bd90a77159f [PR build]

vs-mobiletools-engineering-service2 · 2026-02-12T09:31:58Z

✅ API diff for current PR / commit

NET (empty diffs)

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

NET (empty diffs)

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

ℹ️ Generator diff

Generator Diff: vsdrops (html) vsdrops (raw diff) gist (raw diff) - Please review changes)

Pipeline on Agent
Hash: 05bb49024d21cfcbc9e715ee990c7bd90a77159f [PR build]

vs-mobiletools-engineering-service2 · 2026-02-12T09:56:31Z

✅ [CI Build #05bb490] Build passed (Build macOS tests) ✅

Pipeline on Agent
Hash: 05bb49024d21cfcbc9e715ee990c7bd90a77159f [PR build]

vs-mobiletools-engineering-service2 · 2026-02-12T10:10:09Z

💻 [CI Build #05bb490] Tests on macOS arm64 - Mac Sequoia (15) passed 💻

✅ All tests on macOS arm64 - Mac Sequoia (15) passed.

Pipeline on Agent
Hash: 05bb49024d21cfcbc9e715ee990c7bd90a77159f [PR build]

vs-mobiletools-engineering-service2 · 2026-02-12T10:10:19Z

💻 [CI Build #05bb490] Tests on macOS M1 - Mac Monterey (12) passed 💻

✅ All tests on macOS M1 - Mac Monterey (12) passed.

Pipeline on Agent
Hash: 05bb49024d21cfcbc9e715ee990c7bd90a77159f [PR build]

vs-mobiletools-engineering-service2 · 2026-02-12T10:11:37Z

💻 [CI Build #05bb490] Tests on macOS M1 - Mac Ventura (13) passed 💻

✅ All tests on macOS M1 - Mac Ventura (13) passed.

Pipeline on Agent
Hash: 05bb49024d21cfcbc9e715ee990c7bd90a77159f [PR build]

vs-mobiletools-engineering-service2 · 2026-02-12T10:12:26Z

💻 [CI Build #05bb490] Tests on macOS arm64 - Mac Tahoe (26) passed 💻

✅ All tests on macOS arm64 - Mac Tahoe (26) passed.

Pipeline on Agent
Hash: 05bb49024d21cfcbc9e715ee990c7bd90a77159f [PR build]

vs-mobiletools-engineering-service2 · 2026-02-12T10:13:02Z

💻 [CI Build #05bb490] Tests on macOS X64 - Mac Sonoma (14) passed 💻

✅ All tests on macOS X64 - Mac Sonoma (14) passed.

Pipeline on Agent
Hash: 05bb49024d21cfcbc9e715ee990c7bd90a77159f [PR build]

rolfbjarne requested review from Copilot and jonathanpeppers February 10, 2026 16:16

Copilot started reviewing on behalf of rolfbjarne February 10, 2026 16:17 View session

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[dotnet] Fix support for startup hooks. Fixes #24492. #24664

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[dotnet] Fix support for startup hooks. Fixes #24492. #24664

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vs-mobiletools-engineering-service2 commented Feb 11, 2026

🔥 [CI Build #883f1d7] Test results 🔥

Test results

Failures

❌ dotnettests tests (iOS)

Failed tests

❌ dotnettests tests (MacCatalyst)

Failed tests

❌ dotnettests tests (macOS)

Failed tests

❌ dotnettests tests (tvOS)

Failed tests

❌ monotouch tests (iOS)

Failed tests

Successes

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vs-mobiletools-engineering-service2 commented Feb 12, 2026

✅ [PR Build #05bb490] Build passed (Detect API changes) ✅

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vs-mobiletools-engineering-service2 commented Feb 12, 2026

✅ [CI Build #05bb490] Build passed (Build packages) ✅

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vs-mobiletools-engineering-service2 commented Feb 12, 2026

✅ API diff for current PR / commit