[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.

Copilot · 2026-02-10T16:26:58Z

tests/dotnet/UnitTests/StartupHookTest.cs

+				env = new Dictionary<string, string?> ();
+				ExecuteWithMagicWordAndAssert (appExecutable, env, expectedExitCode: 1); // this should fail


Passing an empty environment dictionary here does not unset DOTNET_STARTUP_HOOKS for the child process; Execution.RunAsync inherits the parent environment unless a key is explicitly removed (value=null). This can make the test flaky if DOTNET_STARTUP_HOOKS is set in the test runner environment. Set DOTNET_STARTUP_HOOKS to null in the env dictionary for the "no startup hook" case instead of using an empty dictionary.

Copilot · 2026-02-10T16:26:59Z

tests/dotnet/UnitTests/StartupHookTest.cs

+				env = new Dictionary<string, string?> ();
+				ExecuteWithMagicWordAndAssert (appExecutable, env, expectedExitCode: 1); // this should fail


Same issue as above: using an empty env dictionary does not clear DOTNET_STARTUP_HOOKS inherited from the parent process. Explicitly set DOTNET_STARTUP_HOOKS=null to ensure startup hooks are disabled for this run.

Copilot · 2026-02-10T16:26:59Z

tests/dotnet/UnitTests/StartupHookTest.cs

+				env = new Dictionary<string, string?> ();
+				ExecuteWithMagicWordAndAssert (appExecutable, env, expectedExitCode: 1); // this should fail


Same issue as above: passing an empty env dictionary won't remove DOTNET_STARTUP_HOOKS from the inherited environment. Set DOTNET_STARTUP_HOOKS=null for the "should fail" execution to make the test deterministic.

Copilot · 2026-02-10T16:26:59Z

tests/dotnet/StartupHookTest/AppDelegate.cs

+	public static bool Initialized { get; private set; }
+	public static void Initialize ()
+	{
+		Console.WriteLine ("STARTUP");


Formatting/style: this repo's C# style uses a space before parentheses in method declarations/calls (e.g. Main (string [] args) in other dotnet test apps). Update Initialize() to Initialize () (and consider aligning brace placement with the surrounding code for consistency).

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.

jonathanpeppers · 2026-02-10T16:30:47Z

tests/dotnet/StartupHookTest/AppDelegate.cs

+	}
+}
+
+class StartupHook {


You may want to actually test this in a separate class library like dotnet watch will use. I got different startup behavior when I did this, because it was a library different than the main app.

vs-mobiletools-engineering-service2 · 2026-02-10T17:07:15Z

✅ [CI Build #0feba87] Build passed (Build packages) ✅

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vs-mobiletools-engineering-service2 · 2026-02-10T17:10:27Z

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vs-mobiletools-engineering-service2 · 2026-02-10T17:49:05Z

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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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macOS: vsdrops gist ( No breaking changes )

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vs-mobiletools-engineering-service2 · 2026-02-10T18:07:36Z

✅ [CI Build #0feba87] Build passed (Build macOS tests) ✅

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vs-mobiletools-engineering-service2 · 2026-02-10T18:20:49Z

💻 [CI Build #0feba87] Tests on macOS M1 - Mac Monterey (12) passed 💻

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

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Hash: 0feba87bcd06d48b9d0132426e08393c282d5205 [PR build]

vs-mobiletools-engineering-service2 · 2026-02-10T18:22:23Z

💻 [CI Build #0feba87] Tests on macOS M1 - Mac Ventura (13) passed 💻

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

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vs-mobiletools-engineering-service2 · 2026-02-10T18:22:43Z

💻 [CI Build #0feba87] Tests on macOS arm64 - Mac Sequoia (15) passed 💻

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

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vs-mobiletools-engineering-service2 · 2026-02-10T18:52:30Z

💻 [CI Build #0feba87] Tests on macOS arm64 - Mac Tahoe (26) passed 💻

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

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vs-mobiletools-engineering-service2 · 2026-02-10T19:20:16Z

🔥 [CI Build #0feba87] Test results 🔥

Test results

❌ Tests failed on VSTS: test results

0 tests crashed, 6 tests failed, 122 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.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 (MacCatalyst)

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 (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.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

❌ linker tests

1 tests failed, 43 tests passed.

Failed tests

link sdk/iOS - simulator/Debug: Crashed

Html Report (VSDrops) Download

❌ monotouch tests (MacCatalyst)

1 tests failed, 14 tests passed.

Failed tests

monotouch-test/Mac Catalyst/Release (NativeAOT): HarnessException (Harness exception for 'monotouch-test': System.Exception: Unable to evaluate the property OutputPath in /Users/builder/azdo/_work/3/s/macios/tests/xharness/bin/Debug/tmp-test-dir/monotouch-test576/monotouch-test.csproj, build failed with exit code 139. Timed out: False
at Xharness.AppBundleLocator.GetPropertyByMSBuildEvaluationAsync(XmlDocument csproj, String projectPath, String evaluateProperty, String dependsOnTargets, Dictionary`2 properties) in /Users/builder/azdo/_work/3/s/macios/tests/xharness/AppBundleLocator.cs:line 121
at Xharness.AppBundleLocator.LocateAppBundle(XmlDocument projectFile, String projectFilePath, TestTarget target, String buildConfiguration) in /Users/builder/azdo/_work/3/s/macios/tests/xharness/AppBundleLocator.cs:line 48
at Xharness.Jenkins.TestTasks.MacExecuteTask.RunTestAsync() in /Users/builder/azdo/_work/3/s/macios/tests/xharness/Jenkins/TestTasks/MacExecuteTask.cs:line 52
at Xharness.Jenkins.TestTasks.RunTest.ExecuteAsync() in /Users/builder/azdo/_work/3/s/macios/tests/xharness/Jenkins/TestTasks/RunTest.cs:line 113
at Xharness.Jenkins.TestTasks.TestTasks.RunInternalAsync() in /Users/builder/azdo/_work/3/s/macios/tests/xharness/Jenkins/TestTasks/TestTask.cs:line 233)

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 4 tests passed. Html Report (VSDrops) Download
✅ monotouch (iOS): All 11 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: 0feba87bcd06d48b9d0132426e08393c282d5205 [PR build]

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

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		env = new Dictionary<string, string?> ();
		ExecuteWithMagicWordAndAssert (appExecutable, env, expectedExitCode: 1); // this should fail

[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 10, 2026

✅ [CI Build #0feba87] Build passed (Build packages) ✅

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✅ [PR Build #0feba87] Build passed (Detect API changes) ✅

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✅ [CI Build #0feba87] Build passed (Build macOS tests) ✅

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vs-mobiletools-engineering-service2 commented Feb 10, 2026

💻 [CI Build #0feba87] Tests on macOS M1 - Mac Monterey (12) passed 💻

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💻 [CI Build #0feba87] Tests on macOS M1 - Mac Ventura (13) passed 💻

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💻 [CI Build #0feba87] Tests on macOS arm64 - Mac Sequoia (15) passed 💻

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vs-mobiletools-engineering-service2 commented Feb 10, 2026

💻 [CI Build #0feba87] Tests on macOS arm64 - Mac Tahoe (26) passed 💻

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🔥 [CI Build #0feba87] 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