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[illink] set $(StartupHookSupport)=false *only* for Release mode#10670

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dev/peppers/startup-hook
Jan 12, 2026
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[illink] set $(StartupHookSupport)=false *only* for Release mode#10670
jonathanpeppers merged 3 commits intomainfrom
dev/peppers/startup-hook

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@jonathanpeppers jonathanpeppers commented Jan 8, 2026

Context: https://github.com/dotnet/runtime/blob/242f7b23752599f22157268de41fee91cb97ef6c/docs/design/features/host-startup-hook.md

.NET has a concept of a "startup hook", which appears to work fine
on Mono if you set:

env.txt
    DOTNET_STARTUP_HOOKS=StartupHook
(MSBuild)
    <StartupHookSupport>true</StartupHookSupport>
    <RuntimeHostConfigurationOption Include="STARTUP_HOOKS" Value="StartupHook" />

The startup hook is a managed assembly with a static method, in the
"global" namespace:

class StartupHook
{
    public static void Initialize()
    {
        // ...
    }
}

When the runtime starts, it will call StartupHook.Initialize() before
any other managed code is executed.

dotnet watch (Hot Reload) relies on this feature.

I added a test to Mono.Android-Tests to verify that the startup hook
was called.

This is not yet working on CoreCLR on Android, investigation ongoing.

@jonathanpeppers jonathanpeppers mentioned this pull request Jan 8, 2026
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jonathanpeppers commented Jan 8, 2026

@rolfbjarne do you know if this works on iOS/Catalyst?

rolfbjarne added a commit to rolfbjarne/macios that referenced this pull request Jan 8, 2026
@rolfbjarne
[dotnet] Fix support for startup hooks.
a7ba492 
startup hooks don't seem to work.

Refs:

* dotnet/android#10670
* https://github.com/dotnet/runtime/blob/242f7b23752599f22157268de41fee91cb97ef6c/docs/design/features/host-startup-hook.md
@rolfbjarne rolfbjarne mentioned this pull request Jan 8, 2026
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rolfbjarne commented Jan 8, 2026

@rolfbjarne do you know if this works on iOS/Catalyst?

I did a quick test, and it doesn't seem to work, so I filed an issue: dotnet/macios#24492

@jonathanpeppers
[illink] set $(StartupHookSupport)=false *only* for Release mode
914127f 
Context: https://github.com/dotnet/runtime/blob/242f7b23752599f22157268de41fee91cb97ef6c/docs/design/features/host-startup-hook.md

.NET has a concept of a "startup hook", which appears to work fine
on Mono if you set:

    env.txt
        DOTNET_STARTUP_HOOKS=StartupHook
    (MSBuild)
        <StartupHookSupport>true</StartupHookSupport>
        <RuntimeHostConfigurationOption Include="STARTUP_HOOKS" Value="StartupHook" />

The startup hook is a managed assembly with a static method, in the
"global" namespace:

    class StartupHook
    {
        public static void Initialize()
        {
            // ...
        }
    }

When the runtime starts, it will call `StartupHook.Initialize()` before
any other managed code is executed.

`dotnet watch` (Hot Reload) relies on this feature.

I added a test to `Mono.Android-Tests` to verify that the startup hook
was called.

This is not yet working on CoreCLR on Android, investigation ongoing.
@jonathanpeppers
[coreclr] call System.StartupHookProvider.ProcessStartupHooks()
17176cd
@jonathanpeppers jonathanpeppers force-pushed the dev/peppers/startup-hook branch from a09103f to 17176cd Compare January 8, 2026 23:23
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jonathanpeppers commented Jan 12, 2026

The one test failure is a flaky test, we can ignore:

image

@jonathanpeppers jonathanpeppers marked this pull request as ready for review January 12, 2026 15:19
rolfbjarne
rolfbjarne approved these changes Jan 12, 2026
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src/Mono.Android/Android.Runtime/JNIEnvInit.cs
return;
}

var method = type.GetMethod (methodName,
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@rolfbjarne rolfbjarne Jan 12, 2026

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Would it make sense to use the [UnsafeAccessor] attribute to avoid using reflection here?

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It looks like you can only do that if a type is public, and it's internal in System.Private.CoreLib.

src/Mono.Android/Android.Runtime/JNIEnvInit.cs
}

[RequiresUnreferencedCode ("Uses reflection to access System.StartupHookProvider.")]
static void RunStartupHooks ()
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Do you have a test to ensure this method is really trimmed away for a Release build (or when startuphook support is not enabled)?

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@jonathanpeppers jonathanpeppers Jan 12, 2026 •
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Yes, we run the new test in both trimmed, AOT, all runtimes, etc.

They have a line in dotnet/runtime to preserve it, too:

https://github.com/dotnet/runtime/blob/eaf635955da0410b94cb8607f6e598c51245f830/src/mono/System.Private.CoreLib/src/ILLink/ILLink.Descriptors.xml#L656-L663

@jonathanpeppers jonathanpeppers merged commit b2332ca into main Jan 12, 2026
57 of 59 checks passed
@jonathanpeppers jonathanpeppers deleted the dev/peppers/startup-hook branch January 12, 2026 17:12
jonathanpeppers added a commit that referenced this pull request Jan 21, 2026
@jonathanpeppers
[illink] set $(StartupHookSupport)=false *only* for Release mode (#…
d15d98e 
…10670)

Context: https://github.com/dotnet/runtime/blob/242f7b23752599f22157268de41fee91cb97ef6c/docs/design/features/host-startup-hook.md

.NET has a concept of a "startup hook", which appears to work fine
on Mono if you set:

    env.txt
        DOTNET_STARTUP_HOOKS=StartupHook
    (MSBuild)
        <StartupHookSupport>true</StartupHookSupport>
        <RuntimeHostConfigurationOption Include="STARTUP_HOOKS" Value="StartupHook" />

The startup hook is a managed assembly with a static method, in the
"global" namespace:

    class StartupHook
    {
        public static void Initialize()
        {
            // ...
        }
    }

When the runtime starts, it will call `StartupHook.Initialize()` before
any other managed code is executed.

`dotnet watch` (Hot Reload) relies on this feature.

I added tests to verify startup hooks are called on Android for
all runtimes.

* [coreclr] call `System.StartupHookProvider.ProcessStartupHooks()`

This feature only works on CoreCLR if you have a `static void Main()`,
which we do not on Android. For now, I called the `StartupHookProvider`
directly with System.Reflection. We can address in dotnet/runtime
in the future.
jonathanpeppers added a commit that referenced this pull request Jan 21, 2026
@jonathanpeppers
[illink] set $(StartupHookSupport)=false *only* for Release mode (#…
14387f2 
…10670)

Context: https://github.com/dotnet/runtime/blob/242f7b23752599f22157268de41fee91cb97ef6c/docs/design/features/host-startup-hook.md

.NET has a concept of a "startup hook", which appears to work fine
on Mono if you set:

    env.txt
        DOTNET_STARTUP_HOOKS=StartupHook
    (MSBuild)
        <StartupHookSupport>true</StartupHookSupport>
        <RuntimeHostConfigurationOption Include="STARTUP_HOOKS" Value="StartupHook" />

The startup hook is a managed assembly with a static method, in the
"global" namespace:

    class StartupHook
    {
        public static void Initialize()
        {
            // ...
        }
    }

When the runtime starts, it will call `StartupHook.Initialize()` before
any other managed code is executed.

`dotnet watch` (Hot Reload) relies on this feature.

I added tests to verify startup hooks are called on Android for
all runtimes.

* [coreclr] call `System.StartupHookProvider.ProcessStartupHooks()`

This feature only works on CoreCLR if you have a `static void Main()`,
which we do not on Android. For now, I called the `StartupHookProvider`
directly with System.Reflection. We can address in dotnet/runtime
in the future.
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@rolfbjarne rolfbjarne rolfbjarne approved these changes

@grendello grendello Awaiting requested review from grendello grendello is a code owner

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