[hot reload] implement `DeployHotReloadAgentConfiguration` target #10699

jonathanpeppers · 2026-01-16T17:19:43Z

Context: dotnet/sdk#52492
Context: https://github.com/dotnet/runtime/blob/79609d7dbef01231b0694a356de1d4e5618785cd/src/libraries/System.Private.CoreLib/src/System/StartupHookProvider.cs#L32
Context: https://github.com/dotnet/runtime/blob/79609d7dbef01231b0694a356de1d4e5618785cd/src/mono/mono/metadata/object.c#L8137-L8140
Context: b2332ca

Added a new DeployHotReloadAgentConfiguration MSBuild target that dotnet-watch can use to configure Hot Reload for Android apps.

Notes on runtimes:

MonoVM currently passes string.Empty to ProcessStartupHooks and relies on reading STARTUP_HOOKS from AppContext.
To fix CoreCLR, we can read DOTNET_STARTUP_HOOKS from environment variables, and pass to ProcessStartupHooks.
NativeAOT likely works as-is.

I added/updated tests to verify these changes.

Context: dotnet/sdk#52492 Context: https://github.com/dotnet/runtime/blob/79609d7dbef01231b0694a356de1d4e5618785cd/src/libraries/System.Private.CoreLib/src/System/StartupHookProvider.cs#L32 Context: https://github.com/dotnet/runtime/blob/79609d7dbef01231b0694a356de1d4e5618785cd/src/mono/mono/metadata/object.c#L8137-L8140 Context: b2332ca Added a new `DeployHotReloadAgentConfiguration` MSBuild target that `dotnet-watch` can use to configure Hot Reload for Android apps. Notes on runtimes: * MonoVM currently passes `string.Empty` to `ProcessStartupHooks` and relies on reading `STARTUP_HOOKS` from `AppContext`. * To fix CoreCLR, we can read `DOTNET_STARTUP_HOOKS` from environment variables, and pass to `ProcessStartupHooks`. * NativeAOT likely works as-is. I added/updated tests to verify these changes.

tmat · 2026-01-20T17:41:10Z

...in.Android.Build.Tasks/Microsoft.Android.Sdk/targets/Microsoft.Android.Sdk.HotReload.targets

+
+    <!-- Parse DotNetHotReloadAgentEnvironment (semicolon-separated KEY=VALUE pairs) -->
+    <ItemGroup Condition=" '$(DotNetHotReloadAgentEnvironment)' != '' ">
+      <_HotReloadEnvironment Include="$(DotNetHotReloadAgentEnvironment)" />


I realized it'd be easier to make DotNetHotReloadAgentEnvironment an item group.

<DotNetHotReloadAgentEnvironment Include="<name>" Value="<value>"/>

That way we don't need to worry about escaping = and ;.

Updated to use an item group now.

jonathanpeppers · 2026-01-20T22:47:11Z

tests/Mono.Android-Tests/Mono.Android-Tests/hotreload.env

@@ -0,0 +1 @@
+DOTNET_STARTUP_HOOKS=StartupHook


Note this wasn't being used in some of our on-device tests, because we had:

<ItemGroup Condition=" '$(Configuration)' == 'Debug' ">  <AndroidEnvironment Include="env.txt" />

I just made a new file for this.

jonathanpeppers · 2026-01-20T22:49:14Z

...in.Android.Build.Tasks/Microsoft.Android.Sdk/targets/Microsoft.Android.Sdk.HotReload.targets

@@ -0,0 +1,84 @@
+<!--
+***********************************************************************************************
+Microsoft.Android.Sdk.HotReload.targets


@rolfbjarne do you see anything problematic in this file for iOS? $(DotNetHotReloadAgentStartupHook) will have the full path to the .dll.

The solution for iOS will be rather different, but it doesn't look impossible.

Introduces a new test to verify that the DOTNET_STARTUP_HOOKS environment variable is set to 'StartupHook'. Also renames an existing test method for clarity.

jonathanpeppers · 2026-01-21T14:59:23Z

tests/Mono.Android-Tests/Mono.Android-Tests/Mono.Android.NET-Tests.csproj

+    <!-- Set STARTUP_HOOKS via RuntimeHostConfigurationOption for MonoVM and NativeAOT (read via AppContext.GetData) -->
+    <RuntimeHostConfigurationOption Include="STARTUP_HOOKS" Value="StartupHook" Condition=" '$(UseMonoRuntime)' == 'true' or '$(PublishAot)' == 'true' " />


Filed this to fix NativeAOT later:

[NativeAOT] env var timing at startup #10705

rolfbjarne · 2026-01-21T15:14:45Z

...in.Android.Build.Tasks/Microsoft.Android.Sdk/targets/Microsoft.Android.Sdk.HotReload.targets

+    - DotNetHotReloadAgentStartupHook (Property): Path to Microsoft.Extensions.DotNetDeltaApplier.dll
+    - DotNetHotReloadAgentEnvironment (ItemGroup): Environment variables as <DotNetHotReloadAgentEnvironment Include="name" Value="value" />
+  -->
+  <Target Name="DeployHotReloadAgentConfiguration"


The caller of this target will have to set the Device property, so that we know which device/simulator to deploy to.

Yes, after we have the initial part working, I hope to share the code with dotnet-run that will prompt for a target framework and device. So, you should get the same prompts and property.

Hot Reload is typically used from an IDE though, and the IDE in question must pass the selected device in this case.

rolfbjarne · 2026-01-21T15:15:23Z

...in.Android.Build.Tasks/Microsoft.Android.Sdk/targets/Microsoft.Android.Sdk.HotReload.targets

+
+    <!-- Add the Hot Reload agent DLL as a reference so it gets deployed -->
+    <ItemGroup>
+      <Reference Include="$(DotNetHotReloadAgentStartupHook)" Condition=" Exists('$(DotNetHotReloadAgentStartupHook)') " />


Does this still work if the app is trimmed? What if the hot reload agent assembly contains references to BCL APIs that were trimmed away?

They have the BCL part setup here:

https://github.com/dotnet/runtime/blob/92d46b9d7f24f1c63c4558904314295ff0d0a919/src/mono/System.Private.CoreLib/src/ILLink/ILLink.Descriptors.xml#L649-L663

I also did this in a test project that seems to work in all runtimes/configurations:

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

We might have to do this, if Debug builds are trimmed on iOS.

Hot Reload isn't supported for trimmed apps.

My usage of @(TrimmerRootAssembly) was to test startup hooks in general.

But yeah, hot reload, would break randomly if they decided to use System.Guid, for example, during a reload and it was trimmed away.

iOS Debug builds run the trimmer, but I bet it is setup to preserve almost everything. We may need to review that behavior.

@rolfbjarne it's also worth noting we are mostly targeting CoreCLR for this -- if that helps anything.

...in.Android.Build.Tasks/Microsoft.Android.Sdk/targets/Microsoft.Android.Sdk.HotReload.targets

jonathanpeppers added 4 commits January 16, 2026 11:16

Update InstallAndRunTests.cs

5a5fc67

Update InstallAndRunTests.cs

38de42f

Update StartupHookTest.cs

8aa3cd7

jonathanpeppers mentioned this pull request Jan 16, 2026

[Epic] vibe-coding .NET MAUI apps works out of the box dotnet/maui#32967

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tmat reviewed Jan 20, 2026

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jonathanpeppers added 3 commits January 20, 2026 16:34

@(DotNetHotReloadAgentEnvironment) is item group

ec61662

Fix DeployHotReloadAgentConfiguration test

081426d

Fix $DOTNET_STARTUP_HOOKS in tests

e2964b4

jonathanpeppers commented Jan 20, 2026

View reviewed changes

jonathanpeppers requested a review from rolfbjarne January 20, 2026 22:48

jonathanpeppers commented Jan 20, 2026

View reviewed changes

jonathanpeppers added 3 commits January 21, 2026 08:30

Rename project to MyStartupHook

e7b1edf

Add test for DOTNET_STARTUP_HOOKS environment variable

81b1f29

Introduces a new test to verify that the DOTNET_STARTUP_HOOKS environment variable is set to 'StartupHook'. Also renames an existing test method for clarity.

Update Mono.Android.NET-Tests.csproj

1bf0c48

jonathanpeppers commented Jan 21, 2026

View reviewed changes

rolfbjarne reviewed Jan 21, 2026

View reviewed changes

		<!-- Set STARTUP_HOOKS via RuntimeHostConfigurationOption for MonoVM and NativeAOT (read via AppContext.GetData) -->
		<RuntimeHostConfigurationOption Include="STARTUP_HOOKS" Value="StartupHook" Condition=" '$(UseMonoRuntime)' == 'true' or '$(PublishAot)' == 'true' " />

		@@ -0,0 +1 @@
		DOTNET_STARTUP_HOOKS=StartupHook

[hot reload] implement DeployHotReloadAgentConfiguration target #10699

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[hot reload] implement `DeployHotReloadAgentConfiguration` target #10699

[hot reload] implement `DeployHotReloadAgentConfiguration` target #10699