[hot reload] implement DeployHotReloadAgentConfiguration target

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

@@ -204,9 +204,11 @@ static void RunStartupHooks ()
 				return;
 			}
 
-			// Pass empty string for diagnosticStartupHooks parameter
-			// The method will read STARTUP_HOOKS from AppContext internally
-			method.Invoke (null, [ "" ]);
+			// ProcessStartupHooks accepts startup hooks directly via parameter.
+			// It will also read STARTUP_HOOKS from AppContext internally.
+			// Pass DOTNET_STARTUP_HOOKS env var value so it works without needing AppContext setup.
+			string? startupHooks = Environment.GetEnvironmentVariable ("DOTNET_STARTUP_HOOKS");
+			method.Invoke (null, [ startupHooks ?? "" ]);
 		}
 
 		static void SetSynchronizationContext () =>

src/Xamarin.Android.Build.Tasks/Microsoft.Android.Sdk/targets/Microsoft.Android.Sdk.After.targets

@@ -33,4 +33,5 @@ This file is imported *after* the Microsoft.NET.Sdk/Sdk.targets.
   <Import Project="Microsoft.Android.Sdk.Publish.targets" />
   <Import Project="Microsoft.Android.Sdk.RuntimeConfig.targets" />
   <Import Project="Microsoft.Android.Sdk.Tooling.targets" />
+  <Import Project="Microsoft.Android.Sdk.HotReload.targets" Condition=" '$(AndroidApplication)' == 'true' " />
 </Project>

src/Xamarin.Android.Build.Tasks/Microsoft.Android.Sdk/targets/Microsoft.Android.Sdk.HotReload.targets

@@ -0,0 +1,84 @@
+<!--
+***********************************************************************************************
+Microsoft.Android.Sdk.HotReload.targets
+
+This file contains targets for Hot Reload support in .NET for Android.
+These targets are invoked by dotnet-watch/IDE when Hot Reload starts.
+
+See: https://github.com/dotnet/sdk/issues/52492
+
+***********************************************************************************************
+-->
+
+<Project>
+
+  <!-- Always enable startup hooks support when Hot Reload agent is configured -->
+  <PropertyGroup Condition=" '$(DotNetHotReloadAgentStartupHook)' != '' ">
+    <StartupHookSupport>true</StartupHookSupport>
+  </PropertyGroup>
+
+  <!--
+    _AndroidHotReloadAgentConfiguration:
+    Configures the Hot Reload agent by:
+    a) Adding the Hot Reload agent DLL as a reference
+    b) Setting up STARTUP_HOOKS via RuntimeHostConfigurationOption (for MonoVM)
+    c) Setting up DOTNET_STARTUP_HOOKS environment variable (for CoreCLR)
+    d) Adding additional environment variables from DotNetHotReloadAgentEnvironment
+  -->
+  <Target Name="_AndroidHotReloadAgentConfiguration"
+          Condition=" '$(DotNetHotReloadAgentStartupHook)' != '' ">
+
+    <PropertyGroup>
+      <_HotReloadAgentAssemblyName>$([System.IO.Path]::GetFileNameWithoutExtension('$(DotNetHotReloadAgentStartupHook)'))</_HotReloadAgentAssemblyName>
+    </PropertyGroup>
+
+    <!-- Add the Hot Reload agent DLL as a reference so it gets deployed -->
+    <ItemGroup>
+      <Reference Include="$(DotNetHotReloadAgentStartupHook)" Condition=" Exists('$(DotNetHotReloadAgentStartupHook)') " />
+    </ItemGroup>
+
+    <!-- Set STARTUP_HOOKS via RuntimeHostConfigurationOption for MonoVM (read by Mono runtime) -->
+    <ItemGroup Condition=" '$(UseMonoRuntime)' == 'true' ">
+      <RuntimeHostConfigurationOption Include="STARTUP_HOOKS" Value="$(_HotReloadAgentAssemblyName)" />
+    </ItemGroup>
+
+    <!-- Generate Hot Reload environment file for other runtimes -->
+    <ItemGroup>
+      <_HotReloadEnvironment Include="DOTNET_STARTUP_HOOKS=$(_HotReloadAgentAssemblyName)" />
+    </ItemGroup>
+
+    <!-- <DotNetHotReloadAgentEnvironment Include="<name>" Value="<value>"/> -->
+    <ItemGroup>
+      <_HotReloadEnvironment Include="@(DotNetHotReloadAgentEnvironment->'%(Identity)=%(Value)')" />
+    </ItemGroup>
+
+    <WriteLinesToFile
+        File="$(IntermediateOutputPath)__hotreload_environment__.txt"
+        Lines="@(_HotReloadEnvironment)"
+        Overwrite="True"
+        WriteOnlyWhenDifferent="True"
+    />
+
+    <ItemGroup>
+      <AndroidEnvironment Include="$(IntermediateOutputPath)__hotreload_environment__.txt" />
+      <FileWrites         Include="$(IntermediateOutputPath)__hotreload_environment__.txt" />
+    </ItemGroup>
+
+  </Target>
+
+  <!--
+    DeployHotReloadAgentConfiguration:
+    Entry point target invoked by dotnet-watch/IDE.
+    Sets up the Hot Reload agent configuration and deploys to the device.
+
+    This target is discovered by checking if the project has a target named
+    "DeployHotReloadAgentConfiguration". If found, dotnet-watch/IDE will
+    set the following:
+    - DotNetHotReloadAgentStartupHook (Property): Path to Microsoft.Extensions.DotNetDeltaApplier.dll
+    - DotNetHotReloadAgentEnvironment (ItemGroup): Environment variables as <DotNetHotReloadAgentEnvironment Include="name" Value="value" />
+  -->
+  <Target Name="DeployHotReloadAgentConfiguration"
+          DependsOnTargets="_AndroidHotReloadAgentConfiguration;DeployToDevice" />
+
+</Project>
+

tests/MSBuildDeviceIntegration/Tests/InstallAndRunTests.cs

@@ -1582,5 +1582,109 @@ public void AppStartsWithManagedMarshalMethodsLookupEnabled ()
 				Path.Combine (Root, builder.ProjectDirectory, "logcat.log"), 30);
 			Assert.IsTrue (didLaunch, "Activity should have started.");
 		}
+
+		[Test]
+		public void DeployHotReloadAgentConfiguration ()
+		{
+			// Create a library project that contains the startup hook
+			var startupHookLib = new XamarinAndroidLibraryProject {
+				ProjectName = "MyStartupHook",
+				Sources = {
+					new BuildItem.Source ("StartupHook.cs") {
+						TextContent = () => @"
+using System;
+
+internal static class StartupHook
+{
+	public static void Initialize ()
+	{
+		Console.WriteLine (""HOTRELOAD_TEST_HOOK_INITIALIZED=true"");
+	}
+}
+"
+					}
+				}
+			};
+
+			var proj = new XamarinAndroidApplicationProject {
+				ProjectName = nameof (DeployHotReloadAgentConfiguration),
+				IsRelease = false,
+				Imports = {
+					// Add a .targets file that simulates what dotnet-watch/IDE would inject
+					new Import (() => "HotReload.targets") {
+						TextContent = () => @"<Project xmlns=""http://schemas.microsoft.com/developer/msbuild/2003"">
+	<PropertyGroup>
+		<DotNetHotReloadAgentStartupHook>MyStartupHook</DotNetHotReloadAgentStartupHook>
+	</PropertyGroup>
+	<ItemGroup>
+		<DotNetHotReloadAgentEnvironment Include=""HOTRELOAD_TEST_VAR"" Value=""TestValue123"" />
+		<DotNetHotReloadAgentEnvironment Include=""ANOTHER_VAR"" Value=""AnotherValue456"" />
+	</ItemGroup>
+</Project>"
+					},
+				}
+			};
+			proj.SetRuntime (AndroidRuntime.CoreCLR);
+			proj.AddReference (startupHookLib);
+			proj.MainActivity = proj.DefaultMainActivity.Replace ("//${AFTER_ONCREATE}", @"
+		Console.WriteLine (""DOTNET_STARTUP_HOOKS="" + Environment.GetEnvironmentVariable(""DOTNET_STARTUP_HOOKS""));
+		Console.WriteLine (""HOTRELOAD_TEST_VAR="" + Environment.GetEnvironmentVariable(""HOTRELOAD_TEST_VAR""));
+		Console.WriteLine (""ANOTHER_VAR="" + Environment.GetEnvironmentVariable(""ANOTHER_VAR""));
+");
+
+			var rootPath = Path.Combine (Root, "temp", TestName);
+			using var libBuilder = CreateDllBuilder (Path.Combine (rootPath, startupHookLib.ProjectName));
+			Assert.IsTrue (libBuilder.Build (startupHookLib), "Library build should have succeeded.");
+
+			using var builder = CreateApkBuilder (Path.Combine (rootPath, proj.ProjectName));
+			builder.Save (proj);
+
+			var projectDirectory = Path.Combine (rootPath, proj.ProjectName);
+			var dotnet = new DotNetCLI (Path.Combine (projectDirectory, proj.ProjectFilePath));
+
+			// Build normally first
+			Assert.IsTrue (dotnet.Build (), "`dotnet build` should succeed");
+
+			// Run the DeployHotReloadAgentConfiguration target (hot reload properties come from HotReload.targets)
+			Assert.IsTrue (dotnet.Build (target: "DeployHotReloadAgentConfiguration"),
+				"`dotnet build -t:DeployHotReloadAgentConfiguration` should succeed");
+
+			// Launch the app using adb
+			ClearAdbLogcat ();
+			var result = AdbStartActivity ($"{proj.PackageName}/{proj.JavaPackageName}.MainActivity");
+			Assert.IsTrue (result.Contains ("Starting: Intent"), $"Activity should have launched. adb output:\n{result}");
+
+			bool didLaunch = WaitForActivityToStart (proj.PackageName, "MainActivity",
+				Path.Combine (projectDirectory, "logcat.log"), 30);
+			Assert.IsTrue (didLaunch, "Activity should have started.");
+
+			var logcatOutput = File.ReadAllText (Path.Combine (projectDirectory, "logcat.log"));
+
+			// Verify the startup hook was set via DOTNET_STARTUP_HOOKS
+			StringAssert.Contains (
+				"DOTNET_STARTUP_HOOKS=MyStartupHook",
+				logcatOutput,
+				"DOTNET_STARTUP_HOOKS should be set to MyStartupHook"
+			);
+
+			// Verify the startup hook was called
+			StringAssert.Contains (
+				"HOTRELOAD_TEST_HOOK_INITIALIZED=true",
+				logcatOutput,
+				"StartupHook.Initialize() should have been called"
+			);
+
+			// Verify the additional env vars from DotNetHotReloadAgentEnvironment were set
+			StringAssert.Contains (
+				"HOTRELOAD_TEST_VAR=TestValue123",
+				logcatOutput,
+				"HOTRELOAD_TEST_VAR should be set from DotNetHotReloadAgentEnvironment"
+			);
+			StringAssert.Contains (
+				"ANOTHER_VAR=AnotherValue456",
+				logcatOutput,
+				"ANOTHER_VAR should be set from DotNetHotReloadAgentEnvironment"
+			);
+		}
 	}
 }

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

@@ -52,11 +52,12 @@
     <StartupHookSupport>true</StartupHookSupport>
   </PropertyGroup>
 
-  <ItemGroup Condition=" '$(Configuration)' == 'Debug' ">
+  <ItemGroup>
     <!-- trying to track:
       JNI ERROR (app bug): accessed deleted Global 0x3056
     -->
-    <AndroidEnvironment Include="env.txt" />
+    <AndroidEnvironment Include="env.txt" Condition=" '$(Configuration)' == 'Debug' " />
+    <AndroidEnvironment Include="hotreload.env" />
   </ItemGroup>
 
   <ItemGroup>
@@ -231,7 +232,8 @@
     <RuntimeHostConfigurationOption Include="test_bool"    Value="true" />
     <RuntimeHostConfigurationOption Include="test_integer" Value="42" />
     <RuntimeHostConfigurationOption Include="test_string"  Value="foo" />
-    <RuntimeHostConfigurationOption Include="STARTUP_HOOKS" Value="StartupHook" />
+    <!-- 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' " />
   </ItemGroup>
 
   <ItemGroup Condition=" '$(AndroidPackageFormat)' != 'aab' ">

tests/Mono.Android-Tests/Mono.Android-Tests/System/StartupHookTest.cs

@@ -7,6 +7,20 @@ namespace SystemTests
 	[TestFixture]
 	public class StartupHookTest
 	{
+		[Test]
+		public void FeatureFlagIsEnabled ()
+		{
+			// NOTE: this is set to true in tests\Mono.Android-Tests\Mono.Android-Tests\Mono.Android.NET-Tests.csproj
+			Assert.IsTrue (Microsoft.Android.Runtime.RuntimeFeature.StartupHookSupport, "RuntimeFeature.StartupHookSupport should be true");
+		}
+
+		[Test]
+		public void EnvironmentVariableIsSet ()
+		{
+			var value = Environment.GetEnvironmentVariable ("DOTNET_STARTUP_HOOKS");
+			Assert.AreEqual ("StartupHook", value, "DOTNET_STARTUP_HOOKS should be set to 'StartupHook'");
+		}
+
 		[Test]
 		public void IsInitialized ()
 		{

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

@@ -1,4 +1,3 @@
 # Environment Variables and system properties
 # debug.mono.log=gref,default
 debug.mono.debug=1
-DOTNET_STARTUP_HOOKS=StartupHook

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

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