Guard Mono-specific AOT targets for CoreCLR runtime, add XA1042 warning

Documentation/docs-mobile/messages/xa1042.md

@@ -0,0 +1,45 @@
+---
+title: .NET for Android warning XA1042
+description: XA1042 warning code
+ms.date: 04/02/2026
+f1_keywords:
+  - "XA1042"
+---
+
+# .NET for Android warning XA1042
+
+## Example messages
+
+```
+warning XA1042: The MSBuild property 'RunAOTCompilation' is not compatible with the CoreCLR runtime and will be ignored. Either remove the property or guard it with a condition: Condition="'$(UseMonoRuntime)' == 'true'"
+```
+
+## Issue
+
+This warning indicates that a Mono-specific MSBuild property is set in
+the project file, but the project is configured to use a non-Mono
+runtime (such as CoreCLR or NativeAOT). The property will have no
+effect with the current runtime and will be ignored.
+
+Properties that trigger this warning include:
+- `RunAOTCompilation` — Mono AOT compilation; CoreCLR uses `PublishReadyToRun` instead.
+- `EnableLLVM` — Enables LLVM for the Mono AOT compiler.
+
+## Solution
+
+Remove the Mono-specific property from your project file or
+`Directory.Build.props`, or guard it with a runtime condition:
+
+```xml
+<PropertyGroup Condition=" '$(UseMonoRuntime)' == 'true' ">
+  <RunAOTCompilation>true</RunAOTCompilation>
+</PropertyGroup>
+```
+
+For CoreCLR, use `PublishReadyToRun` instead of `RunAOTCompilation`:
+
+```xml
+<PropertyGroup Condition=" '$(Configuration)' == 'Release' ">
+  <PublishReadyToRun>true</PublishReadyToRun>
+</PropertyGroup>
+```

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

@@ -21,7 +21,7 @@ They run in a context of an inner build with a single $(RuntimeIdentifier).
     when $(MonoAOTCompilerTasksAssemblyPath) is blank:
     https://github.com/dotnet/runtime/blob/69711860262e44458bbe276393ea3eb9f7a2192a/src/mono/nuget/Microsoft.NET.Workload.Mono.Toolchain.Manifest/WorkloadManifest.targets.in#L20-L25
   -->
-  <ImportGroup Condition=" '$(MonoAOTCompilerTasksAssemblyPath)' == '' and '$(AotAssemblies)' == 'true' ">
+  <ImportGroup Condition=" '$(MonoAOTCompilerTasksAssemblyPath)' == '' and '$(AotAssemblies)' == 'true' and '$(_AndroidRuntime)' == 'MonoVM' ">
     <Import Project="Sdk.props" Sdk="Microsoft.NET.Runtime.MonoAOTCompiler.Task" />
     <Import Project="Sdk.props" Sdk="Microsoft.NETCore.App.Runtime.AOT.Cross.android-x86" />
     <Import Project="Sdk.props" Sdk="Microsoft.NETCore.App.Runtime.AOT.Cross.android-x64" />
@@ -39,7 +39,7 @@ They run in a context of an inner build with a single $(RuntimeIdentifier).
   </Target>
 
   <Target Name="_AndroidAot"
-      Condition=" '$(AotAssemblies)' == 'true' and '$(RuntimeIdentifier)' != '' "
+      Condition=" '$(AotAssemblies)' == 'true' and '$(RuntimeIdentifier)' != '' and '$(_AndroidRuntime)' == 'MonoVM' "
       DependsOnTargets="_CreatePropertiesCache;_AndroidAotInputs;_AndroidAotCompilation">
     <ReadLinesFromFile File="$(_AndroidStampDirectory)_AndroidAot.stamp">
       <Output TaskParameter="Lines" ItemName="_AotCompiledAssemblies" />

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

@@ -119,6 +119,11 @@
     <RunAOTCompilation Condition=" '$(RunAOTCompilation)' == '' and '$(AotAssemblies)' == '' and '$(Configuration)' == 'Release' and '$(_AndroidRuntime)' == 'MonoVM' ">true</RunAOTCompilation>
     <RunAOTCompilation Condition=" '$(RunAOTCompilation)' == '' and '$(AotAssemblies)' == 'true' ">true</RunAOTCompilation>
     <RunAOTCompilation Condition=" '$(RunAOTCompilation)' == '' ">false</RunAOTCompilation>
+    <!-- Warn and reset Mono-specific properties when the runtime is not MonoVM -->
+    <_AndroidXA1042_RunAOTCompilation Condition=" '$(RunAOTCompilation)' == 'true' and '$(_AndroidRuntime)' != 'MonoVM' ">true</_AndroidXA1042_RunAOTCompilation>
+    <_AndroidXA1042_EnableLLVM Condition=" '$(EnableLLVM)' == 'true' and '$(_AndroidRuntime)' != 'MonoVM' ">true</_AndroidXA1042_EnableLLVM>
+    <RunAOTCompilation Condition=" '$(_AndroidXA1042_RunAOTCompilation)' == 'true' ">false</RunAOTCompilation>
+    <EnableLLVM Condition=" '$(_AndroidXA1042_EnableLLVM)' == 'true' ">false</EnableLLVM>
     <_AndroidXA1029 Condition=" '$(AotAssemblies)' != '' ">true</_AndroidXA1029>
     <_AndroidXA1030 Condition=" '$(RunAOTCompilation)' == 'true' and '$(PublishTrimmed)' == 'false' ">true</_AndroidXA1030>
     <AotAssemblies>$(RunAOTCompilation)</AotAssemblies>

src/Xamarin.Android.Build.Tasks/Properties/Resources.resx

@@ -1007,6 +1007,13 @@ To use a custom JDK path for a command line build, set the 'JavaSdkDirectory' MS
     <comment>The following are literal names and should not be translated: .NET.
 {0} - The MSBuild property that has the incorrect value.
 {1} - The current value of the property
+</comment>
+  </data>
+  <data name="XA1042" xml:space="preserve">
+    <value>The MSBuild property '{0}' is not compatible with the {1} runtime and will be ignored. Either remove the property or guard it with a condition: Condition="'$(UseMonoRuntime)' == 'true'"</value>
+    <comment>The following are literal names and should not be translated: MSBuild, UseMonoRuntime.
+{0} - The MSBuild property name (e.g. RunAOTCompilation, EnableLLVM).
+{1} - The name of the .NET runtime (e.g. CoreCLR, NativeAOT).
 </comment>
   </data>
   <data name="XA4241" xml:space="preserve">

src/Xamarin.Android.Build.Tasks/Tests/Xamarin.Android.Build.Tests/AotTests.cs

@@ -492,5 +492,37 @@ public void CheckWhetherLibcAndLibmAreReferencedInAOTLibraries ()
 			}
 		}
 
+		[Test]
+		public void RunAOTCompilationWithCoreClrWarnsAndSkipsMonoAot ()
+		{
+			var proj = new XamarinAndroidApplicationProject {
+				IsRelease = true,
+			};
+			proj.SetRuntime (AndroidRuntime.CoreCLR);
+			proj.SetProperty ("RunAOTCompilation", "true");
+
+			using var b = CreateApkBuilder ();
+			Assert.IsTrue (b.Build (proj), "Build should have succeeded.");
+			StringAssertEx.Contains ("warning XA1042", b.LastBuildOutput, "Build output should contain warning XA1042");
+			StringAssertEx.Contains ("RunAOTCompilation", b.LastBuildOutput, "Build output should mention RunAOTCompilation");
+			StringAssertEx.Contains ("CoreCLR", b.LastBuildOutput, "Build output should mention CoreCLR");
+		}
+
+		[Test]
+		public void EnableLLVMWithCoreClrWarnsAndIsIgnored ()
+		{
+			var proj = new XamarinAndroidApplicationProject {
+				IsRelease = true,
+			};
+			proj.SetRuntime (AndroidRuntime.CoreCLR);
+			proj.SetProperty ("EnableLLVM", "true");
+
+			using var b = CreateApkBuilder ();
+			Assert.IsTrue (b.Build (proj), "Build should have succeeded.");
+			StringAssertEx.Contains ("warning XA1042", b.LastBuildOutput, "Build output should contain warning XA1042");
+			StringAssertEx.Contains ("EnableLLVM", b.LastBuildOutput, "Build output should mention EnableLLVM");
+			StringAssertEx.Contains ("CoreCLR", b.LastBuildOutput, "Build output should mention CoreCLR");
+		}
+
 	}
 }

src/Xamarin.Android.Build.Tasks/Xamarin.Android.Common.targets

@@ -550,6 +550,16 @@ Copyright (C) 2011-2012 Xamarin. All rights reserved.
       ResourceName="XA1030"
       Condition=" $(_AndroidXA1030) == 'true' "
   />
+  <AndroidWarning Code="XA1042"
+      ResourceName="XA1042"
+      FormatArguments="RunAOTCompilation;$(_AndroidRuntime)"
+      Condition=" '$(_AndroidXA1042_RunAOTCompilation)' == 'true' "
+  />
+  <AndroidWarning Code="XA1042"
+      ResourceName="XA1042"
+      FormatArguments="EnableLLVM;$(_AndroidRuntime)"
+      Condition=" '$(_AndroidXA1042_EnableLLVM)' == 'true' "
+  />
   <AndroidError Code="XA1011"
       ResourceName="XA1011"
       Condition=" '$(AndroidLinkTool)' == 'proguard' And '$(AndroidDexTool)' == 'd8' "