From 54b8e3ea28224e3c5b7ad365d25f6b4c1d16e3f3 Mon Sep 17 00:00:00 2001
From: Sven Boemer <sbomer@gmail.com>
Date: Fri, 9 Jan 2026 11:10:31 -0800
Subject: [PATCH 1/2] Migrate WarnOnPreserveAttribute from ILLink step to
 MSBuild task

Replace the ILLink custom step with an MSBuild task that runs before
_RunILLink. This simplifies the build pipeline by removing a dependency
on the ILLink step infrastructure while maintaining the same behavior.

The new CheckForObsoletePreserveAttribute task uses System.Reflection.Metadata
for fast metadata scanning instead of Mono.Cecil.
---
 .../WarnOnPreserveAttribute.cs                | 38 ------------
 .../Microsoft.Android.Sdk.ILLink.targets      | 10 +--
 .../CheckForObsoletePreserveAttribute.cs      | 61 +++++++++++++++++++
 3 files changed, 67 insertions(+), 42 deletions(-)
 delete mode 100644 src/Microsoft.Android.Sdk.ILLink/WarnOnPreserveAttribute.cs
 create mode 100644 src/Xamarin.Android.Build.Tasks/Tasks/CheckForObsoletePreserveAttribute.cs

diff --git a/src/Microsoft.Android.Sdk.ILLink/WarnOnPreserveAttribute.cs b/src/Microsoft.Android.Sdk.ILLink/WarnOnPreserveAttribute.cs
deleted file mode 100644
index ec845dad5ca..00000000000
--- a/src/Microsoft.Android.Sdk.ILLink/WarnOnPreserveAttribute.cs
+++ /dev/null
@@ -1,38 +0,0 @@
-using System;
-using System.Collections;
-using System.Collections.Generic;
-using System.Linq;
-using Mono.Cecil;
-using Mono.Linker;
-using Mono.Linker.Steps;
-using Xamarin.Android.Tasks;
-
-namespace Microsoft.Android.Sdk.ILLink
-{
-	/// <summary>
-	/// This step provides warnings when an assembly references the obsolete PresrveAttribute.
-	/// The PreserveAttribute used to indicate to the linker that a type or member should not be trimmed.
-	/// It had similar functionality to the newer DynamicDependencyAttribute, but was Android-specific and is now obsolete.
-	/// </summary>
-	public class WarnOnPreserveAttribute : BaseStep {
-		protected override void ProcessAssembly (AssemblyDefinition assembly)
-		{
-			foreach (var module in assembly.Modules) {
-				foreach (var tr in module.GetTypeReferences ()) {
-					if (tr is {
-						Namespace: "Android.Runtime",
-						Name: "PreserveAttribute",
-					}) {
-						Context.LogMessage (MessageContainer.CreateCustomWarningMessage(
-							Context,
-							$"Assembly '{assembly.Name.Name}' contains reference to obsolete attribute 'Android.Runtime.PreserveAttribute'. Members with this attribute may be trimmed. Please use System.Diagnostics.CodeAnalysis.DynamicDependencyAttribute instead",
-							6001,
-							new MessageOrigin (),
-							WarnVersion.ILLink0));
-						return;
-					}
-				}
-			}
-		}
-	}
-}
diff --git a/src/Xamarin.Android.Build.Tasks/Microsoft.Android.Sdk/targets/Microsoft.Android.Sdk.ILLink.targets b/src/Xamarin.Android.Build.Tasks/Microsoft.Android.Sdk/targets/Microsoft.Android.Sdk.ILLink.targets
index c0c7ff7de09..f0acc0b574c 100644
--- a/src/Xamarin.Android.Build.Tasks/Microsoft.Android.Sdk/targets/Microsoft.Android.Sdk.ILLink.targets
+++ b/src/Xamarin.Android.Build.Tasks/Microsoft.Android.Sdk/targets/Microsoft.Android.Sdk.ILLink.targets
@@ -9,6 +9,8 @@ This file contains the .NET 5-specific targets to customize ILLink
 
 <Project xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
 
+  <UsingTask TaskName="Xamarin.Android.Tasks.CheckForObsoletePreserveAttribute" AssemblyFile="$(_XamarinAndroidBuildTasksAssembly)" />
+
   <Target Name="_PrepareLinking"
       Condition=" '$(PublishTrimmed)' == 'true' "
       AfterTargets="ComputeResolvedFilesToPublishList"
@@ -39,10 +41,6 @@ This file contains the .NET 5-specific targets to customize ILLink
           Value="$(_ProguardProjectConfiguration)"
       />
       <_TrimmerCustomData Include="SystemIOHashingAssemblyPath" Value="$(_SystemIOHashingAssemblyPath)" />
-      <_TrimmerCustomSteps
-        Include="$(_AndroidLinkerCustomStepAssembly)"
-        BeforeStep="MarkStep"
-        Type="Microsoft.Android.Sdk.ILLink.WarnOnPreserveAttribute" />
 
       <!--
         Used for the <ILLink CustomSteps="@(_TrimmerCustomSteps)" /> value:
@@ -102,6 +100,10 @@ This file contains the .NET 5-specific targets to customize ILLink
     </ItemGroup>
   </Target>
 
+  <Target Name="_CheckForObsoletePreserveAttribute" BeforeTargets="_RunILLink">
+    <CheckForObsoletePreserveAttribute Assemblies="@(ManagedAssemblyToLink)" />
+  </Target>
+
   <Target Name="_FixRootAssembly" AfterTargets="PrepareForILLink">
     <ItemGroup>
       <TrimmerRootAssembly Update="@(TrimmerRootAssembly)" Condition=" '%(RootMode)' == 'EntryPoint' " RootMode="All" />
diff --git a/src/Xamarin.Android.Build.Tasks/Tasks/CheckForObsoletePreserveAttribute.cs b/src/Xamarin.Android.Build.Tasks/Tasks/CheckForObsoletePreserveAttribute.cs
new file mode 100644
index 00000000000..dedd7e8eeda
--- /dev/null
+++ b/src/Xamarin.Android.Build.Tasks/Tasks/CheckForObsoletePreserveAttribute.cs
@@ -0,0 +1,61 @@
+#nullable enable
+
+using System;
+using System.IO;
+using System.Reflection.Metadata;
+using System.Reflection.PortableExecutable;
+using Microsoft.Build.Framework;
+using Microsoft.Android.Build.Tasks;
+
+namespace Xamarin.Android.Tasks
+{
+	/// <summary>
+	/// This task provides warnings when an assembly references the obsolete PreserveAttribute.
+	/// The PreserveAttribute used to indicate to the linker that a type or member should not be trimmed.
+	/// It had similar functionality to the newer DynamicDependencyAttribute, but was Android-specific and is now obsolete.
+	/// </summary>
+	public class CheckForObsoletePreserveAttribute : AndroidTask
+	{
+		public override string TaskPrefix => "COPA";
+
+		public ITaskItem[] Assemblies { get; set; } = [];
+
+		public override bool RunTask ()
+		{
+			foreach (var assembly in Assemblies) {
+				if (HasObsoletePreserveAttribute (assembly.ItemSpec, out string? assemblyName)) {
+					Log.LogCodedWarning ("IL6001", $"Assembly '{assemblyName}' contains reference to obsolete attribute 'Android.Runtime.PreserveAttribute'. Members with this attribute may be trimmed. Please use System.Diagnostics.CodeAnalysis.DynamicDependencyAttribute instead");
+				}
+			}
+			return true; // Warnings don't fail the build
+		}
+
+		static bool HasObsoletePreserveAttribute (string assemblyPath, out string? assemblyName)
+		{
+			assemblyName = null;
+
+			try {
+				using var stream = File.OpenRead (assemblyPath);
+				using var pe = new PEReader (stream);
+
+				if (!pe.HasMetadata)
+					return false;
+
+				var reader = pe.GetMetadataReader ();
+				assemblyName = reader.GetString (reader.GetAssemblyDefinition ().Name);
+
+				foreach (var handle in reader.TypeReferences) {
+					var typeRef = reader.GetTypeReference (handle);
+					var ns = reader.GetString (typeRef.Namespace);
+					var name = reader.GetString (typeRef.Name);
+
+					if (ns == "Android.Runtime" && name == "PreserveAttribute")
+						return true;
+				}
+				return false;
+			} catch (BadImageFormatException) {
+				return false;
+			}
+		}
+	}
+}

From 692d7f0cf7b49935bc9bd10b0d2af26c2e5b7241 Mon Sep 17 00:00:00 2001
From: Sven Boemer <sbomer@gmail.com>
Date: Fri, 9 Jan 2026 15:57:57 -0800
Subject: [PATCH 2/2] Move obsolete PreserveAttribute check to run on all
 builds

Address PR feedback: move _CheckForObsoletePreserveAttribute target from
ILLink.targets to AssemblyResolution.targets so it runs on all builds,
not just Release/trimmed builds. This gives developers earlier feedback
about using the obsolete attribute.
---
 .../Microsoft.Android.Sdk.AssemblyResolution.targets        | 5 +++++
 .../targets/Microsoft.Android.Sdk.ILLink.targets            | 6 ------
 2 files changed, 5 insertions(+), 6 deletions(-)

diff --git a/src/Xamarin.Android.Build.Tasks/Microsoft.Android.Sdk/targets/Microsoft.Android.Sdk.AssemblyResolution.targets b/src/Xamarin.Android.Build.Tasks/Microsoft.Android.Sdk/targets/Microsoft.Android.Sdk.AssemblyResolution.targets
index 9cc9b0e77b7..b1717d2b0fe 100644
--- a/src/Xamarin.Android.Build.Tasks/Microsoft.Android.Sdk/targets/Microsoft.Android.Sdk.AssemblyResolution.targets
+++ b/src/Xamarin.Android.Build.Tasks/Microsoft.Android.Sdk/targets/Microsoft.Android.Sdk.AssemblyResolution.targets
@@ -10,6 +10,7 @@ _ResolveAssemblies MSBuild target.
 
 <Project xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
 
+  <UsingTask TaskName="Xamarin.Android.Tasks.CheckForObsoletePreserveAttribute" AssemblyFile="$(_XamarinAndroidBuildTasksAssembly)" />
   <UsingTask TaskName="Xamarin.Android.Tasks.ProcessAssemblies"      AssemblyFile="$(_XamarinAndroidBuildTasksAssembly)" />
   <UsingTask TaskName="Xamarin.Android.Tasks.ProcessNativeLibraries" AssemblyFile="$(_XamarinAndroidBuildTasksAssembly)" />
   <UsingTask TaskName="Xamarin.Android.Tasks.ProcessRuntimePackLibraryDirectories" AssemblyFile="$(_XamarinAndroidBuildTasksAssembly)" />
@@ -226,6 +227,10 @@ _ResolveAssemblies MSBuild target.
     </ItemGroup>
   </Target>
 
+  <Target Name="_CheckForObsoletePreserveAttribute" AfterTargets="_PrepareAssemblies">
+    <CheckForObsoletePreserveAttribute Assemblies="@(_ResolvedUserAssemblies)" />
+  </Target>
+
   <Target Name="_IncludeNativeSystemLibraries">
     <PropertyGroup>
       <_AndroidIncludeSystemGlobalizationNative Condition=" '$(_AndroidIncludeSystemGlobalizationNative)' == '' ">true</_AndroidIncludeSystemGlobalizationNative>
diff --git a/src/Xamarin.Android.Build.Tasks/Microsoft.Android.Sdk/targets/Microsoft.Android.Sdk.ILLink.targets b/src/Xamarin.Android.Build.Tasks/Microsoft.Android.Sdk/targets/Microsoft.Android.Sdk.ILLink.targets
index f0acc0b574c..d5411d79619 100644
--- a/src/Xamarin.Android.Build.Tasks/Microsoft.Android.Sdk/targets/Microsoft.Android.Sdk.ILLink.targets
+++ b/src/Xamarin.Android.Build.Tasks/Microsoft.Android.Sdk/targets/Microsoft.Android.Sdk.ILLink.targets
@@ -9,8 +9,6 @@ This file contains the .NET 5-specific targets to customize ILLink
 
 <Project xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
 
-  <UsingTask TaskName="Xamarin.Android.Tasks.CheckForObsoletePreserveAttribute" AssemblyFile="$(_XamarinAndroidBuildTasksAssembly)" />
-
   <Target Name="_PrepareLinking"
       Condition=" '$(PublishTrimmed)' == 'true' "
       AfterTargets="ComputeResolvedFilesToPublishList"
@@ -100,10 +98,6 @@ This file contains the .NET 5-specific targets to customize ILLink
     </ItemGroup>
   </Target>
 
-  <Target Name="_CheckForObsoletePreserveAttribute" BeforeTargets="_RunILLink">
-    <CheckForObsoletePreserveAttribute Assemblies="@(ManagedAssemblyToLink)" />
-  </Target>
-
   <Target Name="_FixRootAssembly" AfterTargets="PrepareForILLink">
     <ItemGroup>
       <TrimmerRootAssembly Update="@(TrimmerRootAssembly)" Condition=" '%(RootMode)' == 'EntryPoint' " RootMode="All" />