microsoft
diff --git a/‎.github/copilot-instructions.md‎
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diff --git a/‎.github/skills/interop-generator/SKILL.md‎
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diff --git a/‎docs/usage.md‎
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diff --git a/‎nuget/Microsoft.Windows.CsWinRT.CsWinRTGen.targets‎
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diff --git a/‎nuget/readme.md‎
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diff --git a/‎src/WinRT.Generator.Core/Errors/WellKnownGeneratorMessage.cs‎
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diff --git a/‎src/WinRT.Generator.Core/Parsing/ResponseFileBuilder.cs‎
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diff --git a/‎src/WinRT.Generator.Core/Parsing/ResponseFileParser.cs‎
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@@ -711,7 +711,7 @@ All five .NET build tools (`cswinrtprojectionrefgen`, `cswinrtprojectiongen`, `c
 | Projection Generator (host) | `CSWINRTPROJECTIONGENxxxx` | `0001`–`0011`, `9999` |
 | Projection Writer (library) | `CSWINRTPROJECTIONGEN5xxx` | `5003`–`5021`, `9999` (shares the `CSWINRTPROJECTIONGEN` prefix with the host; the writer reserves the 5000+ range so the two never collide) |
 | Impl Generator | `CSWINRTIMPLGENxxxx` | `0001`–`0014`, `9999` |
-| Interop Generator | `CSWINRTINTEROPGENxxxx` | `0001`–`0097`, `9999` |
+| Interop Generator | `CSWINRTINTEROPGENxxxx` | `0001`–`0100`, `9999` |
 | WinMD Generator | `CSWINRTWINMDGENxxxx` | `0001`–`0010`, `9999` |
 | Runtime (obsolete markers) | `CSWINRT3xxx` | `CSWINRT3001` |
 
 
@@ -61,8 +61,9 @@ WinRT.Interop.Generator/
 ├── Errors/                                 # Error/warning infrastructure
 │   ├── UnhandledInteropException.cs        # Wraps unexpected errors with phase context
 │   ├── WellKnownInteropException.cs        # Structured errors with CSWINRTINTEROPGEN### codes
-│   ├── WellKnownInteropExceptions.cs       # Factory for all error/warning codes (80+ codes)
-│   └── WellKnownInteropWarning.cs          # Non-fatal warnings (can be promoted to errors)
+│   ├── WellKnownInteropExceptions.cs       # Factory for all error/warning/message codes (100+ codes)
+│   ├── WellKnownInteropWarning.cs          # Non-fatal warnings (can be promoted to errors)
+│   └── WellKnownInteropMessage.cs          # Informational messages (never promoted to errors)
 ├── Extensions/                             # Extension methods (23 files)
 │   ├── WindowsRuntimeExtensions.cs         # Core: IsProjectedWindowsRuntimeType, IsBlittable, etc.
 │   ├── TypeSignatureExtensions.cs          # GetAbiType, EnumerateAllInterfaces, etc.
@@ -230,6 +231,8 @@ The generator is invoked via a response file: `cswinrtinteropgen.exe @path/to/re
 | `--winrt-component-assembly-path` | `string?` | `WinRT.Component.dll` path (for authored components) |
 | `--generated-assembly-directory` | `string` | Output folder for `WinRT.Interop.dll` |
 | `--use-windows-ui-xaml-projections` | `bool` | Use UWP XAML (`Windows.UI.Xaml`) instead of WinUI |
+| `--marshalling-mode` | `CsWinRTMarshallingMode` | Which assemblies to analyze for discovery: `All`, `Minimal` (default; skip BCL), or `Strict` (only WinRT-referencing) |
+| `--marshalling-enabled-assembly-names` | `string[]` | Assembly names (`.dll`/directory ignored) always analyzed regardless of mode (from the `CsWinRTMarshallingEnabledAssembly` item) |
 | `--validate-winrt-runtime-assembly-version` | `bool` | Check version compatibility |
 | `--validate-winrt-runtime-dll-version-2-references` | `bool` | Reject CsWinRT 2.x references |
 | `--enable-incremental-generation` | `bool` | Enable incremental generation (caching) |
@@ -282,6 +285,15 @@ The generator processes two categories of assemblies:
 - `WinRT.Projection.dll` — 3rd-party component projections (optional)
 - `WinRT.Component.dll` — Authored component projections (optional)
 
+**Marshalling mode** (`--marshalling-mode`, `ShouldProcessModule` in `InteropGenerator.Discover.cs`): controls which modules are analyzed for discovery. Modules referencing the Windows Runtime assembly (i.e. those built targeting a Windows TFM) are **always** analyzed; the mode only affects modules that don't reference any CsWinRT assembly:
+- `All` — analyze every module, even plain `.NET` assemblies (including the BCL).
+- `Minimal` (default) — analyze every module except those from the BCL (detected via well-known .NET/Microsoft public key tokens, see `BaseClassLibraryIdentity` in `WinRT.Generator.Core` and the `ModuleDefinition.IsBaseClassLibraryModule` extension), to reduce binary size. This still lets projects that don't target a Windows TFM (e.g. a class library with just MVVM viewmodels) contribute the marshalling code their types need.
+- `Strict` — only analyze modules referencing the Windows Runtime assembly (the historical behavior). Corresponds to `!ReferencesWindowsRuntimeAssembly && !IsWindowsRuntimeModule → skip`.
+
+**Legacy-runtime skip** (`ModuleDefinition.TargetsLegacyRuntime` in `InteropGenerator.Discover.cs`): regardless of mode, modules targeting a legacy/portable runtime (i.e. a `netstandard` or `mscorlib` corlib scope) are **never** analyzed. The entire generator infrastructure identifies well-known types (including custom-mapped types such as `IEnumerable<T>`) by comparing against type references scoped to the modern .NET corlib (e.g. `System.Runtime`), which the emit phase also uses. Legacy-runtime assemblies declare those same types against a different corlib (`netstandard`/`mscorlib`), which `AsmResolver`'s `SignatureComparer` treats as a distinct scope — so the generator can't match them, and generating marshalling code for them would fail with `CSWINRTINTEROPGEN0055`/`0020`. Such assemblies could in principle still use custom-mapped types that need marshalling, but for simplicity they're skipped entirely.
+
+**Opt-in assemblies** (`--marshalling-enabled-assembly-names`, from the `CsWinRTMarshallingEnabledAssembly` MSBuild item): assemblies listed here are **always** analyzed (matched by simple name, ignoring `.dll`/directory), regardless of the mode — checked first in `ShouldProcessModule`. This lets users fine-tune size (e.g. `strict` mode + a few opted-in assemblies). It does **not** override the legacy-runtime skip (legacy assemblies still can't be marshalled). `ValidateMarshallingEnabledAssemblies` emits three diagnostics: `CSWINRTINTEROPGEN0098` (warning: entry doesn't match any referenced assembly), `CSWINRTINTEROPGEN0099` (message: entry already targets Windows, redundant), and `CSWINRTINTEROPGEN0100` (message: mode is `all`, all entries redundant).
+
 **Type exclusions** (`Helpers/TypeExclusions.cs`):
 - `System.Threading.Tasks.Task<T>` — Cannot be marshalled across Windows Runtime boundary
 - `System.Collections.Concurrent.ConditionalWeakTable<,>` — Memory semantics conflict
 
@@ -136,10 +136,47 @@ Below are the most commonly used MSBuild properties. For a full list, refer to t
 | `CsWinRTGenerateProjection` | `true` | Generate C# projection sources from `.winmd` metadata. |
 | `CsWinRTGenerateReferenceProjection` | `false` | Generate reference-only projections (for NuGet distribution). |
 | `CsWinRTComponent` | `false` | Enable Windows Runtime component authoring mode. |
+| `CsWinRTMarshallingMode` | `minimal` | Controls which assemblies the interop generator analyzes for marshalling code (`all`, `minimal`, or `strict`). See below. |
 | `CsWinRTIncludes` | *(empty)* | Semicolon-separated namespaces to include in the projection. |
 | `CsWinRTExcludes` | `Windows;Microsoft` | Semicolon-separated namespaces to exclude from the projection. |
 | `CsWinRTMessageImportance` | `normal` | Build message verbosity (`normal` or `high`). |
 
+### Marshalling mode
+
+By default (`minimal`), the interop generator analyzes **every** assembly in the app to discover user-defined
+(CCW) types and generic instantiations that need marshalling code — even assemblies that don't target a
+Windows TFM and don't reference any CsWinRT assembly — with the exception of the .NET base class library
+(BCL). This means a plain `.NET` class library (e.g. one holding MVVM viewmodels) no longer needs to target
+a `-windows` TFM just so its types can be marshalled to native code, while the BCL is skipped to keep the
+generated interop assembly small.
+
+`CsWinRTMarshallingMode` controls this behavior:
+
+| Value | Behavior |
+|-------|----------|
+| `all` | Analyze all assemblies, including those from the .NET base class library (BCL). |
+| `minimal` (default) | Same as `all`, but skip assemblies from the .NET base class library (BCL) to reduce binary size. |
+| `strict` | Only analyze assemblies referencing the Windows Runtime assembly (i.e. those targeting a Windows TFM). |
+
+Assemblies that reference the Windows Runtime assembly are always analyzed, regardless of the mode. Assemblies targeting a legacy or portable runtime (i.e. .NET Standard or .NET Framework) are never analyzed, since the interop generator can only marshal types declared against a modern .NET runtime.
+
+### Opting in specific assemblies
+
+The `CsWinRTMarshallingEnabledAssembly` item lets you force the interop generator to analyze specific assemblies, regardless of the marshalling mode. This is useful for fine-tuning binary size: for example, you can keep the `strict` mode (the smallest option) while opting in just the few assemblies you know need marshalling support:
+
+```xml
+<PropertyGroup>
+  <CsWinRTMarshallingMode>strict</CsWinRTMarshallingMode>
+</PropertyGroup>
+
+<ItemGroup>
+  <CsWinRTMarshallingEnabledAssembly Include="MyApp.ViewModels" />
+  <CsWinRTMarshallingEnabledAssembly Include="MyApp.Models" />
+</ItemGroup>
+```
+
+Each item is an assembly name (the `.dll` extension is optional). The interop generator reports a warning if an entry doesn't match any referenced assembly, and an informational message if an entry is redundant (e.g. it already targets Windows, or the mode is `all` and thus already analyzes everything).
+
 ## Author and consume a C#/WinRT component
 
 This is coming soon for CsWinRT 3.0.
 
@@ -45,6 +45,19 @@ Copyright (C) Microsoft Corporation. All rights reserved.
     <CsWinRTGeneratorStandardErrorImportance Condition="'$(CsWinRTGeneratorStandardErrorImportance)' == ''">High</CsWinRTGeneratorStandardErrorImportance>
     <CsWinRTGeneratorLogStandardErrorAsError Condition="'$(CsWinRTGeneratorLogStandardErrorAsError)' == ''">true</CsWinRTGeneratorLogStandardErrorAsError>
 
+    <!--
+      The marshalling mode controls which assemblies 'cswinrtinteropgen' analyzes to discover
+      user-defined/CCW/generic types that need marshalling code:
+        - 'minimal' (default): analyze all assemblies (even those not referencing any CsWinRT
+          assembly), except those from the .NET base class library (BCL). This lets projects that
+          don't target a Windows TFM (e.g. a class library with just MVVM viewmodels) contribute
+          the marshalling code their types need, while skipping the BCL to reduce binary size.
+        - 'all': same as 'minimal', but also analyze assemblies from the .NET base class library (BCL).
+        - 'strict': only analyze assemblies referencing the Windows Runtime assembly (i.e. those
+          targeting a Windows TFM).
+    -->
+    <CsWinRTMarshallingMode Condition="'$(CsWinRTMarshallingMode)' == ''">minimal</CsWinRTMarshallingMode>
+
     <!-- Emits a SetEntryAssembly module initializer into WinRT.Component.dll to enable JIT TypeMap discovery.
          Not needed for AOT (handled via a separate exe-project workaround). Will become unnecessary once
          the TypeMappingEntryAssembly MSBuild property is available. -->
@@ -133,6 +146,8 @@ Copyright (C) Microsoft Corporation. All rights reserved.
       <_RunCsWinRTGeneratorInputsCacheToHash Include="$(CsWinRTGeneratorInteropAssemblyDirectory)" />
       <_RunCsWinRTGeneratorInputsCacheToHash Include="$(CsWinRTGeneratorDebugReproDirectory)" />
       <_RunCsWinRTGeneratorInputsCacheToHash Include="$(CsWinRTUseWindowsUIXamlProjections)" />
+      <_RunCsWinRTGeneratorInputsCacheToHash Include="$(CsWinRTMarshallingMode)" />
+      <_RunCsWinRTGeneratorInputsCacheToHash Include="@(CsWinRTMarshallingEnabledAssembly)" />
       <_RunCsWinRTGeneratorInputsCacheToHash Include="$(CsWinRTGeneratorValidateWinRTRuntimeAssemblyVersion)" />
       <_RunCsWinRTGeneratorInputsCacheToHash Include="$(CsWinRTGeneratorValidateWinRTRuntimeDllVersion2References)" />
       <_RunCsWinRTGeneratorInputsCacheToHash Include="$(CsWinRTGeneratorEnableIncrementalGeneration)" />
@@ -185,6 +200,8 @@ Copyright (C) Microsoft Corporation. All rights reserved.
       CsWinRTToolsDirectory="$(CsWinRTInteropGenEffectiveToolsDirectory)"
       CsWinRTToolsArchitecture="$(CsWinRTToolsArchitecture)"
       UseWindowsUIXamlProjections="$(CsWinRTUseWindowsUIXamlProjections)"
+      MarshallingMode="$(CsWinRTMarshallingMode)"
+      MarshallingEnabledAssemblies="@(CsWinRTMarshallingEnabledAssembly)"
       ValidateWinRTRuntimeAssemblyVersion="$(CsWinRTGeneratorValidateWinRTRuntimeAssemblyVersion)"
       ValidateWinRTRuntimeDllVersion2References="$(CsWinRTGeneratorValidateWinRTRuntimeDllVersion2References)"
       EnableIncrementalGeneration="$(CsWinRTGeneratorEnableIncrementalGeneration)"
 
@@ -33,6 +33,8 @@ C#/WinRT behavior can be customized with these project properties:
 | CsWinRTGenerateReferenceProjection | true \| *false | Generate reference-only projections (for NuGet distribution) |
 | CsWinRTGenerateInteropAssembly | auto | Generate interop assemblies at build time (defaults to `true` for Exe/WinExe, or Library with `PublishAot=true`) |
 | CsWinRTComponent | true \| *false | Enable Windows Runtime component authoring mode |
+| CsWinRTMarshallingMode | all \| *minimal \| strict | Controls which assemblies the interop generator analyzes for marshalling code. `all` analyzes every assembly (including the .NET base class library), `minimal` analyzes every assembly except the .NET base class library (BCL), and `strict` only analyzes assemblies referencing the Windows Runtime assembly |
+| CsWinRTMarshallingEnabledAssembly | *(item)* | An item listing specific assemblies (by name, `.dll` optional) to always analyze for marshalling code, regardless of `CsWinRTMarshallingMode`. Useful for fine-tuning binary size (e.g. using `strict` and opting in a few assemblies) |
 | CsWinRTUseWindowsUIXamlProjections | true \| *false | Use UWP XAML (`Windows.UI.Xaml`) instead of WinUI (`Microsoft.UI.Xaml`) |
 | CsWinRTMergeReferencedActivationFactories | true \| *false | Makes `DllGetActivationFactory` forward activation calls to all referenced WinRT components, allowing them to be merged into a single executable |
 | CsWinRTIncludes | "" | Semicolon-separated namespaces to include in projection output |
 
@@ -0,0 +1,48 @@
+// Copyright (c) Microsoft Corporation.
+// Licensed under the MIT License.
+
+using System;
+
+namespace WindowsRuntime.Generator.Errors;
+
+/// <summary>
+/// A well-known informational message shared by all generators.
+/// </summary>
+/// <remarks>
+/// Unlike a warning, a message is purely informational: it is never promoted to an error, and it is surfaced
+/// by MSBuild as a build message rather than a warning (the emitted line intentionally omits the
+/// <c>warning</c>/<c>error</c> category keyword that MSBuild's output parser looks for). It is distinct from
+/// <see cref="WellKnownGeneratorMessages"/>, which holds the message templates for the shared logical errors.
+/// </remarks>
+internal sealed class WellKnownGeneratorMessage
+{
+    /// <summary>
+    /// The id of the message.
+    /// </summary>
+    private readonly string _id;
+
+    /// <summary>
+    /// The message text.
+    /// </summary>
+    private readonly string _message;
+
+    /// <summary>
+    /// Creates a new <see cref="WellKnownGeneratorMessage"/> instance with the specified parameters.
+    /// </summary>
+    /// <param name="id">The id of the message.</param>
+    /// <param name="message">The message text.</param>
+    public WellKnownGeneratorMessage(string id, string message)
+    {
+        _id = id;
+        _message = message;
+    }
+
+    /// <summary>
+    /// Logs the message through the provided logger.
+    /// </summary>
+    /// <param name="log">The logger to write the message to (typically <c>ConsoleApp.Log</c> from ConsoleAppFramework).</param>
+    public void Log(Action<string> log)
+    {
+        log($"{_id}: {_message}");
+    }
+}
@@ -89,6 +89,13 @@ private static string FormatValue(object value)
             return string.Join(',', array);
         }
 
+        // Enum values are serialized by their member name (e.g. 'All'), matching the parser's
+        // case-insensitive, name-based enum handling so the round-trip stays deterministic.
+        if (value is Enum enumValue)
+        {
+            return enumValue.ToString();
+        }
+
         // All other primitive values use invariant culture, matching the parser's
         // invariant 'Convert.ChangeType' so the round-trip is deterministic.
         return Convert.ToString(value, CultureInfo.InvariantCulture) ?? string.Empty;
 
@@ -261,6 +261,26 @@ private static bool TryConvert(string rawValue, Type targetType, out object? con
             return true;
         }
 
+        // Enum-typed arguments are parsed by name (case-insensitively), so that MSBuild values such
+        // as 'all' round-trip to the matching enum member (e.g. 'All'). Numeric values are rejected,
+        // as they carry no meaning in a response file and would silently accept out-of-range inputs.
+        if (targetType.IsEnum)
+        {
+            if (rawValue.Length > 0 &&
+                !char.IsAsciiDigit(rawValue[0]) &&
+                rawValue[0] != '-' &&
+                Enum.TryParse(targetType, rawValue, ignoreCase: true, out object? parsedEnum))
+            {
+                converted = parsedEnum;
+
+                return true;
+            }
+
+            converted = null;
+
+            return false;
+        }
+
         // For primitives ('bool', 'int', etc.) we rely on the standard 'Convert.ChangeType', which
         // is AOT-safe for the built-in primitives and uses invariant culture for deterministic parsing.
         try