Prepare .NET libraries for trimming

The .NET SDK makes it possible to reduce the size of self-contained apps by trimming, removing unused code from the app and its dependencies. Not all code is compatible with trimming, so .NET 6 provides trim analysis warnings to detect patterns that may break trimmed apps. This article describes how to prepare libraries for trimming with the aid of these warnings, including recommendations for fixing some common cases.

Trim warnings in apps

In .NET 6+, when publishing an app, the PublishTrimmed project file element produces trim analysis warnings for patterns that are not statically understood to be compatible with trimming, including patterns in your code and in dependencies.

You will encounter detailed warnings originating from your own code and ProjectReference dependencies. You may also see warnings like warning IL2104: Assembly 'SomeAssembly' produced trim warnings for PackageReference libraries. This warning means that the library contained patterns that are not guaranteed to work in the context of the trimmed app, and may result in a broken app. Consider contacting the author to see if the library can be annotated for trimming.

To resolve warnings originating from the app code, see resolving trim warnings. If you are interested in making your own ProjectReference libraries trim friendly, follow the instructions to enable library trim warnings.

If your app only uses parts of a library that are compatible with trimming, consider enabling trimming if it's not already being trimmed. By enabling trimming, .NET only produces warnings if your app uses problematic parts of the library. (You can also show detailed warnings for the library to see which parts of it are problematic.)

Enable library trim warnings

These instructions show how to enable and resolve static analysis warnings to prepare a library for trimming. Follow these steps if you are authoring a library and either want to proactively make your library trimmable, or have been contacted by app authors who encountered trim warnings from your library.

Tip

Ensure you're using the .NET 6 SDK or later for these steps. They will not work correctly in previous versions.

Set IsTrimmable

Set <IsTrimmable>true</IsTrimmable> (in .NET 6+) in your library project. This will mark your assembly as "trimmable". Being trimmable means when your library is used in a trimmed application the assembly can have its unused members trimmed in the final output.

Setting <IsTrimmable>true</IsTrimmable> enables a Roslyn analyzer for trim compatibility. The Roslyn analyzer is useful for quick feedback in your IDE, but it's currently incomplete. It doesn't cover all trim analysis warnings, but the set of patterns it understands will improve over time to give more complete coverage. The Roslyn analyzer also isn't able to analyze the implementations of reference assemblies that you depend on. It's important to follow the steps outlined in the rest of this article to ensure that your library is fully compatible with trimming.

Alternatively, you can just set <EnableTrimAnalyzer>true</EnableTrimAnalyzer> (in .NET 6+) in your library project. This will not have any effect on the output, but it will enable trim analysis during build via the Roslyn analyzer.

Show all warnings

To show all analysis warnings for your library, including warnings about dependencies, create a separate app project like the following that references your library, and publish it with PublishTrimmed.

The extra step of creating an app project just to get complete warnings for a library is necessary because the implementations of dependencies are not generally available during dotnet build, and reference assemblies don't contain enough information to determine whether they are compatible with trimming. Publishing a self-contained app ensures that the library is analyzed in a context where its dependencies are available, so that you are alerted if your library uses any code from dependencies that could break a trimmed app.

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net6.0</TargetFramework>
    <!-- Use a RID of your choice. -->
    <RuntimeIdentifier>linux-x64</RuntimeIdentifier>
    <PublishTrimmed>true</PublishTrimmed>
    <!-- Prevent warnings from unused code in dependencies -->
    <TrimmerDefaultAction>link</TrimmerDefaultAction>
  </PropertyGroup>

  <ItemGroup>
    <ProjectReference Include="path/to/MyLibrary.csproj" />
    <!-- Analyze the whole library, even if attributed with "IsTrimmable" -->
    <TrimmerRootAssembly Include="MyLibrary" />
  </ItemGroup>

</Project>
dotnet publish -c Release
  • TrimmerRootAssembly ensures that every part of the library is analyzed. This is necessary in case the library has [AssemblyMetadata("IsTrimmable", "True")], which would otherwise let trimming remove the unused library without analyzing it.

  • <TrimmerDefaultAction>link</TrimmerDefaultAction> ensures that only used parts of dependencies are analyzed. Without this option, you would see warnings originating from any part of a dependency that doesn't set [AssemblyMetadata("IsTrimmable", "True")], including parts that are unused by your library.

You can also follow the same pattern for multiple libraries, adding them all to the same project as ProjectReference and TrimmerRootAssembly item to see trim analysis warnings for more than one library at a time, but note that this will warn about dependencies if any of the root libraries use a trim-unfriendly API in a dependency. To see warnings that have to do with only a particular library, reference that library only.

Note

The analysis results depend on the implementation details of your dependencies. If you update to a new version of a dependency, this may introduce analysis warnings if the new version added non-understood reflection patterns, even if there were no API changes. In other words, introducing trim analysis warnings to a library is a breaking change when the library is used with PublishTrimmed.

Resolve trim warnings

The above steps will produce warnings about code that may cause problems when used in a trimmed app. Here are a few examples of the most common kinds of warnings you may encounter, with recommendations for fixing them.

RequiresUnreferencedCode

using System.Diagnostics.CodeAnalysis;

public class MyLibrary
{
    public static void Method()
    {
        // warning IL2026 : MyLibrary.Method: Using method 'MyLibrary.DynamicBehavior' which has
        // 'RequiresUnreferencedCodeAttribute' can break functionality
        // when trimming application code.
        DynamicBehavior();
    }

    [RequiresUnreferencedCode("DynamicBehavior is incompatible with trimming.")]
    static void DynamicBehavior()
    {
    }
}

This means the library calls a method that has explicitly been annotated as incompatible with trimming, using RequiresUnreferencedCodeAttribute. To get rid of the warning, consider whether Method needs to call DynamicBehavior to do its job. If so, annotate the caller Method with RequiresUnreferencedCode as well; this will "bubble up" the warning so that callers of Method get a warning instead:

// Warn for calls to Method, but not for Method's call to DynamicBehavior.
[RequiresUnreferencedCode("Calls DynamicBehavior.")]
public static void Method()
{
    DynamicBehavior(); // OK. Doesn't warn now.
}

Once you have "bubbled up" the attribute all the way to public APIs (so that these warnings are produced only for public methods, if at all), you are done. Apps that call your library will now get warnings if they call those public APIs, but these will no longer produce warnings like IL2104: Assembly 'MyLibrary' produced trim warnings.

DynamicallyAccessedMembers

using System.Diagnostics.CodeAnalysis;

public class MyLibrary
{
    static void UseMethods(Type type)
    {
        // warning IL2070: MyLibrary.UseMethods(Type): 'this' argument does not satisfy
        // 'DynamicallyAccessedMemberTypes.PublicMethods' in call to 'System.Type.GetMethods()'.
        // The parameter 't' of method 'MyLibrary.UseMethods(Type)' does not have matching annotations.
        foreach (var method in type.GetMethods())
        {
            // ...
        }
    }
}

Here, UseMethods is calling a reflection method that has a DynamicallyAccessedMembersAttribute requirement. The requirement states that the type's public methods are available. In this case, you can satisfy the requirement by adding the same requirement to the parameter of UseMethods.

static void UseMethods(
    // State the requirement in the UseMethods parameter.
    [DynamicallyAccessedMembers(DynamicallyAccessedMemberTypes.PublicMethods)]
    Type type)
{
    // ...
}

Now any calls to UseMethods will produce warnings if they pass in values that don't satisfy the PublicMethods requirement. Like with RequiresUnreferencedCode, once you have bubbled up such warnings to public APIs, you are done.

Here is another example where an unknown Type flows into the annotated method parameter, this time from a field:

static Type type;

static void UseMethodsHelper()
{
    // warning IL2077: MyLibrary.UseMethodsHelper(Type): 'type' argument does not satisfy
    // 'DynamicallyAccessedMemberTypes.PublicMethods' in call to 'MyLibrary.UseMethods(Type)'.
    // The field 'System.Type MyLibrary::type' does not have matching annotations.
    UseMethods(type);
}

Similarly, here the problem is that the field type is passed into a parameter with these requirements. You can fix it by adding DynamicallyAccessedMembers to the field. This will warn about code that assigns incompatible values to the field instead. Sometimes this process will continue until a public API is annotated, and other times it will end when a concrete type flows into a location with these requirements. For example:

[DynamicallyAccessedMembers(DynamicallyAccessedMembers.PublicMethods)]
static Type type;

static void InitializeTypeField()
{
    MyLibrary.type = typeof(System.Tuple);
}

In this case the trim analysis will simply keep public methods of System.Tuple, and will not produce further warnings.

Recommendations

In general, try to avoid reflection if possible. When using reflection, limit it in scope so that it is reachable only from a small part of the library.

  • Avoid using non-understood patterns in places like static constructors that will result in the warning propagating to all members of the class.
  • Avoid annotating virtual methods or interface methods, which will require all overrides to have matching annotations.
  • In some cases, you will be able to mechanically propagate warnings through your code without issues. Sometimes this will result in much of your public API being annotated with RequiresUnreferencedCode, which is the right thing to do if the library indeed behaves in ways that can't be understood statically by the trim analysis.
  • In other cases, you might discover that your code uses patterns that can't be expressed in terms of the DynamicallyAccessedMembers attributes, even if it only uses reflection to operate on statically known types. In these cases, you may need to reorganize some of your code to make it follow an analyzable pattern.
  • Sometimes the existing design of an API will render it mostly trim-incompatible, and you may need to find other ways to accomplish what it is doing. A common example is reflection-based serializers. In these cases, consider adopting other technology like source generators to produce code that is more easily statically analyzed.

Resolve warnings for non-analyzable patterns

It's better to resolve warnings by expressing the intent of your code using RequiresUnreferencedCode and DynamicallyAccessedMembers when possible. However, in some cases you may be interested in enabling trimming of a library that uses patterns that can't be expressed with those attributes, or without refactoring existing code. This section describes some advanced ways to resolve trim analysis warnings.

Warning

These techniques might break your code if used incorrectly.

When suppressing warnings, you are responsible for guaranteeing the trim compatibility of your code based on invariants that you know to be true by inspection. Be careful with these annotations, because if they are incorrect, or if invariants of your code change, they might end up hiding real issues.

UnconditionalSuppressMessage

If the intent of your code can't be expressed with the annotations, but you know that the warning doesn't represent a real issue at run time, you can suppress the warnings using UnconditionalSuppressMessageAttribute. This is similar to SuppressMessageAttribute, but it's persisted in IL and respected during trim analysis. For example:

class TypeCollection
{
    Type[] types;

    // Ensure that only types with ctors are stored in the array
    [DynamicallyAccessedMembers(DynamicallyAccessedMemberTypes.PublicParameterlessConstructor)]
    public Type this[int i]
    {
        // warning IL2063: TypeCollection.Item.get: Value returned from method 'TypeCollection.Item.get'
        // can not be statically determined and may not meet 'DynamicallyAccessedMembersAttribute' requirements.
        get => types[i];
        set => types[i] = value;
    }
}

class TypeCreator
{
    TypeCollection types;

    public void CreateType(int i)
    {
        types[i] = typeof(TypeWithConstructor);
        Activator.CreateInstance(types[i]); // No warning!
    }
}

class TypeWithConstructor
{
}

Here, the indexer property has been annotated so that the returned Type meets the requirements of CreateInstance. This already ensures that the TypeWithConstructor constructor is kept, and that the call to CreateInstance doesn't warn. Furthermore, the indexer setter annotation ensures that any types stored in the Type[] have a constructor. However, the analysis isn't able to see this, and still produces a warning for the getter, because it doesn't know that the returned type has its constructor preserved.

If you are sure that the requirements are met, you can silence this warning by adding UnconditionalSuppressMessage to the getter:

[DynamicallyAccessedMembers(DynamicallyAccessedMemberTypes.PublicParameterlessConstructor)]
public Type this[int i]
{
    [UnconditionalSuppressMessage("ReflectionAnalysis", "IL2063",
        Justification = "The list only contains types stored through the annotated setter.")]
    get => types[i];
    set => types[i] = value;
}