Trying to follow example provided on page but still getting "Trim analysis error IL2026"

[perlun]

Hi,

While preparing the migration to .NET 6 (RC2), I started seeing a bunch of IL2026 errors like these:

/home/per/git/perlang/src/Perlang.Interpreter/PerlangInterpreter.cs(120,34): Trim analysis error IL2026: Perlang.Interpreter.PerlangInterpreter.<>c.<RegisterGlobalClasses>b__16_0(Assembly): Using member 'System.Reflection.Assembly.GetTypes()' which has 'RequiresUnreferencedCodeAttribute' can break functionality when trimming application code. Types might be removed. [/home/per/git/perlang/src/Perlang.ConsoleApp/Perlang.ConsoleApp.csproj]
/home/per/git/perlang/src/Perlang.Interpreter/PerlangInterpreter.cs(1010,21): Trim analysis error IL2026: Perlang.Interpreter.PerlangInterpreter.VisitBinaryExpr(Expr.Binary): Using member 'Microsoft.CSharp.RuntimeBinder.Binder.BinaryOperation(CSharpBinderFlags,ExpressionType,Type,IEnumerable<CSharpArgumentInfo>)' which has 'RequiresUnreferencedCodeAttribute' can break functionality when trimming application code. Using dynamic types might cause types or members to be removed by trimmer. [/home/per/git/perlang/src/Perlang.ConsoleApp/Perlang.ConsoleApp.csproj]

I found https://github.com/dotnet/designs/blob/main/accepted/2020/linking-libraries.md#trimming-compatibility and the examples on this page, but I am unable to get them working in my code; the trimming error still persists. What could I be doing wrong? 🤔 Here's the WIP PR which can be used to reproduce the problem: perlang-org/perlang#223.

(To perform the linking & trimming in that project, run make release on a Linux machine with` .NET 6 RC2 installed. The CI logs are also available in that PR in case anyone's interested.)

        /// <summary>
        /// Registers global classes defined in native .NET code.
        /// </summary>
        /// <exception cref="PerlangInterpreterException">Multiple classes with the same name was encountered.</exception>
        [UnconditionalSuppressMessage("AssemblyLoadTrimming", "IL2026:RequiresUnreferencedCode", Justification = "Perlang.Stdlib is manually preserved")]
        private void RegisterGlobalClasses()
        {
            var globalClassesQueryable = AppDomain.CurrentDomain.GetAssemblies()
                .SelectMany(a => a.GetTypes())
                .Select(t => new
                {
                    Type = t,
                    ClassAttribute = t.GetCustomAttribute<GlobalClassAttribute>()
                })
                .Where(t => t.ClassAttribute != null);

            // ...
        }

---

Document Details

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• ID: 9c90ba1c-aa4b-8071-0ebc-6564054aa795
• Version Independent ID: 3e26a47b-b849-88c8-b473-db31a2d09f34
• Content: Introduction to trim warnings - .NET
• Content Source: docs/core/deploying/trimming/fixing-warnings.md
• Product: dotnet-fundamentals
• GitHub Login: @agocke
• Microsoft Alias: angocke

[perlun]

Here's a direct link to the CI failure, with more details: https://github.com/perlang-org/perlang/runs/4121928902?check_suite_focus=true

[agocke]

Thanks for the feedback! Unfortunately you've hit on a rather unfortunate part of the difference between analyzing C# code and analyzing IL -- in this case the warning you're getting is inside a lambda. The attribute you're adding is to the parent method. The C# compiler actually compiles the lambda down into a different method, so at the IL level, the attribute is no longer on the right method.

We're trying to work to make the linker smarter here, but it won't be ready for a while. In the meantime, you should be able to workaround this using local functions, e.g.

        /// <summary>
        /// Registers global classes defined in native .NET code.
        /// </summary>
        /// <exception cref="PerlangInterpreterException">Multiple classes with the same name was encountered.</exception>
        [UnconditionalSuppressMessage("AssemblyLoadTrimming", "IL2026:RequiresUnreferencedCode", Justification = "Perlang.Stdlib is manually preserved")]
        private void RegisterGlobalClasses()
        {
            var globalClassesQueryable = AppDomain.CurrentDomain.GetAssemblies()
                .SelectMany(GetTypes)
                .Select(t => new
                {
                    Type = t,
                    ClassAttribute = GetCustomAttribute(t)
                })
                .Where(t => t.ClassAttribute != null);

            [UnconditionalSuppressMessage("AssemblyLoadTrimming", "IL2026:RequiresUnreferencedCode", Justification = "Perlang.Stdlib is manually preserved")]
            IEnumerable<Type> GetTypes(Assembly a) => a.GetTypes();

            [UnconditionalSuppressMessage("AssemblyLoadTrimming", "IL2026:RequiresUnreferencedCode", Justification = "Perlang.Stdlib is manually preserved")]
            CustomAttribute GetCustomAttribute(Type t) => t.GetCustomAttribute<GlobalClassAttribute>();


            // ...
        }

[perlun]

@agocke Thanks for a very helpful & fast answer, greatly appreciated! 🙏 You also helped me learn about a new C# (7.0) feature; I almost feel ashamed to mention it but I wasn't even aware of the concept of local functions. 😊 (Justification = "My day-job is working as Java developer." 😁)

With the above in place, the warning is indeed silenced as intended. 👍

Would it be worth mentioning this on the docs page? I guess others could potentially run into this particular problem as well, given how prevalent lambdas are in e.g. LINQ-query-operator-based code.

[agocke]

Yup, we should definitely have a docs section about this. It's not an obvious limitation

[jtschuster]

The linker now propagates UnconditionalSuppressMessage and RequiresUnreferencedCode to local and lambda functions. dotnet/linker#2689