metalama

name

metalama

description

Complete Metalama documentation for aspect-oriented programming and compile-time code generation in C#, covering aspects, T# templates, fabrics, architecture validation, and the Metalama.Patterns libraries (caching, contracts, observability). Use whenever writing, reviewing, or debugging Metalama code; when a build reports LAMA diagnostics or errors point into transformed/generated code; when asked to eliminate repetitive C# (logging, INotifyPropertyChanged, validation, retry, equality, builders) in a project that references Metalama packages; when configuring Metalama MSBuild properties; when testing aspects; or when migrating from PostSharp.

Metalama Documentation Skill

This skill contains the complete Metalama documentation including conceptual guides, API reference, and sample code.

This skill pertains to Metalama 2026.1.

Before you write code (read this first)

Three habits prevent most mistakes when writing Metalama code. Do them in this order.

1. Look up the real API — don't guess signatures

The complete, authoritative Metalama API is on disk in this skill under api/*.yml, indexed by api/.manifest. Metalama's fluent advising and code-model APIs (meta.*, IAspectBuilder, IMethod, builder.Advice.*, etc.) are easy to get subtly wrong from memory. Before writing an aspect, look up the type or member (see API Reference below for details):

python scripts/find-api.py OverrideMethodAspect   # lists matching UIDs and prints their docs
python scripts/find-api.py IntroduceMethod

Every script has a PowerShell twin for environments without Python (works on Windows PowerShell 5.1 and pwsh):

powershell -File scripts/find-api.ps1 OverrideMethodAspect

As a last resort, grep the manifest and read the listed .yml file:

grep -i "OverrideMethodAspect" api/.manifest      # find the YML file

If the user's project references a different Metalama version than this skill, prefer the actual referenced version. Its XML docs ship in the NuGet cache (e.g. ~/.nuget/packages/metalama.framework/2026.1/lib/**/*.xml).

2. See the generated code — you have no IDE diff preview

Metalama transforms code at compile time. Unlike a developer using Visual Studio (CodeLens, Metalama Diff), an agent must read the generated C# from disk. This is your primary way to verify an aspect does what you intended.

Emit the transformed code with any of these — recipe 1 is the default:

# 1. Simplest: the built-in LamaDebug configuration emits formatted output
dotnet build -c LamaDebug

# 2. Any configuration, without switching config
dotnet build -p:MetalamaEmitCompilerTransformedFiles=True -p:MetalamaFormatOutput=True

# 3. Equivalent master switch (also formats)
dotnet build -p:MetalamaDebugTransformedCode=True

Then read the generated .cs files under:

obj/<Configuration>/<TargetFramework>/metalama/

For example obj/LamaDebug/net10.0/metalama/. These are the actual files the compiler produced from your source plus the aspects.

• MetalamaDebugTransformedCode=True (what LamaDebug sets) implies MetalamaEmitCompilerTransformedFiles=True and defaults MetalamaFormatOutput=True (readable, pretty-printed output).
• Expect warning LAMA0066 ("Formatting of generated code is enabled...") in these builds — it only announces that formatting slows the build down. It is benign; don't chase it.
• On Windows, if dotnet run fails in a deeply nested directory (the apphost path exceeds the path-length limit), run the built assembly directly: dotnet bin/<Configuration>/<tfm>/App.dll.
• Do not confuse this with MetalamaDebugCompiler — that property attaches a debugger to the compiler process and is unrelated to reading generated code.
• In this skill's code/ samples, Name.t.cs files are pre-baked expected output. In a real user project you generate fresh output using the commands above.

3. Build to verify — the compiler is your correctness oracle

Metalama reports eligibility problems, template errors, and transformation issues as compiler diagnostics (LAMAxxxx and CS codes). After writing or changing an aspect, run dotnet build and read the diagnostics; treat a clean build as the pass condition, exactly like a test suite.

A clean build proves the transformation applied; it does not prove behavior. When feasible, also run the program (or a quick test) to verify the woven code does what was asked — e.g. that a retry aspect actually retries.

For stronger verification, write an aspect test (Metalama.Testing.AspectTesting): an input .cs whose transformed output is snapshot-asserted. This is the ideal agent verification harness — deterministic and diffable, no IDE required. See content/conceptual/aspects/testing/ and content/conceptual/aspects/testing/debugging-aspects.md.

[!TIP] assets/scratch-project/ contains a minimal LamaDebug-ready project (csproj + Program.cs) you can copy to try an aspect outside the user's solution.

Directory Structure

Directory	Contents
content/conceptual/	Conceptual documentation (aspects, templates, fabrics, validation, etc.)
content/patterns/	Pattern libraries (contracts, caching, observability, memoization, DI)
content/api/	API documentation overview pages
code/	Sample code (.cs = source, .t.cs = transformed output, .Aspect.cs = aspect implementation)
api/	API reference YML files from DocFx (trimmed to the documentation content)
api/migration/	Legacy PostSharp API docs — consult only when migrating PostSharp code to Metalama
scripts/	Helper scripts: find-api.py/.ps1 (API lookup), find-doc.py/.ps1 (article search)
assets/scratch-project/	Minimal ready-to-build project for trying aspects in isolation

Finding Information

1. Search the article index:

   python scripts/find-doc.py caching invalidation
   # or, without Python:
   powershell -File scripts/find-doc.ps1 caching invalidation
   

   Last resort — grep index.yml with -B3 (each entry has: name, path, summary, keywords, so -B3 captures the path):

   grep -i -B3 "caching" index.yml
   

2. Browse by topic: Navigate the content/ directory structure.
3. API lookup:

   python scripts/find-api.py OverrideMethodAspect
   # or, without Python:
   powershell -File scripts/find-api.ps1 OverrideMethodAspect
   

   Last resort — grep api/.manifest, then read the listed YML file:

   grep -i "OverrideMethodAspect" api/.manifest
   

Key Entry Points

Topic	File	Description
Getting Started	content/conceptual/using/using-metalama.md	How to use Metalama in your projects
Creating Aspects	content/conceptual/aspects/aspects.md	Overview of aspect creation
Overriding Methods	content/conceptual/aspects/simple-aspects/overriding-methods.md	Basic method interception
Templates	content/conceptual/aspects/templates/templates.md	T# template syntax and patterns
Fabrics	content/conceptual/using/fabrics/fabrics.md	Bulk aspect application
Aspect Ordering	content/conceptual/aspects/multiple-instances/ordering.md	Ordering multiple aspects on the same declaration
Contracts	content/patterns/contracts/contract-patterns.md	Parameter/property validation
Caching	content/patterns/caching/caching.md	Method result caching
Observability	content/patterns/observability/observability.md	INotifyPropertyChanged implementation
Debugging Aspects	content/conceptual/aspects/testing/debugging-aspects.md	Debug compile-time code, breakpoints, meta.DebugBreak()
Debugging User Code	content/conceptual/using/debugging-aspect-oriented-code.md	Debug run-time transformed code, LamaDebug configuration

Common Patterns

Base Classes

Base Class	Target	Key Override	Use When
OverrideMethodAspect	Methods	OverrideMethod()	Wrap/intercept methods
OverrideFieldOrPropertyAspect	Fields/Properties	OverrideProperty	Wrap property access
ContractAspect	Parameters/Fields/Properties	Validate(dynamic? value)	Validate values
TypeAspect	Types	BuildAspect()	Introduce members, implement interfaces
MethodAspect	Methods	BuildAspect()	Programmatic method transformation
TypeFabric	Single type	AmendType()	Bulk changes to one type
ProjectFabric	Project	AmendProject()	Apply aspects across project

Template Fundamentals

[!IMPORTANT] T# templates look like C# but have different semantics. Code that works in normal C# may not work identically in a template. Always read the full template documentation at content/conceptual/aspects/templates/ before writing template code.

public override dynamic? OverrideMethod()
{
    // Pre-logic
    try
    {
        return meta.Proceed();  // Calls original method
    }
    finally
    {
        // Post-logic (always runs)
    }
}

• dynamic? handles any return type (void returns null)
• meta.Proceed() auto-transforms to await for async targets
• Use meta.Target.* to access compile-time information about the target declaration
• To debug templates, use meta.DebugBreak() (not Debugger.Break())

Debugging Quick Reference

Debugging compile-time code (aspects, fabrics, templates):

1. Add Debugger.Break() in BuildAspect/fabrics, or meta.DebugBreak() in templates
2. Build with: dotnet build -p:MetalamaDebugCompiler=True -p:MetalamaConcurrentBuildEnabled=False
3. Attach debugger when prompted, then set breakpoints in transformed code (obj/.../metalama/)

Debugging run-time code (transformed output):

1. Create a LamaDebug build configuration in Visual Studio
2. Use F11 to step into code, or add Debugger.Break()
3. Set breakpoints in transformed files under obj/<Config>/<TFM>/metalama/

Common Pitfalls

Mistake	Correct Approach
Using Debugger.Break() in templates	Use meta.DebugBreak() in templates; Debugger.Break() only works in BuildAspect and fabrics
Setting breakpoints in source files	Breakpoints don't work in Metalama-transformed projects; use Debugger.Break()/meta.DebugBreak() then set breakpoints in transformed code
Using nameof() for introduced members	Use string literals; nameof() resolves at aspect compile-time, not target compile-time
Filtering all types by namespace in fabrics	Use GlobalNamespace.GetDescendant("Ns") or a NamespaceFabric instead of SelectTypes().Where(t => t.Namespace...)
Forgetting partial on target classes	Classes receiving introduced members need the partial modifier
Assuming AspectOrder lists application order	Application order and run-time execution order are opposite. Prefer [assembly: AspectOrder(AspectOrderDirection.RunTime, ...)] and list aspects in run-time order, outermost first — see content/conceptual/aspects/multiple-instances/ordering.md

Sample Code Conventions

• Name.cs - Target code receiving the aspect
• Name.Aspect.cs - Aspect implementation
• Name.t.cs - Transformed output (what the compiler generates)
• Name.Dependency.cs - Referenced project code for multi-project examples

Metalama Markdown Directives

The documentation uses custom [!metalama-*] directives to include code samples. These are processed at build time to generate HTML, but in the skill files you see the raw directives.

Reading Directive References

When you see a directive in a Markdown file, extract the file path and read the referenced file directly.

Directive	Purpose	Example
[!metalama-file PATH]	Shows a single source file	[!metalama-file ~/code/Project/File.cs]
[!metalama-test PATH]	Shows test with input/output	[!metalama-test ~/code/Project/Test.cs]
[!metalama-compare PATH]	Shows side-by-side diff	[!metalama-compare ~/code/Project/File.cs]
[!metalama-vimeo ID]	Embeds Vimeo video	[!metalama-vimeo 842168905]

Path Resolution

• ~/ resolves to the SKILLS.md directory
• Example: ~/code/Metalama.Documentation.SampleCode.AspectFramework/GettingStarted/GettingStarted.cs → Read code/Metalama.Documentation.SampleCode.AspectFramework/GettingStarted/GettingStarted.cs

How to Read Referenced Code

When you encounter a directive like [!metalama-file ~/code/Project/File.cs]:

1. Read the main file: code/Project/File.cs
2. Check for related files (same name, different suffix):
   • File.Aspect.cs - Aspect implementation
   • File.t.cs - Transformed output
   • File.Fabric.cs - Fabric code

Markers: If you see marker="NAME", look for code between // [<snippet NAME>] and // [<endsnippet NAME>] in the file.

API Reference

The api/ directory contains DocFx-generated YML files for all public APIs. The files are trimmed to documentation content (items: with summary, syntax, parameters, returns); the DocFx references: rendering section is stripped at build time.

Finding API Documentation

Preferred: python scripts/find-api.py <name-or-uid> (or powershell -File scripts/find-api.ps1 <name-or-uid> without Python) — resolves the manifest and prints the documentation block for the matching UIDs directly, so you don't need to read the whole YML file.

Manual route — use api/.manifest, a JSON index mapping all UIDs (types, members, overloads) to their YML files:

1. Search the manifest for the type or member name:

   "Metalama.Framework.Aspects.OverrideMethodAspect": "Metalama.Framework.Aspects.OverrideMethodAspect.yml"
   "Metalama.Framework.Aspects.OverrideMethodAspect.OverrideMethod": "Metalama.Framework.Aspects.OverrideMethodAspect.yml"
   "Metalama.Framework.Code.IMethod": "Metalama.Framework.Code.IMethod.yml"
   

2. Read the corresponding YML file to get full documentation.

3. Naming conventions:

   • Types: Namespace.TypeName.yml
   • Generic types: backtick becomes dash, e.g., IAspectBuilder<T> → Metalama.Framework.Aspects.IAspectBuilder-1.yml
   • All members of a type are in the same YML file as the type

YML File Structure

Each type's YML file contains all members of that type in a single file:

items:
- uid: Namespace.TypeName           # Type definition
  commentId: T:Namespace.TypeName
  type: Class|Interface|Enum|...
  summary: Type description
  remarks: Detailed explanation
  syntax:
    content: public class TypeName : BaseClass
  children:                         # List of member UIDs
  - Namespace.TypeName.Method1
  - Namespace.TypeName.Property1

- uid: Namespace.TypeName.Method1   # Member definition
  commentId: M:Namespace.TypeName.Method1
  type: Method|Property|Field|...
  summary: Member description
  syntax:
    content: public void Method1()
  parameters: [...]                 # For methods
  return: { type: ..., description: ... }

Key Namespaces

Namespace	Purpose
Metalama.Framework.Aspects	Aspect base classes, attributes, meta API
Metalama.Framework.Code	Code model interfaces (IMethod, IType, IParameter, etc.)
Metalama.Framework.Advising	Advice APIs for introducing members, implementing interfaces
Metalama.Framework.Eligibility	Eligibility builders for aspect targeting
Metalama.Framework.Diagnostics	Reporting warnings and errors
Metalama.Framework.Fabrics	Fabric base classes
Metalama.Patterns.Contracts	Contract validation aspects
Metalama.Patterns.Caching	Caching aspects and configuration
Metalama.Patterns.Observability	INotifyPropertyChanged implementation
Metalama.Extensions.DependencyInjection	Dependency injection
Metalama.Extensions.Architecture	Architecture enforcement/validation
Flashtrace	Logging facade used by Metalama.Patterns.Caching and logging patterns

Namespaces to avoid in user code

Namespace	Why
Metalama.Framework.Engine	Internal compiler engine. Only relevant when building Metalama SDK extensions (custom weavers — see content/conceptual/sdk/). Never reference it from aspects or user code.
PostSharp.* (in api/migration/)	Documentation of the legacy PostSharp API, kept only so you can map PostSharp constructs to their Metalama equivalents when migrating (see content/conceptual/migration/). Never write new code against PostSharp APIs.

Linking to Documentation

When referencing documentation articles, provide links to the live documentation at https://doc.metalama.net.

URL format: https://doc.metalama.net/<path> where <path> is derived from the file path under content/:

• Remove content/ prefix
• Remove .md suffix
• For index files where the filename matches the parent folder (e.g., path/leaf/leaf.md), use just path/leaf

Examples:

File Path	URL
content/conceptual/aspects/aspects.md	https://doc.metalama.net/conceptual/aspects
content/conceptual/aspects/templates/templates.md	https://doc.metalama.net/conceptual/aspects/templates
content/patterns/caching/caching.md	https://doc.metalama.net/patterns/caching
content/conceptual/aspects/simple-aspects/overriding-methods.md	https://doc.metalama.net/conceptual/aspects/simple-aspects/overriding-methods

External Resources

• Live documentation: https://doc.metalama.net
• Source repository: https://github.com/metalama/Metalama
• Samples repository: https://github.com/metalama/Metalama.Samples