Reflection, Attributes, and Metadata
Inspect .NET metadata deliberately, define attributes with narrow semantics, cache reflection work, and avoid turning compile-time contracts into fragile string conventions.
Before this lesson
Read type and member metadata
Design a focused custom attribute
Control reflection cost and trimming risk
The short answer
Reflection reads type and member metadata at runtime; attributes attach declarative metadata to those elements. Use them at infrastructure boundaries where types are not known statically, validate discovered shapes early, and cache stable lookup results.
Build the runtime mental model
Assemblies contain metadata describing types, members, signatures, attributes, and references. Type and MemberInfo APIs expose it. Invoking members reflectively trades compile-time checking for runtime discovery, so the boundary needs explicit validation and useful diagnostics.
Advanced C# work improves when you separate language syntax, runtime behavior, and application policy. Write down which layer owns the guarantee in this lesson. Then identify the observable evidence—a compiler rejection, test result, generated query, trace, or measurement—that would prove the model correct.
Design the boundary deliberately
An attribute should describe metadata, not perform work. A serializer, validator, test runner, or plugin loader interprets it. Decide whether the attribute is inherited, repeated, and valid on classes, methods, or properties through AttributeUsage.
The starter isolates one part of the mental model so it can run in the browser. The exercise moves the same rule into a current local .NET project where packages, framework hosting, diagnostics, and multi-file tests are available.
using System;
[AttributeUsage(AttributeTargets.Class)]
class FeatureAttribute : Attribute
{
public string Name { get; private set; }
public FeatureAttribute(string name) { Name = name; }
}
[Feature("reports")]
class ReportFeature { }
class Program
{
static void Main()
{
object[] attributes = typeof(ReportFeature).GetCustomAttributes(false);
Console.WriteLine(attributes.Length);
}
}Expected output
1
Diagnose failure and misuse
Repeated scanning and invocation can allocate and be slower than direct calls, though measurement matters. Private-member access breaks encapsulation. Reflection by string is vulnerable to renames, and trimming or ahead-of-time compilation may remove undiscovered members.
Classify each failure as a contract violation, transient operational failure, permanent dependency response, concurrency conflict, or programmer defect. That classification determines whether to reject, retry, compensate, cancel, or fail fast. A generic catch-and-continue policy destroys the information needed to make that decision.
| Question | Evidence to inspect | Decision |
|---|---|---|
| Is the input valid? | Validation result and boundary examples | Reject with a stable contract |
| Is the failure transient? | Typed status, exception, and policy context | Retry only when bounded and safe |
| Is state still consistent? | Invariant and transaction outcome | Commit, compensate, or abort |
| Is performance acceptable? | Representative latency and allocation data | Keep simple or optimize one cause |
Apply the concept in production
Scan once at startup when possible, reject duplicate registrations, cache compiled accessors, and document deployment requirements. Prefer source generation or explicit registration where performance, trimming, or auditability dominates dynamic discovery.
Finish by making the result operable. Add structured diagnostics at the boundary, propagate cancellation, avoid sensitive data, and record SDK and dependency versions. Test the public behavior instead of private implementation details. If a framework or provider performs translation, serialization, concurrency, or I/O, include at least one test against the real production technology.
A senior-level review should be able to answer four questions: what contract is promised, who owns lifetime and cleanup, how failures become visible, and what evidence supports the design. If any answer depends on “the framework probably handles it,” inspect the documentation or runtime behavior and turn the assumption into a checked decision.
Quick knowledge check
Answer before you reveal.
01Why should reflection discovery happen near startup?
It centralizes validation and caching so shape errors fail early instead of appearing on a rarely used request path.
02What must happen before adding complexity to this design?
State the requirement, preserve a correct baseline, collect evidence, and explain how the proposed mechanism improves a specific quality.
Exercise
Practice challenge
Create a command attribute and registry that discovers command handlers once, rejects duplicate names, and reports invalid signatures clearly.
Requirements
- The implementation states its contract and ownership boundary explicitly
- Automated checks cover the successful path and at least two meaningful failures
- Diagnostics expose failure context without secrets or swallowed exceptions
- The project documents required SDK, packages, setup, run, and test commands
Optional extension: Measure or load-test the critical path and record whether the evidence justifies another optimization or abstraction.
Open in C# compilerLesson checkpoint
One small step locks it in
Mark this lesson complete, then keep the momentum going.
Clear up the details
Frequently asked questions
When should I use reflection, attributes, and metadata?
Use it when its explicit tradeoff solves a measured requirement or clarifies an owned boundary. Keep the simpler design when the additional mechanism does not improve correctness, operability, or changeability.
Does the browser compiler cover the complete production setup?
No. It runs the focused starter program. Framework, package, database, benchmark, and multi-project work requires a current local .NET SDK and the project commands described in the exercise.
What evidence should I keep after the exercise?
Keep the acceptance cases, automated tests, diagnostic or benchmark output where relevant, and a short decision note describing the chosen boundary and rejected alternative.