Dependency Injection, Configuration, and Logging

The built-in container constructs an object graph from registrations. Transient creates per resolution, scoped shares within a scope such as a request, and singleton lives for the application. A longer-lived service must not capture a shorter-lived dependency.

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.

Use options classes for related configuration and validate required values, ranges, and relationships during startup. Logging templates preserve named fields for filtering and aggregation. Correlation identifiers connect events across a request.

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;

interface IClock { DateTime UtcNow { get; } }
class SystemClock : IClock { public DateTime UtcNow { get { return DateTime.UtcNow; } } }

class ReportService
{
  private readonly IClock clock;
  public ReportService(IClock clock) { this.clock = clock; }
  public int CurrentYear() { return clock.UtcNow.Year; }
}

class Program
{
  static void Main() { Console.WriteLine(new ReportService(new SystemClock()).CurrentYear() > 2020); }
}

True

Injecting IServiceProvider and resolving arbitrary services hides dependencies. Singleton mutable state creates contention and cross-request leaks. Logging tokens, connection strings, personal data, or entire request bodies creates security and compliance risk.

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.

Keep the composition root near startup, fail fast on invalid configuration, document environment overrides, and test registrations by building the provider and resolving important roots. Define log levels and redaction rules.

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.