microsoft-extensions-configuration

1# Microsoft.Extensions Configuration Patterns
2 
3## When to Use This Skill
4 
5Use this skill when:
6- Binding configuration from appsettings.json to strongly-typed classes
7- Validating configuration at application startup (fail fast)
8- Implementing complex validation logic for settings
9- Designing configuration classes that are testable and maintainable
10- Understanding IOptions<T>, IOptionsSnapshot<T>, and IOptionsMonitor<T>
11 
12## Reference Files
13 
14- [advanced-patterns.md](advanced-patterns.md): Validators with dependencies, named options, complete production example (AkkaSettings), and testing validators
15 
16## Why Configuration Validation Matters
17 
18**The Problem:** Applications often fail at runtime due to misconfiguration - missing connection strings, invalid URLs, out-of-range values. These failures happen deep in business logic, far from where configuration is loaded.
19 
20**The Solution:** Validate configuration at startup. If invalid, fail immediately with a clear error message.
21 
22```csharp
23// BAD: Fails at runtime when someone tries to use the service
24public class EmailService
25{
26    public EmailService(IOptions<SmtpSettings> options)
27    {
28        var settings = options.Value;
29        // Throws NullReferenceException 10 minutes into production
30        _client = new SmtpClient(settings.Host, settings.Port);
31    }
32}
33 
34// GOOD: Fails at startup with clear error
35// "SmtpSettings validation failed: Host is required"
36```
37 
38---
39 
40## Pattern 1: Basic Options Binding
41 
42### Define a Settings Class
43…