Microsoft Extensions Configuration

This handles configuration setup in .NET apps using Microsoft.Extensions.Configuration, with a strong focus on validation at startup instead of runtime failures. You get patterns for binding appsettings.json to strongly-typed classes, using Data Annotations for simple rules, and implementing IValidateOptions for complex cross-property checks. The core idea is fail fast: if your SMTP host is missing or your production database points to localhost, you want the app to crash on startup with a clear message, not 10 minutes into a user session. Also covers the differences between IOptions, IOptionsSnapshot, and IOptionsMonitor, plus how to inject dependencies into validators when you need environment-specific logic.