The Options Pattern

An `appsettings.json` section stores `MaxRetries: 3` as text, but a C# class wants it as a real `int`. One line pours those keys into typed properties, and how you read them back has three answers.

▸Bound values arrive typed, but a property with no setter, or a field, silently keeps its default and never throws.
▸The wrapper you inject, not the class, sets freshness: IOptions freezes at startup, IOptionsSnapshot rebinds per request, IOptionsMonitor reads live.
▸Injecting the scoped IOptionsSnapshot into a singleton is illegal; IOptionsMonitor, itself a singleton, is the singleton-safe live reader.

BUILDS ON

#18 The Configuration System

Bind a Section onto a Class

Reading config["RetryPolicy:MaxRetries"] gives you a string every time, so every consumer parses "3" into an int by hand and hopes the key is spelled right. The options pattern replaces that with a plain class:

public sealed class RetryPolicy
{
    public int MaxRetries { get; set; }
    public TimeSpan Delay { get; set; }
}

One registration line binds a named section onto it:

builder.Services.Configure<RetryPolicy>(
    builder.Configuration.GetSection("RetryPolicy"));

Configure<T> matches each key under the "RetryPolicy" section to a property of the same name and assigns it, converting the stored string to the property's type. Against this JSON:

{ "RetryPolicy": { "MaxRetries": 3, "Delay": "00:00:05" } }

MaxRetries arrives as an int and Delay as a TimeSpan, parsed once at binding, not at every read. The section name is just a string argument: GetSection("Retry") would bind a section named Retry to the same class, so the class name and the section name are independent.

STRING KEYS LAND AS TYPED PROPERTIES

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