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The .NET Options pattern turns related configuration values into typed classes that you bind and register with dependency injection. Choose IOptions<T> for fixed settings, IOptionsSnapshot<T> for a scope-specific view, or IOptionsMonitor<T> when code needs current values or change notifications. Validate settings at startup when invalid configuration should prevent the application from running.
What the .NET Options pattern does
Microsoft describes the pattern as using classes to provide strongly typed access to groups of related settings. Instead of reading individual configuration keys throughout an application, define a class for a coherent settings group, bind a configuration section to it, register it with dependency injection, and inject an options interface where the values are needed.
Keep each options class focused on the scenario it serves. This supports encapsulation and separation of concerns: a service can depend on the settings it uses without taking responsibility for unrelated configuration.
Bind a configuration section and register it
A configuration section name does not have to match the options class name. The registration connects the section to the type; nameof is simply a convenient way to avoid repeating a string when the names do match.
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using Microsoft.Extensions.Options;
var builder = WebApplication.CreateBuilder(args);
builder.Services
.AddOptions<TransientFaultHandlingOptions>()
.Bind(builder.Configuration.GetSection("FaultHandling"));
var app = builder.Build();Here, the FaultHandling section is bound to TransientFaultHandlingOptions, even though the names differ. A common alternative is builder.Services.Configure<TransientFaultHandlingOptions>(builder.Configuration.GetSection("FaultHandling")). The options type can then be injected as IOptions<TransientFaultHandlingOptions>, IOptionsSnapshot<TransientFaultHandlingOptions>, or IOptionsMonitor<TransientFaultHandlingOptions>, depending on the consumer’s lifetime and update requirements.
For most applications, start with section binding and the options builder. More customized pipelines can use IOptionsFactory<TOptions>, which creates instances by applying registered configuration and post-configuration, or IOptionsMonitorCache<TOptions>, which stores monitor instances and supports removing or clearing cached instances so they can be recomputed.
Choose the options interface for the consumer
No interface is universally best. Decide whether the consumer needs a stable value, a consistent view within a scope, or access to changes after startup. Also consider whether it needs multiple named configurations and whether it must respond to change notifications.
| Interface | Lifetime and scope | Updates and named options | Best fit |
|---|---|---|---|
IOptions<T> |
Singleton; may be injected into services of any lifetime | Does not provide updated configuration after startup; does not support named options | Simple settings that remain fixed for the application |
IOptionsSnapshot<T> |
Scoped; unsuitable for injection into a singleton | Supports named options. Values are computed on access and cached for the scope | Scoped or transient consumers that need a scope-specific view |
IOptionsMonitor<T> |
Singleton; may be injected into services of any lifetime | Supports named options, current values, change notifications, and cache invalidation | Singleton consumers or code that needs to retrieve current values or react to changes |
Use IOptions<T> for fixed settings
Inject IOptions<T> when the application can use the configured value established at startup and does not need named instances. It is the simplest accessor, but it is not a mechanism for observing later configuration changes.
Use IOptionsSnapshot<T> for a scoped view
A snapshot is scoped and caches the options instance for that scope. It suits scoped or transient services that should use a coherent view during a request or other dependency-injection scope. Because a singleton outlives a scope, it cannot consume this scoped service directly.
Use IOptionsMonitor<T> for current values or notifications
A monitor is a singleton accessor that supports named options, exposes current values, and lets code register change notifications. Use it when a long-lived consumer needs to obtain updated settings or perform work when they change. A monitor’s ability to observe a change still depends on the configuration source and deployment environment actually reporting that change.
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Named options represent multiple configurations of one type
When one options type represents several configurations, register named options and retrieve the desired instance by name through an interface that supports them: IOptionsSnapshot<T> or IOptionsMonitor<T>. IOptions<T> does not support named options. Use the named configuration APIs for registration, and make the selected name explicit where the consumer retrieves its settings.
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Understand when configuration reload works
Reload is not guaranteed for every provider or filesystem. Microsoft documents change tracking for file-based providers including JSON, INI, XML, Key per File, and User Secrets. Whether a monitor sees a change depends on the provider and the environment’s ability to report it.
Some Docker and network filesystem setups may not reliably send file-change notifications. Microsoft documents a polling alternative for those environments: set DOTNET_USE_POLLING_FILE_WATCHER; the documented polling interval is four seconds. That interval applies to the documented polling approach, not to all configuration reloads.
Validate options before relying on them
Options validation can use data annotations, custom validators, or class-level IValidatableObject validation. For example, data annotations can express constraints on individual properties, while a custom validator or IValidatableObject can check rules involving multiple values.
Use ValidateOnStart when the application should check settings as the host starts rather than waiting until an options value is first requested. This makes invalid configuration a startup failure instead of deferring discovery until a particular service accesses the settings.
Know the synchronous validation limit
The .NET 10 ASP.NET Core options documentation says standard options value access, snapshots, and reloads observed through IOptionsMonitor<T> remain synchronous; these paths do not invoke asynchronous validators. The .NET 10 IOptionsMonitor<TOptions> API reference likewise states that the default monitor recreates and validates options synchronously after change notifications and does not call ValidateAsync.
As a result, an asynchronous validator can cause a reload to fail and prevent change listeners from being called. Do not rely on the monitor to provide asynchronous validation or an asynchronous last-known-good fallback.
Quick Recap
References
- Microsoft Learn: Options pattern in .NET
- Microsoft Learn: Options pattern in ASP.NET Core (.NET 10)
- Microsoft Learn: IOptionsMonitor<TOptions> API reference (.NET 10)
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