To use feature flags in ASP.NET Core, install Microsoft.FeatureManagement.AspNetCore, register feature management, define named flags in configuration, and evaluate them with a feature gate or IFeatureManager. You can start with appsettings.json; Azure App Configuration is optional when you need centralized flag management and provider-based refresh.
How feature flags work in ASP.NET Core
A feature flag is a named configuration setting that controls whether an application feature is available. Feature management is built on .NET’s IConfiguration, so flags can come from appsettings.json or another configuration provider. Microsoft describes the practice as decoupling feature release from code deployment, allowing availability to be changed without changing the application code.
A basic flag is either enabled or disabled. Optional feature filters make its state conditional—for example, on a percentage rollout, a time window, or a user/group targeting rule. You do not need Azure to use flags; Azure App Configuration is one option for centrally managing them.
Add a basic feature flag
- Install the ASP.NET Core integration. Add the
Microsoft.FeatureManagement.AspNetCorepackage to the project. Microsoft documents the package and integration points in its ASP.NET Core feature-flag quickstart. - Register feature management. In the application’s service-registration setup, add
services.AddFeatureManagement();(or the equivalent service registration for the hosting style used by the app). This makes the feature-management services available through dependency injection. - Define the flag in configuration. Microsoft’s documented JSON format uses a
feature_managementsection with afeature_flagsarray. For example:{ "feature_management": { "feature_flags": [ { "id": "Beta", "enabled": true } ] } }A flag enabled without conditions is on. Keep the identifier, such as
Beta, consistent wherever the flag is referenced. - Choose where to enforce it. Use a feature gate to protect a web surface, or inject
IFeatureManagerand check the flag in application logic when only part of a workflow should change. - Register any configured filters. If a flag names a feature filter, ensure the corresponding filter is registered and its alias matches the configuration. Evaluation throws if a configured filter is missing, unless the application opts to ignore missing filters.
Microsoft’s .NET feature management reference documents the JSON schema and evaluation behavior. Configuration-provider conventions can determine how configuration is loaded and overridden; verify the effective flag state in the environment where the app runs.
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Choose a feature gate or an in-code check
Gate an entire page, route, or action
ASP.NET Core integration supports feature gates for MVC controller actions, Razor Pages, views, routes, and middleware. A gate prevents the protected endpoint or page from running while its flag is off. Microsoft’s Razor Pages quickstart, for example, decorates a page with [FeatureGate("Beta")].
Use a gate when the whole surface should be unavailable unless the flag is on. Consider what users should see when access is blocked, and ensure navigation or links do not imply that the gated feature is available.
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Branch within application logic
Inject IFeatureManager and evaluate the named flag when a smaller section of a handler, service, or view model needs conditional behavior. This keeps the route available while allowing only the relevant operation or presentation to vary. Prefer one clearly scoped check over scattering unrelated flag checks throughout the code.
Use filters for staged activation
Feature filters determine when a flag is on. Microsoft documents percentage, time-window, and targeting filters. They let a team make availability conditional rather than treating every flag as a permanent all-or-nothing switch.
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- Percentage: expose a feature to a configured share of users for a staged rollout.
- Time window: activate a feature during a configured schedule.
- Targeting: select users or groups, such as internal testers or a named audience.
When a flag has multiple filters, the requirement controls how results combine: Any means at least one filter must return true; All means every filter must return true. Microsoft’s documented default requirement when conditions are present is Any. That filter-combination rule is distinct from the percentage or audience settings inside a targeting rule.
Target users and groups correctly
Targeting needs identity and group information at evaluation time. The ASP.NET Core integration provides a default ITargetingContextAccessor that reads targeting details from HttpContext.User, including the identity name and role claims. If your application represents audiences differently, implement a custom accessor and register it so targeting evaluates the context your rules expect.
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Check that authenticated identities have stable identifiers suitable for targeting; a missing or changing identity can make audience assignment unreliable. In a targeting filter, exclusions take precedence over the rest of the targeting configuration. See Microsoft’s targeting filter reference for the documented behavior.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Use Azure App Configuration when flags need central management
Keep flags in local or deployment-provided configuration when that operating model is sufficient. Choose Azure App Configuration when a team needs a centralized place to manage flags and change availability without changing and redeploying the application.
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For the ASP.NET Core targeting walkthrough, Microsoft uses Microsoft.Azure.AppConfiguration.AspNetCore alongside Microsoft.FeatureManagement.AspNetCore, connects to the App Configuration store, calls UseFeatureFlags(), registers feature management, and adds UseAzureAppConfiguration() middleware to enable refresh. The walkthrough recommends Microsoft Entra ID authentication with DefaultAzureCredential; it also demonstrates a connection-string option. Keep credentials out of source code and use an authentication approach suitable for the deployment environment. Follow the ASP.NET Core walkthrough for the setup details.
The .NET configuration provider requires explicitly loading flags with UseFeatureFlags(). Its documented default refresh interval is 30 seconds, configurable with SetRefreshInterval; selection can also be scoped by key, label, and tag. This is provider refresh behavior, not a promise that every running app observes a change immediately. Review Microsoft’s provider refresh documentation when setting expectations for propagation.
When to use variants and telemetry
A binary flag is appropriate when the choice is simply whether a feature is enabled. Use variants when different segments should receive different feature configurations; a variant can hold a string, number, Boolean, or configuration object. Feature-management telemetry can emit evaluation information for analysis.
Variants and telemetry do not establish that one version performs better. A useful experiment still needs a defined hypothesis, relevant outcome measures, and enough observations to support a conclusion. Microsoft’s feature management overview describes feature flags, variants, and telemetry.
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Practical operating checks
- Use stable, descriptive flag identifiers and keep references aligned across configuration and code.
- Check that all filter aliases referenced by configuration have registered implementations.
- Decide which application boundary should be gated, and separately consider whether the interface communicates the feature’s availability clearly.
- For targeting, verify which identity and group claims the accessor supplies and whether they match the targeting rules.
- For remote configuration, test the refresh path and plan for its configured interval rather than assuming an instant update.
- Remove or retire temporary rollout flags when they are no longer needed, so old branches do not become permanent complexity.
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