Deploying new features directly to production can be risky. Even well-tested code may behave differently under real-world workloads, and rolling back an entire deployment because of a single problematic feature can be costly.
Feature flags (also known as feature toggles) allow developers to deploy code without immediately exposing new functionality to users. Features can be enabled or disabled at runtime, making it possible to perform gradual rollouts, A/B testing, and rapid rollbacks without redeploying the application.
ASP.NET Core provides first-class support for feature management through the Microsoft Feature Management libraries, making feature flags easy to integrate into modern .NET applications.
In this article, you'll learn how feature flags work, how to implement them in ASP.NET Core, and the best practices for managing feature lifecycles.
What Are Feature Flags?
A feature flag is a configuration value that determines whether a particular feature is enabled.
Instead of deploying code and immediately making it available:
Deploy
│
Users
The deployment flow becomes:
Deploy
│
Feature Flag
│
Users
This separates deployment from feature release, giving teams more control over production changes.
Why Use Feature Flags?
Feature flags provide several advantages for modern development teams.
Common use cases include:
Instead of waiting for a large release, teams can continuously deploy code while deciding when users gain access to new functionality.
Installing Feature Management
Install the required package:
dotnet add package Microsoft.FeatureManagement.AspNetCore
Register feature management:
builder.Services.AddFeatureManagement();
Configure a feature in appsettings.json:
{
"FeatureManagement": {
"NewCheckout": true
}
}
The feature is now available throughout the application.
Using Feature Flags in Controllers
Inject the feature manager:
using Microsoft.FeatureManagement;
public class CheckoutController : ControllerBase
{
private readonly IFeatureManager _featureManager;
public CheckoutController(
IFeatureManager featureManager)
{
_featureManager = featureManager;
}
[HttpGet]
public async Task<IActionResult> Checkout()
{
if (await _featureManager.IsEnabledAsync("NewCheckout"))
{
return Ok("New checkout experience");
}
return Ok("Classic checkout");
}
}
Changing the configuration controls which implementation is used without modifying application code.
Feature Gates
Feature gates protect entire controllers or endpoints.
Example:
using Microsoft.FeatureManagement.Mvc;
[FeatureGate("NewCheckout")]
[ApiController]
[Route("checkout")]
public class CheckoutController : ControllerBase
{
}
If the feature is disabled, requests to the controller automatically receive an appropriate response.
This keeps feature checks out of controller logic and improves maintainability.
Gradual Rollouts
One of the biggest advantages of feature flags is progressive deployment.
Instead of enabling a feature for everyone:
100% Users
Use staged rollouts:
10%
│
25%
│
50%
│
100%
This allows teams to monitor production behavior before expanding availability.
If issues arise, the feature can be disabled immediately without rolling back the deployment.
Azure App Configuration Integration
For cloud-based applications, Azure App Configuration centralizes feature management.
Benefits include:
Runtime configuration changes
Centralized feature management
Environment-specific settings
Integration with Azure services
No application restart required for configuration updates
This approach is particularly useful when multiple services share the same feature flags.
Feature Flags for A/B Testing
Feature flags can also support experimentation.
For example:
Users are assigned to different feature variants, allowing teams to measure:
Conversion rates
User engagement
Performance
Error rates
Unlike traditional deployments, experiments can be enabled or disabled without redeploying the application.
Managing Feature Lifecycles
Feature flags should not remain in the codebase indefinitely.
A typical lifecycle looks like this:
Development
│
Testing
│
Gradual Rollout
│
Full Release
│
Remove Feature Flag
Once a feature is fully adopted, remove the associated flag and obsolete code paths.
Leaving unused feature flags in place increases complexity and technical debt.
Feature Flag Strategies Compared
| Strategy | Best Use Case | Benefits |
|---|
| Boolean flag | Enable or disable a feature | Simple implementation |
| Percentage rollout | Gradual deployment | Reduces release risk |
| User targeting | Beta programs | Enables controlled access |
| Environment-based | Different deployment stages | Simplifies testing |
| Time-based activation | Scheduled releases | Automates feature availability |
Choosing the appropriate strategy depends on the business and deployment requirements.
Best Practices
Keep feature flags focused on a single capability.
Use descriptive names that clearly indicate their purpose.
Remove feature flags after successful rollout.
Monitor feature performance during staged deployments.
Store production feature configuration outside application code.
Test both enabled and disabled scenarios.
Document feature ownership and planned removal dates.
Common Mistakes
Keeping Feature Flags Forever
Feature flags are intended to support temporary rollout and experimentation. Leaving obsolete flags in the codebase increases maintenance costs and makes application logic harder to understand.
Combining Multiple Features Under One Flag
Each flag should control a single feature. Bundling unrelated functionality behind one toggle reduces flexibility and complicates deployments.
Ignoring the Disabled Path
Developers often focus only on the enabled experience. Ensure both enabled and disabled code paths are tested to prevent unexpected production issues.
Using Feature Flags for Configuration
Feature flags determine whether functionality is available—they are not a replacement for application configuration. Keep operational settings and feature toggles separate.
Conclusion
Feature flags are a powerful technique for reducing deployment risk in modern .NET applications. By separating deployment from feature release, they enable gradual rollouts, safer experimentation, and rapid rollback without requiring a new deployment.
ASP.NET Core's feature management libraries, combined with services such as Azure App Configuration, make it straightforward to implement feature flags that scale across environments and services. Whether you're introducing a new user experience, testing experimental functionality, or performing canary releases, feature flags provide the flexibility needed for continuous delivery.
Like any architectural tool, feature flags require discipline. Regularly remove obsolete flags, keep them focused on individual features, and monitor production behavior during rollouts. When managed effectively, feature flags become an essential part of building reliable, scalable, and continuously deployable .NET applications.