FeatureFlags.app

Deploy anytime. Release when you're ready. FeatureFlags.app gives .NET developers cloud-agnostic feature management with no user tracking, no vendor lock-in, and seamless Microsoft integration. Built for Modern .NET Applications, it's the perfect solution for development teams seeking a simple, no-frills feature flag management system. FeatureFlags.Client is the client library that integrates your application with FeatureFlags.app.

Get started at https://featureflags.app, or if you want details first and vibes later, keep reading.

Contents

• What This Library Does
• Package And Runtime
• Quick Start
• Using Flags In Code
• Cache Behavior
• Filter Support
• Failure Semantics
• Common Issues And Fixes
• Local Validation
• Related References
• Support And Issue Tracking

What This Library Does

• Registers feature management services in ASP.NET Core via a single AddFeatureFlags() call.
• Fetches feature definitions from our API using an API key header (x-api-key).
• Exposes IFeatureManager/IFeatureManagerSnapshot usage patterns you already know from Microsoft.FeatureManagement.
• Includes a deterministic percentage filter (Acmi.FeatureFlags.ConsistentPercentage) and targeting support.

Package And Runtime

• Package: Acmi.FeatureFlags.Client
• Namespace: Acmi.FeatureFlags.Client
• Target framework: net10.0
• Core dependency: Microsoft.FeatureManagement.AspNetCore

Quick Start

1. Install package

dotnet add package Acmi.FeatureFlags.Client

2. Register services in Program.cs

using Acmi.FeatureFlags.Client;

var builder = WebApplication.CreateBuilder(args);

builder.Services.AddControllersWithViews();
builder.AddFeatureFlags();

var app = builder.Build();
// ... middleware and endpoints ...
await app.RunAsync();

3. Configure endpoint + API key

{
  "FeatureFlags": {
	"ApiBaseEndpoint": "https://featureflags.app/api/",
	"ApiKey": "[from secrets]",
	"CacheExpirationInMinutes": 15
  }
}

Required keys:

• FeatureFlags:ApiBaseEndpoint
• FeatureFlags:ApiKey

Optional key:

• FeatureFlags:CacheExpirationInMinutes (default: 15)

Using Flags In Code

Use IFeatureManagerSnapshot exactly like standard Microsoft feature management.

using Microsoft.FeatureManagement;

public class ProductController : Controller {
	private readonly IFeatureManagerSnapshot _featureManager;

	public ProductController(IFeatureManagerSnapshot featureManager) {
		_featureManager = featureManager;
	}

	public async Task<IActionResult> Index() {
		if (await _featureManager.IsEnabledAsync("NewProductPage")) {
			return View("NewProductPage");
		}

		return View("ProductPage");
	}
}

Also works with the normal ASP.NET Core feature management integrations:

• FeatureGate attributes
• Razor feature tag helpers
• Filter-based evaluations from feature definitions

Cache Behavior

HttpFeatureFlagClient caches all retrieved definitions in memory under a single cache entry.

• First request fetches from remote API.
• Subsequent requests read from cache until expiration.
• Expiration defaults to 15 minutes.
• You can evict cache manually via IFeatureFlagClient.ClearCache().

Example:

public class AdminController : Controller {
	private readonly IFeatureFlagClient _featureFlagClient;

	public AdminController(IFeatureFlagClient featureFlagClient) {
		_featureFlagClient = featureFlagClient;
	}

	[HttpPost]
	public IActionResult RefreshFlags() {
		_featureFlagClient.ClearCache();
		return Ok(new { message = "Feature flag cache cleared." });
	}
}

Filter Support

Service registration wires up:

• AddScopedFeatureManagement()
• .AddFeatureFilter<ConsistentPercentageFilter>()
• .WithTargeting()

Consistent percentage filter

Alias: FeatureFlags.ConsistentPercentage

Behavior:

• If user identity name exists, result is deterministic per user.
• If user identity name is missing, it falls back to random evaluation.
• If configured percentage value is invalid (< 0), evaluation returns false.

Use this filter when you want stable rollout behavior for authenticated users instead of request-by-request randomness.

Failure Semantics

When remote API calls fail:

• Client logs an error.
• GetAllFeatureDefinitionsAsync() returns an empty list.
• GetFeatureDefinitionByNameAsync() returns null.

In practice this means feature checks degrade to "off" unless your app defines alternate behavior. This is generally safer than throwing exceptions into request pipelines and setting your pager on fire.

Common Issues And Fixes

1. AddFeatureFlags() throws at startup

Symptoms:

• FeatureFlags:ApiBaseEndpoint is not configured.
• FeatureFlags:ApiKey is not configured.

Fix:

• Add both required keys to configuration.
• Confirm environment-specific config is loaded (appsettings.{Environment}.json, user secrets, env vars).

2. Flags always evaluate to false

Possible causes:

• API key invalid or missing permissions.
• API unavailable (client degrades to empty definitions).
• Flag name mismatch ("NewDashboard" vs "NewDashbaord", yes this typo happens a lot).

Fix:

• Verify API key and endpoint.
• Check app logs for "Error fetching feature definitions".
• Centralize flag names in constants to avoid string-literal drift.

3. Rollout percentages look random per request

Cause:

• User identity name is missing, so consistent percentage filter uses random fallback.

Fix:

• Ensure authenticated identity with stable User.Identity.Name.
• If anonymous traffic dominates, choose filter strategy accordingly.

4. Flag updates not visible immediately

Cause:

• Cached definitions not yet expired.

Fix:

• Lower CacheExpirationInMinutes for development.
• Call IFeatureFlagClient.ClearCache() after admin updates when immediate refresh is required.

Local Validation

This repository includes:

• src/FeatureFlags.Client (library)
• src/FeatureFlags.Client.Tests (unit tests)
• src/FeatureFlags.Demo (demo MVC app)

Run tests:

dotnet test src/FeatureFlags.Client.Tests/FeatureFlags.Client.Tests.csproj

Run demo:

dotnet run --project src/FeatureFlags.Demo/FeatureFlags.Demo.csproj

Related References

• Hosted app and docs entry point: https://featureflags.app
• Microsoft feature management overview: https://learn.microsoft.com/azure/azure-app-configuration/feature-management-overview
• ASP.NET Core quickstart concepts: https://learn.microsoft.com/azure/azure-app-configuration/quickstart-feature-flag-aspnet-core

Support And Issue Tracking

If something breaks or behaves strangely:

• Browse/search issues: https://github.com/avlcodemonkey-industries/FeatureFlags.Client/issues
• Open a new issue: https://github.com/avlcodemonkey-industries/FeatureFlags.Client/issues/new

Include:

• .NET version
• Package version
• Sanitized FeatureFlags configuration
• Relevant logs/exceptions
• Repro steps

That gives maintainers a chance to help quickly instead of reenacting a detective novel.