C# Feature Flags

Overview

Feature flags (aka feature toggles) are a technique for enabling or disabling functionality in a system without deploying new code. They decouple feature release from code deployment by wrapping new or experimental logic in conditional checks.

"Feature flags allow us to ship code to production early and safely control its visibility to users, teams, or environments through configuration."

They’re widely used in continuous delivery, A/B testing, canary releases, beta features, and gradual rollouts.

Quick Description

"Feature flags allow us to deploy code continuously while controlling feature exposure via config. We wrap features in conditional logic so we can enable or disable them without modifying or redeploying the application. It supports safer releases, experimentation, and rollback strategies — and in .NET, it integrates easily via config files or flag management libraries."

Example: Feature Flags in C#

1. Simple AppSettings-Based Flag

// appsettings.json
{
  "FeatureFlags": {
    "EnableNewCheckout": true
  }
}

// Feature flag check in service/controller
public class CheckoutService
{
    private readonly IConfiguration _config;

    public CheckoutService(IConfiguration config)
    {
        _config = config;
    }

    public void ProcessCheckout()
    {
        bool isEnabled = _config.GetValue<bool>("FeatureFlags:EnableNewCheckout");

        if (isEnabled)
        {
            // New feature logic
            RunNewCheckoutFlow();
        }
        else
        {
            // Old logic
            RunLegacyCheckout();
        }
    }
}

2. Strongly-Typed Flag Options with IOptions<T>

public class FeatureFlags
{
    public bool EnableNewCheckout { get; set; }
}

// Program.cs
builder.Services.Configure<FeatureFlags>(builder.Configuration.GetSection("FeatureFlags"));

public class CheckoutService
{
    private readonly FeatureFlags _flags;

    public CheckoutService(IOptions<FeatureFlags> options)
    {
        _flags = options.Value;
    }

    public void ProcessCheckout()
    {
        if (_flags.EnableNewCheckout)
            RunNewCheckoutFlow();
        else
            RunLegacyCheckout();
    }
}

Benefits of Using Feature Flags

Benefit	Description
Decouple release from deploy	Push code to production but release later
Roll back quickly	Turn off a feature without redeploying
Targeted releases	Enable for specific users or environments
Support experimentation	A/B testing, beta features, etc.
Safer deployments	Gradual rollout, monitor before full exposure

Drawbacks and Considerations

Drawback	Mitigation
Code clutter*	Isolate with clear naming or abstraction
Flag rot	Schedule cleanup of unused flags
Test complexity	Ensure tests cover both flag states
Security risks	Don’t use flags for access control unless enforced server-side

Tools & Libraries

Tool	Description
LaunchDarkly	Enterprise feature flag service
Azure App Configuration	Feature management as a service
Microsoft.FeatureManagement	Lightweight, open-source NuGet package for flags
Unleash	Open-source feature toggle platform

When to Use Feature Flags

Scenario	Use Feature Flag?	Why
Gradual rollout of new UI	✅ Yes	Control release by audience
Experimental logic or A/B testing	✅ Yes	Test without impact
Sensitive logic changes	✅ Yes	Roll back instantly if needed
Clean-up of legacy features	❌ No	Remove via versioning instead
Business-level config (non-code)	✅ Yes	Empower non-devs to toggle

Summary for Interviews

"Feature flags give teams the ability to control feature exposure at runtime, enabling safer deployments, faster feedback, and gradual rollouts. I’ve used them in C# via appsettings.json, IOptions<T>, and Microsoft’s feature management NuGet package. They’re essential in any mature CI/CD pipeline — especially when building resilient, testable, user-focused applications."