DEV Community: Scott Tippett

Already using LaunchDarkly or Flagsmith? Here's how to try FtrIO on a single flag

Scott Tippett — Sun, 21 Jun 2026 18:44:13 +0000

Thinking about trying FtrIO? The new CLI makes it easy to start with just one feature flag

If you've been curious about FtrIO (the .NET feature toggle library that replaces if (featureFlags.IsEnabled(...)) with a [Toggle] attribute woven directly into your compiled IL) but weren't sure where to start, the experimental release of FtrIO.onetwo just made that first step a lot smaller.

You don't need to commit to a full migration. You don't need to rip out your existing flag library. You just need one method and 20 minutes.

What FtrIO.onetwo does

FtrIO.onetwo is a .NET CLI audit tool. Its default mode scans your source tree, finds every FtrIO toggle reference, and tells you exactly what's live right now:

dotnet tool install --global FtrIO.onetwo --version 1.1.1-experimental
ftrio.onetwo --source C:\Projects\MyApp

But in this experimental release it does two new things:

ftrio.onetwo import: pulls your current flag state from LaunchDarkly, Flagsmith, flagd, environment variables, or an HTTP endpoint directly into appsettings.json. Your existing flag library keeps working unchanged.

ftrio.onetwo migrate: scans your .cs files for LaunchDarkly or Flagsmith SDK call patterns using Roslyn, cross-references them against your live flag state, and generates a report showing exactly what each flag would look like in FtrIO and how to migrate it.

The "try it on one flag" workflow

The migrate report categorises every flag it finds:

✅ Ready to migrate: boolean flag, no targeting rules, straightforward [Toggle] replacement
⚠️ Needs review: targeting rules, number flags, needs a decision
❌ Cannot migrate: JSON flags, recommend moving to IConfiguration

For every ready flag it shows the suggested refactor. Something like:

new-checkout-flow → NewCheckoutFlow
File: Services\OrderService.cs:42

Current code:
  if (client.BoolVariation("new-checkout-flow", user, false))
  {
      ValidateCart();
      ApplyDiscounts();
      ProcessPayment();
  }

Suggested action:
  Extract the if block into a parent method named NewCheckoutFlow,
  then decorate it with [Toggle]. The inner methods remain unchanged.

  [Toggle]
  public void NewCheckoutFlow()
  {
      ValidateCart();
      ApplyDiscounts();
      ProcessPayment();
  }

  Add to appsettings.json:
  "NewCheckoutFlow": false

Pick one of those ready flags. Do the refactor. Run the app. That's it: you've got FtrIO running in your codebase alongside your existing flag library, on exactly one feature, with everything else completely unchanged.

If you like how it feels, pick another. If you don't, remove the [Toggle] attribute and you're back to where you started.

Why the parent method suggestion matters

The report nudges toward a pattern that's worth calling out explicitly: one named parent method per toggled feature.

Instead of this:

if (client.BoolVariation("send-welcome-email", user, false))
{
    ValidateEmailAddress(user.Email);
    BuildEmailTemplate(user);
    SendViaSmtp(user.Email);
    LogEmailSent(user.Id);
}

FtrIO works best like this:

[Toggle]
public void SendWelcomeEmail()
{
    ValidateEmailAddress(_user.Email);
    BuildEmailTemplate(_user);
    SendViaSmtp(_user.Email);
    LogEmailSent(_user.Id);
}

The feature has a single, named, discoverable entry point. The toggle is a gate on the feature, not scattered through its implementation. The inner methods have no idea a toggle exists. This is better code regardless of FtrIO; the migration report teaches the pattern rather than just doing a mechanical find-and-replace.

Running the migration report

# With live flag state from LaunchDarkly
ftrio.onetwo migrate \
  --from launchdarkly \
  --api-key sdk-xxx \
  --project my-project \
  --env production \
  --source C:\Projects\MyApp \
  --markdown plan.md

# Code scan only, no API key needed
ftrio.onetwo migrate \
  --from launchdarkly \
  --source C:\Projects\MyApp

The --api-key is optional; without it the report still finds all your call sites and shows the suggested refactors, just with placeholder values in the appsettings.json snippets instead of live flag states.

This is experimental

Both import and migrate are in an experimental branch. To try it you need to explicitly request the experimental version — the default install will give you the stable release which doesn't include these commands yet:

dotnet tool install --global FtrIO.onetwo --version 1.1.1-experimental

Functional but not yet on a stable release. Feedback is genuinely welcome, especially on the migration report format and the flag mapping logic.

The full FtrIO ecosystem:

FtrIO: the core library: dotnet add package FtrIO
FtrIO.Toaster: self-hosted Docker UI for managing toggles live
FtrIO.onetwo: this CLI: dotnet tool install -g FtrIO.onetwo
Docs: ftronoff.github.io/FtrIO
GitHub: github.com/FtrOnOff

If you try it on a flag or two I'd love to hear how it goes; drop a comment or open an issue.

I got tired of writing ifs everywhere — so I built an ecosystem

Scott Tippett — Sun, 21 Jun 2026 14:46:02 +0000

I got tired of writing `if (featureFlags.IsEnabled(...))` everywhere — so I built an ecosystem

I've been writing C# for over a decade and feature flags have always annoyed me in the same way.

Not the concept — the concept is great. The ability to ship code dark, roll out to a percentage of traffic, or flip a switch without a redeploy is genuinely powerful. What annoyed me was the noise. Every time I added a feature flag I'd end up with this scattered through the codebase:

if (featureFlags.IsEnabled("SendWelcomeEmail"))
{
    SendWelcomeEmail();
}

Multiply that across a real codebase and you've got if statements everywhere, magic strings that drift out of sync with config, and no easy way to know what's actually live at any given moment without opening files manually.

So I built something about it. Then I built something else. Then I hadn't slept and had consumed several root beers and I had an ecosystem.

The problem with feature flags in .NET

Most feature flag libraries for .NET — including Microsoft's own Microsoft.FeatureManagement — solve the storage and evaluation problem well. But they don't solve the noise problem. You still have to wrap every call site, inject a service everywhere, and remember to clean up the if statements when a flag is retired.

LaunchDarkly and Flagsmith go further with SaaS dashboards and targeting rules, but now you've got a vendor dependency, a network call on your hot path, and a subscription fee.

What I wanted was something that felt like a natural part of the language — where toggling a feature on or off was as simple as adding or removing an attribute, and the checking happened automatically without touching every call site.

FtrIO — the core library

The idea behind FtrIO is simple: decorate a method with [Toggle] and it becomes config-gated by its own name.

[Toggle]
public void SendWelcomeEmail()
{
    // send the email
}

SendWelcomeEmail(); // only runs if "SendWelcomeEmail": true in appsettings.json

That's it. No if, no injected service, no wrapper. The method calls exactly like a normal method. If the toggle is off, it just doesn't run.

How it actually works

[Toggle] is an AspectInjector aspect. At compile time, AspectInjector weaves the gating check directly into the method's IL. The gate is applied at the IL level, so it works regardless of how the method is called — direct call, reflection, delegate, anything.

Config lives in appsettings.json:

{
  "Toggles": {
    "SendWelcomeEmail": true,
    "NewCheckoutFlow": false
  }
}

Compile-time validation

The thing I'm most proud of: FtrIO ships a Roslyn analyzer that catches missing config entries at build time.

If you decorate a method with [Toggle] but forget to add the matching entry to appsettings.json, the build fails with FTRIO001 — not a silent runtime failure, not a logged warning, a proper compiler error:

error FTRIO001: 'NewCheckoutFlow' is decorated with [Toggle] but has no entry in Toggles

No other feature flag library I know of does this, at any price.

Async support

For async methods, [ToggleAsync] returns Task.CompletedTask or Task.FromResult(default) when the toggle is off — always safely awaitable:

[ToggleAsync]
public async Task SendWelcomeEmailAsync()
{
    await emailClient.SendAsync(...);
}

await SendWelcomeEmailAsync(); // safely awaitable whether the toggle is on or off

Beyond true/false — strategy-based decisions

FtrIO isn't limited to boolean toggles. StrategyToggleParser routes raw config values through a chain of strategies:

{
  "Toggles": {
    "NewCheckout": "20%",
    "PaymentV2": "blue"
  }
}

ToggleParserProvider.Configure(new StrategyToggleParser(
    new PercentageRolloutStrategy(),      // "20%" runs ~1 in 5 calls
    new BlueGreenStrategy("blue", "blue", "green")  // runs only on the blue slot
));

Percentage rollouts, blue-green deployment switching, or any custom logic you implement via IToggleDecisionStrategy.

Dynamic providers

For toggle state driven by external sources — HTTP endpoints, Azure App Config, environment variables — FtrIO uses a provider pipeline that writes into appsettings.json in the background. The read path always comes from the file.

This means if a remote provider goes offline, the last known state serves automatically from disk. No fallback code, no circuit breaker, no stale-cache TTL to configure.

dotnet add package FtrIO.Providers.Http
dotnet add package FtrIO.Providers.AzureAppConfig

Installation

dotnet add package FtrIO

Targets .NET 6, 8, and 10.

FtrIO.Toaster — the management UI

Once FtrIO was in use, I had a new problem: flipping toggles meant editing appsettings.json by hand. Across multiple environments. That got old quickly.

So I built FtrIO.Toaster — a lightweight Dockerized web UI for managing toggles live.

The name has two origins: toast is binary (toasted or not, much like a feature toggle), and it's a nod to the Dungeon Master who runs our D&D sessions. Every good campaign needs someone deciding what's enabled and what isn't.

What it does

Boolean on/off toggles
Percentage rollout — slider and number input in sync
Blue/green deployment switching
Change toggle type at any time
Add and delete toggles
Multi-environment support — manage any number of environments from a single UI instance via a dropdown
Audit log — every change recorded with timestamp, environment, toggle key, old value, new value, and the acting user
Basic Auth built in, with an OAuth2 Proxy sidecar option for SSO (Google, GitHub, Microsoft, GitLab, OIDC)

How it fits with FtrIO core

Toaster implements FtrIO's own ToggleProviderBuffer internally. Changes are staged in memory and flushed atomically to appsettings.json on the configured interval — exactly as a native FtrIO provider would. Your running app picks up changes via ReloadOnChange with no restart.

Getting started

No clone required — pull from Docker Hub:

services:
  toaster:
    image: thescottbot/ftrio:latest
    ports:
      - "8000:8000"
    environment:
      APP_NAME: "My Application"
      APPSETTINGS_PATH: /data/appsettings.json
    volumes:
      - type: bind
        source: /path/to/your/appsettings.json
        target: /data/appsettings.json
    restart: unless-stopped

docker compose up -d

Open http://localhost:8000.

FtrIO.onetwo — the audit CLI

The third problem: as the codebase grew, there was no easy way to know what was actually live right now without opening appsettings.json and cross-referencing it with the source code manually.

FtrIO.onetwo is a .NET global tool that does that cross-referencing for you:

dotnet tool install -g FtrIO.onetwo
ftrio.onetwo --source C:\Projects\MyApp

It walks your source tree, finds every [Toggle], [ToggleAsync], and manual call, and outputs a table:

── Staging C:\Projects\MyApp\appsettings.Staging.json
╭──────────────────┬──────────────────┬────────────┬─────────┬───────────────────┬──────╮
│ Toggle Key       │ Method           │ Source     │  State  │ File              │ Line │
├──────────────────┼──────────────────┼────────────┼─────────┼───────────────────┼──────┤
│ NewCheckoutFlow  │ NewCheckoutFlow  │ [Toggle]   │   50%   │ Services\Order.cs │    9 │
│ PaymentV2        │ PaymentV2        │ [Toggle]   │  BLUE   │ Services\Pay.cs   │    6 │
│ SendWelcomeEmail │ SendWelcomeEmail │ [Toggle]   │   ON    │ Services\Email.cs │   22 │
│ UnknownFeature   │ UnknownFeature   │ ManualCall │ MISSING │ Controllers\Ho... │   42 │
╰──────────────────┴──────────────────┴────────────┴─────────┴───────────────────┴──────╯
4 toggle(s). 1 ON, 0 OFF, 1 PERCENTAGE, 1 BLUE/GREEN, 1 MISSING.

The MISSING state is the one I find most useful — it catches toggles that exist in code but have no config entry, across every environment, before they cause a silent runtime failure. No other feature flag tooling I know of does this.

Supports --env Staging to target a specific environment overlay, and --markdown to write the output to a markdown file for documentation or CI artefacts.

How they fit together

All three tools share appsettings.json as the single source of truth. No coupling between them — use any combination without changing your call sites:

┌─────────────────────────────────────────────────────┐
│  Your code                                          │
│  [Toggle] public void SendWelcomeEmail() { ... }    │
└───────────────────┬─────────────────────────────────┘
                    │ compile-time weaving
                    ▼
┌─────────────────────────────────────────────────────┐
│  FtrIO core                                         │
│  gates method execution at runtime                  │
└───────────────────┬─────────────────────────────────┘
                    │ reads
                    ▼
┌─────────────────────────────────────────────────────┐
│  appsettings.json  — source of truth                │
└──────────┬──────────────────────────┬───────────────┘
           │ writes live              │ reads & audits
           ▼                          ▼
  FtrIO.Toaster                 FtrIO.onetwo
  (web UI — manage toggles)     (CLI — audit state)

How it compares

	FtrIO	LaunchDarkly	Microsoft.FeatureManagement	Flagsmith
Call-site syntax	`[Toggle]` attribute, zero noise	SDK call at every site	`if (await _fm.IsEnabledAsync(...))`	SDK call at every site
Works offline	✅ always (file-backed)	❌ needs SDK fallback config	✅	❌ needs SDK fallback config
Compile-time validation	✅ Roslyn analyzer	❌	❌	❌
Codebase audit / drift detection	✅ onetwo CLI	❌	❌	❌
Management UI	✅ Toaster, self-hosted	✅ SaaS dashboard	❌	✅ SaaS dashboard
Percentage rollout	✅	✅	✅	✅
Self-hosted / no vendor	✅	❌ paid SaaS	✅	✅ (or SaaS)
Cost	Free, OSS	Paid SaaS	Free, OSS	Free tier / paid SaaS

Where to find it

Everything lives under the FtrOnOff org on GitHub:

FtrIO (core library) — dotnet add package FtrIO
FtrIO.Toaster (Docker UI) — docker compose up -d
FtrIO.onetwo (audit CLI) — dotnet tool install -g FtrIO.onetwo
Full docs — ftronoff.github.io/FtrIO

All free, all open source, all built on no sleep and root beer. 🍺

If you try it I'd love to know what you think — drop a comment or open an issue.