Strongly-typed feature flags generated from config — no more magic strings.

A Roslyn source generator that reads feature flag definitions from appsettings.json or flags.json at compile time and generates:

• A strongly-typed IFeatureFlags interface with bool properties
• Extension methods for IFeatureManager
• DI registration via AddFeatureFlags()
• String constants for flag names

Microsoft.FeatureManagement uses magic strings:

// Before: runtime errors on typos, invisible to refactoring
if (await featureManager.IsEnabledAsync("DarkMode")) { ... }

FeatureFlagGen gives you compile-time safety:

// After: IntelliSense, refactorable, compile-time checked
if (flags.IsDarkModeEnabled) { ... }

dotnet add package FeatureFlagGen

{
  "FeatureManagement": {
    "DarkMode": true,
    "NewCheckout": false
  }
}

In your .csproj:

<ItemGroup>
  <AdditionalFiles Include="appsettings.json" />
</ItemGroup>

builder.Services.AddFeatureManagement();
builder.Services.AddFeatureFlags(); // generated extension method

public class MyService
{
    private readonly IFeatureFlags _flags;

    public MyService(IFeatureFlags flags)
    {
        _flags = flags;
    }

    public void DoWork()
    {
        if (_flags.IsDarkModeEnabled)
        {
            // dark mode logic
        }
    }
}

Nest flags in JSON to create grouped access:

{
  "FeatureManagement": {
    "Ui": {
      "DarkMode": true,
      "SideBar": false
    }
  }
}

Access via nested properties:

if (flags.Ui.IsDarkModeEnabled) { ... }

FeatureFlagGen produces five source files:

For gradual migration, use the generated extension methods directly on IFeatureManager:

if (await featureManager.IsDarkModeEnabledAsync()) { ... }

Access flag name strings via generated constants:

var name = FeatureFlagNames.DarkMode; // "DarkMode"
var grouped = FeatureFlagNames.Ui.DarkMode; // "Ui.DarkMode"

Filter-style flag definitions are fully supported:

{
  "FeatureManagement": {
    "Beta": {
      "EnabledFor": [
        { "Name": "Percentage", "Parameters": { "Value": 50 } }
      ]
    }
  }
}

These are treated as flat flags and generate IsBetaEnabled properties.

The generator recognizes these file names as AdditionalFiles:

• appsettings.json
• appsettings.*.json (e.g., appsettings.Development.json)
• flags.json

• .NET 8.0 or later
• Microsoft.FeatureManagement package in your application

1. Fork the repository
2. Create a feature branch (git checkout -b feat/my-feature)
3. Write tests for your changes
4. Ensure all tests pass: dotnet test
5. Ensure formatting: dotnet format --verify-no-changes
6. Submit a pull request

MIT — see LICENSE for details.