Microsoft Feature Toggle Feature Flag Library: A First Look

As the creator of the .NET FeatureToggle library that has over half a million downloads on NuGet, I recently learned (thanks @OzBobWA) with some interest that Microsoft is working on a feature toggle / feature flag library.

It’s in its infancy at the moment and is currently in early preview on NuGet and as I understand it, the library is being developed by the Azure team and is not currently open sourced/available on GitHub.

Essentially the library allows you to configure whether a feature is on or off depending on a configuration setting, such as defined in the configuration JSON file or in Azure configuration.

Whilst the development of the library appears to be currently focused on enabling feature toggling in ASP.NET Core, I though it would be interesting to see if I could get it to work in a .NET Core console app. I hacked together the following code to demonstrate:

using System;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.FeatureManagement;

namespace ConsoleApp1
{
    class Program
    {
        static readonly IFeatureManager FeatureManager;

        static Program()
        {
            // Setup configuration to come from config file 
            IConfigurationBuilder builder = new ConfigurationBuilder();
            builder.AddJsonFile("appsettings.json");
            var configuration = builder.Build();

            // Register services (including feature management)
            var serviceCollection = new ServiceCollection();
            serviceCollection.AddSingleton<IConfiguration>(configuration);
            serviceCollection.AddFeatureManagement();

            // build the service and get IFeatureManager instance
            var serviceProvider = serviceCollection.BuildServiceProvider();
            FeatureManager = serviceProvider.GetService<IFeatureManager>();
        }

        static void Main(string[] args)
        {
            if (FeatureManager.IsEnabled("SayHello"))
            {
                Console.WriteLine("Hello World!");
            }


            Console.WriteLine("Press any key to exit...");
            Console.ReadLine();
        } 
    }
}

This code requires the following NuGets:

    <PackageReference Include="Microsoft.Extensions.Configuration.Json" Version="3.0.0-preview6.19304.6" />
    <PackageReference Include="Microsoft.Extensions.DependencyInjection" Version="2.2.0" />
    <PackageReference Include="Microsoft.FeatureManagement" Version="1.0.0-preview-009000001-1251" />

The console app attempts to read from the appsettings.json file and looks in the FeatureManagement section to see whether features are enabled or not:

{
  "FeatureManagement": {
    "SayHello": true
  }
}

In the code, strings are used to evaluate whether a feature is enabled or not, e.g. the line: if (FeatureManager.IsEnabled("SayHello"))  looks for a configuration value called “SayHello”.

If this value is false, the Console.WriteLine("Hello World!"); will not execute; if it is true the text “Hello World!” will be output.

Comparing Microsoft.FeatureManagement to FeatureToggle

When compared to the FeatureToggle library there are some interesting differences, for example the same app created using the FeatureToggle library would like the following:

using System;
using FeatureToggle;

namespace ConsoleApp2
{
    class SayHelloFeature : SimpleFeatureToggle { }

    class Program
    {
        static void Main(string[] args)
        {
            if (Is<SayHelloFeature>.Enabled)
            {
                Console.WriteLine("Hello World!");
            }

            Console.WriteLine("Press any key to exit...");
            Console.ReadLine();            
        }
    }
}

Notice in the preceding code that there are no magic strings to read the toggle value, instead you define a class that represents the feature and then by convention, the name of the class is used to locate the toggle value in configuration which looks like:

{
  "FeatureToggle": {
    "SayHelloFeature": "false"
  }
}

Overall ,whilst it is still early days for the library, it is cool that Microsoft may have their own supported feature toggling library in the future and help bring the concept of feature toggles as an alternative/adjunct to feature branches in source control to “the masses”.

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