OpenFeature .NET SDK

Quick start

Requirements

• .NET 6+
• .NET Core 6+
• .NET Framework 4.6.2+

Note that the packages will aim to support all current .NET versions. Refer to the currently supported versions .NET and .NET Framework excluding .NET Framework 3.5

Install

Use the following to initialize your project:

dotnet new console

and install OpenFeature:

dotnet add package OpenFeature

Usage

public async Task Example()
{
    // Register your feature flag provider
    await Api.Instance.SetProviderAsync(new InMemoryProvider());

    // Create a new client
    FeatureClient client = Api.Instance.GetClient();

    // Evaluate your feature flag
    bool v2Enabled = await client.GetBooleanValueAsync("v2_enabled", false);

    if ( v2Enabled )
    {
        //Do some work
    }
}

Features

Status	Features	Description
✅	Providers	Integrate with a commercial, open source, or in-house feature management tool.
✅	Targeting	Contextually-aware flag evaluation using evaluation context.
✅	Hooks	Add functionality to various stages of the flag evaluation life-cycle.
✅	Tracking	Associate user actions with feature flag evaluations.
✅	Logging	Integrate with popular logging packages.
✅	Domains	Logically bind clients with providers.
✅	Eventing	React to state changes in the provider or flag management system.
✅	Shutdown	Gracefully clean up a provider during application shutdown.
✅	Extending	Extend OpenFeature with custom providers and hooks.
🔬	DependencyInjection	Integrate OpenFeature with .NET's dependency injection for streamlined provider setup.

Implemented: ✅ | In-progress: ⚠️ | Not implemented yet: ❌ | Experimental: 🔬

Providers

Providers are an abstraction between a flag management system and the OpenFeature SDK. Here is a complete list of available providers.

If the provider you're looking for hasn't been created yet, see the develop a provider section to learn how to build it yourself.

Once you've added a provider as a dependency, it can be registered with OpenFeature like this:

await Api.Instance.SetProviderAsync(new MyProvider());

In some situations, it may be beneficial to register multiple providers in the same application. This is possible using domains, which is covered in more detail below.

Targeting

Sometimes, the value of a flag must consider some dynamic criteria about the application or user such as the user's location, IP, email address, or the server's location. In OpenFeature, we refer to this as targeting. If the flag management system you're using supports targeting, you can provide the input data using the evaluation context.

// set a value to the global context
EvaluationContextBuilder builder = EvaluationContext.Builder();
builder.Set("region", "us-east-1");
EvaluationContext apiCtx = builder.Build();
Api.Instance.SetContext(apiCtx);

// set a value to the client context
builder = EvaluationContext.Builder();
builder.Set("region", "us-east-1");
EvaluationContext clientCtx = builder.Build();
var client = Api.Instance.GetClient();
client.SetContext(clientCtx);

// set a value to the invocation context
builder = EvaluationContext.Builder();
builder.Set("region", "us-east-1");
EvaluationContext reqCtx = builder.Build();

bool flagValue = await client.GetBooleanValuAsync("some-flag", false, reqCtx);

Hooks

Hooks allow for custom logic to be added at well-defined points of the flag evaluation life-cycle. Look here for a complete list of available hooks. If the hook you're looking for hasn't been created yet, see the develop a hook section to learn how to build it yourself.

Once you've added a hook as a dependency, it can be registered at the global, client, or flag invocation level.

// add a hook globally, to run on all evaluations
Api.Instance.AddHooks(new ExampleGlobalHook());

// add a hook on this client, to run on all evaluations made by this client
var client = Api.Instance.GetClient();
client.AddHooks(new ExampleClientHook());

// add a hook for this evaluation only
var value = await client.GetBooleanValueAsync("boolFlag", false, context, new FlagEvaluationOptions(new ExampleInvocationHook()));

Logging

The .NET SDK uses Microsoft.Extensions.Logging. See the manual for complete documentation.

Domains

Clients can be assigned to a domain. A domain is a logical identifier which can be used to associate clients with a particular provider. If a domain has no associated provider, the default provider is used.

// registering the default provider
await Api.Instance.SetProviderAsync(new LocalProvider());

// registering a provider to a domain
await Api.Instance.SetProviderAsync("clientForCache", new CachedProvider());

// a client backed by default provider
FeatureClient clientDefault = Api.Instance.GetClient();

// a client backed by CachedProvider
FeatureClient scopedClient = Api.Instance.GetClient("clientForCache");

Domains can be defined on a provider during registration. For more details, please refer to the providers section.

Eventing

Events allow you to react to state changes in the provider or underlying flag management system, such as flag definition changes, provider readiness, or error conditions. Initialization events (PROVIDER_READY on success, PROVIDER_ERROR on failure) are dispatched for every provider. Some providers support additional events, such as PROVIDER_CONFIGURATION_CHANGED.

Please refer to the documentation of the provider you're using to see what events are supported.

Example usage of an Event handler:

public static void EventHandler(ProviderEventPayload eventDetails)
{
    Console.WriteLine(eventDetails.Type);
}

EventHandlerDelegate callback = EventHandler;
// add an implementation of the EventHandlerDelegate for the PROVIDER_READY event
Api.Instance.AddHandler(ProviderEventTypes.ProviderReady, callback);

It is also possible to register an event handler for a specific client, as in the following example:

EventHandlerDelegate callback = EventHandler;

var myClient = Api.Instance.GetClient("my-client");

var provider = new ExampleProvider();
await Api.Instance.SetProviderAsync(myClient.GetMetadata().Name, provider);

myClient.AddHandler(ProviderEventTypes.ProviderReady, callback);

Tracking

The tracking API allows you to use OpenFeature abstractions and objects to associate user actions with feature flag evaluations. This is essential for robust experimentation powered by feature flags. For example, a flag enhancing the appearance of a UI component might drive user engagement to a new feature; to test this hypothesis, telemetry collected by a hook(#hooks) or provider(#providers) can be associated with telemetry reported in the client's track function.

var client = Api.Instance.GetClient();
client.Track("visited-promo-page", trackingEventDetails: new TrackingEventDetailsBuilder().SetValue(99.77).Set("currency", "USD").Build());

Note that some providers may not support tracking; check the documentation for your provider for more information.

Shutdown

The OpenFeature API provides a close function to perform a cleanup of all registered providers. This should only be called when your application is in the process of shutting down.

// Shut down all providers
await Api.Instance.ShutdownAsync();

Extending

Develop a provider

To develop a provider, you need to create a new project and include the OpenFeature SDK as a dependency. This can be a new repository or included in the existing contrib repository available under the OpenFeature organization. You’ll then need to write the provider by implementing the FeatureProvider interface exported by the OpenFeature SDK.

public class MyProvider : FeatureProvider
{
    public override Metadata GetMetadata()
    {
        return new Metadata("My Provider");
    }

    public override Task<ResolutionDetails<bool>> ResolveBooleanValueAsync(string flagKey, bool defaultValue, EvaluationContext? context = null, CancellationToken cancellationToken = default)
    {
        // resolve a boolean flag value
    }

    public override Task<ResolutionDetails<string>> ResolveStringValueAsync(string flagKey, string defaultValue, EvaluationContext? context = null, CancellationToken cancellationToken = default)
    {
        // resolve a string flag value
    }

    public override Task<ResolutionDetails<int>> ResolveIntegerValueAsync(string flagKey, int defaultValue, EvaluationContext context = null)
    {
        // resolve an int flag value
    }

    public override Task<ResolutionDetails<double>> ResolveDoubleValueAsync(string flagKey, double defaultValue, EvaluationContext? context = null, CancellationToken cancellationToken = default)
    {
        // resolve a double flag value
    }

    public override Task<ResolutionDetails<Value>> ResolveStructureValueAsync(string flagKey, Value defaultValue, EvaluationContext? context = null, CancellationToken cancellationToken = default)
    {
        // resolve an object flag value
    }
}

Develop a hook

To develop a hook, you need to create a new project and include the OpenFeature SDK as a dependency. This can be a new repository or included in the existing contrib repository available under the OpenFeature organization. Implement your own hook by conforming to the Hook interface. To satisfy the interface, all methods (Before/After/Finally/Error) need to be defined.

public class MyHook : Hook
{
  public ValueTask<EvaluationContext> BeforeAsync<T>(HookContext<T> context,
      IReadOnlyDictionary<string, object> hints = null)
  {
    // code to run before flag evaluation
  }

  public ValueTask AfterAsync<T>(HookContext<T> context, FlagEvaluationDetails<T> details,
      IReadOnlyDictionary<string, object> hints = null)
  {
    // code to run after successful flag evaluation
  }

  public ValueTask ErrorAsync<T>(HookContext<T> context, Exception error,
      IReadOnlyDictionary<string, object> hints = null)
  {
    // code to run if there's an error during before hooks or during flag evaluation
  }

  public ValueTask FinallyAsync<T>(HookContext<T> context, IReadOnlyDictionary<string, object> hints = null)
  {
    // code to run after all other stages, regardless of success/failure
  }
}

Built a new hook? Let us know so we can add it to the docs!

DependencyInjection

Note

The OpenFeature.DependencyInjection and OpenFeature.Hosting packages are currently experimental. They streamline the integration of OpenFeature within .NET applications, allowing for seamless configuration and lifecycle management of feature flag providers using dependency injection and hosting services.

Installation

To set up dependency injection and hosting capabilities for OpenFeature, install the following packages:

dotnet add package OpenFeature.DependencyInjection
dotnet add package OpenFeature.Hosting

Usage Examples

For a basic configuration, you can use the InMemoryProvider. This provider is simple and well-suited for development and testing purposes.

Basic Configuration:

builder.Services.AddOpenFeature(featureBuilder => {
    featureBuilder
        .AddHostedFeatureLifecycle() // From Hosting package
        .AddContext((contextBuilder, serviceProvider) => { /* Custom context configuration */ })
        .AddInMemoryProvider();
});

Domain-Scoped Provider Configuration:
To set up multiple providers with a selection policy, define logic for choosing the default provider. This example designates name1 as the default provider:

builder.Services.AddOpenFeature(featureBuilder => {
    featureBuilder
        .AddHostedFeatureLifecycle()
        .AddContext((contextBuilder, serviceProvider) => { /* Custom context configuration */ })
        .AddInMemoryProvider("name1")
        .AddInMemoryProvider("name2")
        .AddPolicyName(options => {
            // Custom logic to select a default provider
            options.DefaultNameSelector = serviceProvider => "name1";
        });
});

Registering a Custom Provider

You can register a custom provider, such as InMemoryProvider, with OpenFeature using the AddProvider method. This approach allows you to dynamically resolve services or configurations during registration.

services.AddOpenFeature()
        .AddProvider(provider =>
        {
            // Resolve services or configurations as needed
            var configuration = provider.GetRequiredService<IConfiguration>();
            var flags = new Dictionary<string, Flag>
            {
                { "feature-key", new Flag<bool>(configuration.GetValue<bool>("FeatureFlags:Key")) }
            };

            // Register a custom provider, such as InMemoryProvider
            return new InMemoryProvider(flags);
        });

Adding a Domain-Scoped Provider

You can also register a domain-scoped custom provider, enabling configurations specific to each domain:

services.AddOpenFeature()
        .AddProvider("my-domain", (provider, domain) =>
        {
            // Resolve services or configurations as needed for the domain
            var flags = new Dictionary<string, Flag>
            {
                { $"{domain}-feature-key", new Flag<bool>(true) }
            };

            // Register a domain-scoped custom provider such as InMemoryProvider
            return new InMemoryProvider(flags);
        });