.NET feature flags installation

The PostHog package supports any .NET platform that targets .NET Standard 2.1 or .NET 8+, including MAUI, Blazor, and console applications. The PostHog.AspNetCore package provides additional conveniences for ASP.NET Core applications such as streamlined registration, request-scoped caching, and integration with .NET Feature Management.

Note: We actively test with ASP.NET Core. Other platforms should work but haven't been specifically tested. If you encounter issues, please report them on GitHub.

Not supported: Classic UWP (requires .NET Standard 2.0 only). Microsoft has deprecated UWP in favor of the Windows App SDK. For Unity projects, see our dedicated Unity SDK (currently in beta).

dotnet add package PostHog.AspNetCore

In your Program.cs (or Startup.cs for ASP.NET Core 2.x) file, add the following code:

using PostHog;

var builder = WebApplication.CreateBuilder(args);

// Add PostHog to the dependency injection container as a singleton.
builder.AddPostHog();

Make sure to configure PostHog with your project API key, instance address, and optional personal API key. For example, in appsettings.json:

{
  "PostHog": {
    "ProjectApiKey": "<ph_project_api_key>",
    "HostUrl": "https://us.i.posthog.com"
  }
}

Note: If the host is not specified, the default host https://us.i.posthog.com is used.

Use a secrets manager to store your personal API key. For example, when developing locally you can use the UserSecrets feature of the dotnet CLI:

dotnet user-secrets init
dotnet user-secrets set "PostHog:PersonalApiKey" "phx_..."

You can find your project API key and instance address in the project settings page in PostHog.

Working with .NET Feature Management

PostHog.AspNetCore supports .NET Feature Management. This enables you to use the <feature /> tag helper and the FeatureGateAttribute in your ASP.NET Core applications to gate access to certain features using PostHog feature flags.

To use feature flags with the .NET Feature Management library, you'll need to implement the IPostHogFeatureFlagContextProvider interface. The quickest way to do that is to inherit from the PostHogFeatureFlagContextProvider class and override the GetDistinctId and GetFeatureFlagOptionsAsync methods.

public class MyFeatureFlagContextProvider(IHttpContextAccessor httpContextAccessor)
    : PostHogFeatureFlagContextProvider
{
    protected override string? GetDistinctId()
        => httpContextAccessor.HttpContext?.User.Identity?.Name;
    
    protected override ValueTask<FeatureFlagOptions> GetFeatureFlagOptionsAsync()
    {
        // In a real app, you might get this information from a 
        // database or other source for the current user.
        return ValueTask.FromResult(
            new FeatureFlagOptions
            {
                PersonProperties = new Dictionary<string, object?>
                {
                    ["email"] = "some-test@example.com"
                },
                OnlyEvaluateLocally = true
            });
    }
}

Then, register your implementation in Program.cs (or Startup.cs):

var builder = WebApplication.CreateBuilder(args);
builder.AddPostHog(options => {
    options.UseFeatureManagement<MyFeatureFlagContextProvider>();
});

With this in place, you can now use feature tag helpers in your Razor views:

<feature name="awesome-new-feature">
    <p>This is the new feature!</p>
</feature>
<feature name="awesome-new-feature" negate="true">
    <p>Sorry, no awesome new feature for you.</p>
</feature>

Multivariate feature flags are also supported:

<feature name="awesome-new-feature" value="variant-a">
    <p>This is the new feature variant A!</p>
</feature>
<feature name="awesome-new-feature" value="variant-b">
    <p>This is the new feature variant B!</p>
</feature>

You can also use the FeatureGateAttribute to gate access to controllers or actions:

[FeatureGate("awesome-new-feature")]
public class NewFeatureController : Controller
{
    public IActionResult Index()
    {
        return View();
    }
}

Using the core package without ASP.NET Core

If you're not using ASP.NET Core (for example, in a console application, MAUI app, or Blazor WebAssembly), install the PostHog package instead of PostHog.AspNetCore. This package has no ASP.NET Core dependencies and can be used in any .NET project targeting .NET Standard 2.1 or .NET 8+.

dotnet add package PostHog

The PostHogClient class must be implemented as a singleton in your project. For PostHog.AspNetCore, this is handled by the builder.AddPostHog(); method. For the PostHog package, you can do the following if you're using dependency injection:

builder.Services.AddPostHog();

If you're not using a builder (such as in a console application), you can do the following:

using PostHog;

var services = new ServiceCollection();
services.AddPostHog();
var serviceProvider = services.BuildServiceProvider();
var posthog = serviceProvider.GetRequiredService<IPostHogClient>();

The AddPostHog methods accept an optional Action<PostHogOptions> parameter that you can use to configure the client.

If you're not using dependency injection, you can create a static instance of the PostHogClient class and use that everywhere in your project:

using PostHog;

public static readonly PostHogClient PostHog = new(new PostHogOptions {
    ProjectApiKey = "<ph_project_api_key>",
    HostUrl = new Uri("https://us.i.posthog.com"),
    PersonalApiKey = Environment.GetEnvironmentVariable(
      "PostHog__PersonalApiKey")
});

Debug mode

If you're not seeing the expected events being captured, the feature flags being evaluated, or the surveys being shown, you can enable debug mode to see what's happening.

To see detailed logging, set the log level to Debug or Trace in appsettings.json:

{
  "DetailedErrors": true,
  "Logging": {
    "LogLevel": {
      "Default": "Information",
      "Microsoft.AspNetCore": "Warning",
      "PostHog": "Trace"
    }
  },
  ...
}

Next steps