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--- title: Enable conditional features with a custom filter in an ASP.NET Core application titleSuffix: Azure App Configuration description: Find out how to implement a custom feature filter so you can enable conditional feature flags for your ASP.NET Core application. ms.service: azure-app-configuration ms.devlang: csharp ms.custom: devx-track-csharp author: zhiyuanliang-ms ms.author: zhiyuanliang ms.topic: how-to ms.date: 9/1/2026 ai-usage: ai-assisted # customer intent: As a developer, I want to implement a custom feature filter in my ASP.NET Core application so that I can turn on features conditionally in my app. --- # Enable conditional features with a custom filter in an ASP.NET Core application Feature flags can use feature filters to enable features conditionally. For more information about feature filters, see [Enable conditional features with feature filters](./howto-feature-filters.md). This guide shows you how to implement a custom feature filter in an ASP.NET Core application and use the feature filter to enable features conditionally. ## Prerequisites - The ASP.NET Core application with a *Beta* feature flag that you create when you complete the steps in [Quickstart: Add feature flags to an ASP.NET Core app](./quickstart-feature-flag-aspnet-core.md). This guide builds on that feature management quickstart. - A custom feature filter called `Random`, added to the *Beta* feature flag in your Azure App Configuration store. For instructions, see [Add a custom feature filter](./howto-feature-filters.md#add-a-custom-feature-filter). ## Implement a custom feature filter In your App Configuration store, the *Beta* feature flag has a custom feature filter named `Random`. That filter has a `Percentage` parameter. To implement the feature filter, you enable the *Beta* feature flag based on the chance defined by the `Percentage` parameter. 1. Go to the folder that contains the ASP.NET Core application project from the feature management quickstart listed in [Prerequisites](#prerequisites). 1. Add a *RandomFilter.cs* file that contains the following code: ```csharp using Microsoft.FeatureManagement; namespace TestAppConfig { [FilterAlias("Random")] public class RandomFilter : IFeatureFilter { private readonly Random _random; public RandomFilter() { _random = new Random(); } public Task<bool> EvaluateAsync(FeatureFilterEvaluationContext context) { int percentage = context.Parameters.GetSection("Percentage").Get<int>(); int randomNumber = _random.Next(100); return Task.FromResult(randomNumber <= percentage); } } } ``` The `RandomFilter` class in the preceding code implements the `IFeatureFilter` interface from the `Microsoft.FeatureManagement` library. The `IFeatureFilter` interface has a single method named `EvaluateAsync`, which is called whenever a feature flag is evaluated. In `EvaluateAsync`, a feature filter enables a feature flag by returning `true`. In this code, the `RandomFilter` class is decorated with `FilterAliasAttribute`, which gives your filter the alias `Random`. That alias matches the filter name in the *Beta* feature flag in App Configuration. 1. Open the *Program.cs* file. Register `RandomFilter` by adding a call to the `AddFeatureFilter` method, as shown in the following code: ```csharp // Existing code in Program.cs // ... ... // Add feature management to the container of services. builder.Services.AddFeatureManagement() .AddFeatureFilter<RandomFilter>(); // The rest of the existing code in Program.cs // ... ... ``` ## Apply the feature filter 1. Build your app by using the `dotnet build` command. Then run it by using `dotnet run`. 1. In the output of the `dotnet run` command, find a URL that the web app is listening on. Open a browser and go to that URL. 1. Refresh the browser a few times. The **Beta** menu sometimes appears on the webpage, and sometimes it doesn't. The *Beta* feature flag isn't being manually turned on and off. The variation is caused by the random number that the `RandomFilter` implementation of `EvaluateAsync` generates when the feature flag is evaluated. :::image type="content" source="./media/quickstarts/aspnet-core-feature-flag-local-before.png" alt-text="Screenshot of a browser open to localhost:7207. Text on the page states Data from Azure App Configuration. No Beta menu is visible."::: :::image type="content" source="./media/quickstarts/aspnet-core-feature-flag-local-after.png" alt-text="Screenshot of a browser open to localhost:7207. Text on the page states Data from Azure App Configuration. A Beta menu is highlighted."::: ## Next steps To find out more about built-in feature filters, continue to the following documents: > [!div class="nextstepaction"] > [Enable features on a schedule](./howto-timewindow-filter.md) > [!div class="nextstepaction"] > [Roll out features to targeted audiences](./howto-targetingfilter.md) For the full feature rundown of the .NET feature management library, continue to the following document: > [!div class="nextstepaction"] > [.NET feature management](./feature-management-dotnet-reference.md)