Enable conditional features with a custom filter in an ASP.NET Core application

Feature flags can use feature filters to enable features conditionally. To learn more about feature filters, see Enable conditional features with feature filters.

The example used in this guide is based on the ASP.NET Core application introduced in the feature management quickstart. Before proceeding further, complete the quickstart to create an ASP.NET Core application with a Beta feature flag. Once completed, you must add a custom feature filter to the Beta feature flag in your App Configuration store.

In this guide, you'll learn how to implement a custom feature filter and use the feature filter to enable features conditionally.

Prerequisites

• Create an ASP.NET Core app with a feature flag.
• Add a custom feature filter to the feature flag

Implement a custom feature filter

You've added a custom feature filter named Random with a Percentage parameter for your Beta feature flag in the prerequisites. Next, you'll implement the feature filter to enable the Beta feature flag based on the chance defined by the Percentage parameter.

1. Add a RandomFilter.cs file with the following code.

   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);
           }
       }
   }
   

   You added a RandomFilter class that 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.

   You decorated a FilterAliasAttribute to the RandomFilter to give your filter an alias Random, which matches the filter name you set in the Beta feature flag in Azure App Configuration.

2. Open the Program.cs file and register the RandomFilter by calling the AddFeatureFilter method.

   // The rest of existing code in Program.cs
   // ... ...
   
   // Add feature management to the container of services.
   builder.Services.AddFeatureManagement()
                   .AddFeatureFilter<RandomFilter>();
   
   // The rest of existing code in Program.cs
   // ... ...
   

Feature filter in action

Relaunch the application and refresh the browser a few times. Without manually toggling the feature flag, you will see that the Beta menu sometimes appears and sometimes doesn't.

Next steps

To learn more about the built-in feature filters, continue to the following documents.

For the full feature rundown of the .NET feature management library, continue to the following document.