Simple Feature Flag Service - blob post https://devblog.gosso.se/?p=1410

FeatureFlagService.cs

using AlloyTemplates.Business;
using EPiServer;
using EPiServer.Core;
using EPiServer.Personalization.VisitorGroups;
using EPiServer.ServiceLocation;
using EPiServer.Web;
using System;
using System.Configuration;
using System.Reflection;
using System.Web;

namespace Gosso.Features.FeatureFlags
{
    [ServiceConfiguration(typeof(IFeatureFlagService), Lifecycle = ServiceInstanceScope.Hybrid)]
    public class FeatureFlagService : IFeatureFlagService
    {
        protected const string PreFixKey = "FeatureFlag";
        protected readonly string Flag1Key = $"{PreFixKey}:Flag1";
        private ISiteSettings siteSettings;
        private bool _flag1 = false;//the default value is set here
        private readonly IContentLoader _contentLoader;
        public FeatureFlagService(IContentLoader contentLoader)
        {
            _contentLoader = contentLoader;
        }

        private void EvalRules()
        {
            //get the ISiteSettings each time to get the latest updated values
            siteSettings = _contentLoader.Get<ISiteSettings>(SiteDefinition.Current.StartPage, LanguageSelector.AutoDetect());

            //Rule one, appsettings default
            _flag1 = (ConfigurationManager.AppSettings[Flag1Key] == "true");

            //Rule two, from site settings
            if (siteSettings.NewFeatureEnabled)
                _flag1 = siteSettings.NewFeatureEnabled;

            //Rule Three, cookies per browser is set by using querystring  ?featureflag:flag1=true
            if (HttpContext.Current != null)
            {
                string cookieValue = HttpContext.Current.Request.Cookies.Get(Flag1Key)?.Value;
                _flag1 = (cookieValue == "true") ? true : (cookieValue == "false") ? false : _flag1;
            }

            //rule four, if there exists a visitor group with this name and its true, use it.
            _flag1 = EPiServer.Security.PrincipalInfo.CurrentPrincipal.IsInRole("FeatureFlag:Flag1", EPiServer.Security.SecurityEntityType.VisitorGroup) == true ? true: _flag1;
        }

        public bool IsFlag1Enabled { get { EvalRules(); return _flag1; } set { _flag1 = value; } }

        /// <summary>
        /// Used to set FeatureFlag Cookie, is called from a PageViewContextFactory or IResultFilter or what ever a page layout model is created
        /// </summary>
        /// <param name="key"></param>
        /// <param name="value"></param>
        public void SetValue(string key, bool value)
        {
            Type t = typeof(FeatureFlagService);
            string propName = "_" + key;
            var field = t.GetField(propName.ToLowerInvariant(), BindingFlags.NonPublic | BindingFlags.Instance);
            if (field != null)// Change the private property value if it exists, also dont set cookie if no flag exists
            {
                field.SetValue(this, value);
                if (HttpContext.Current != null) // set session persistant cookie
                {
                    var c = new HttpCookie($"{PreFixKey}:{key}".ToLowerInvariant(), value.ToString().ToLower()) { /*Expires = DateTime.UtcNow.AddDays(7)*/ };
                    HttpContext.Current.Response.Cookies.Set(c);
                }
            }
        }
    }

    public interface IFeatureFlagService
    {
        bool IsFlag1Enabled { get; set; }
        void SetValue(string key, bool value);
    }
}



//call this from Creation of LayoutModel or IResultFilter
SaveFeatureFlags(requestContext.HttpContext.Request.QueryString);
/// <summary>
/// checks if querystring key if feature flag
/// </summary>
/// <param name="q"></param>
private void SaveFeatureFlags(NameValueCollection q)
{
    if (q.AllKeys.Any(x => x != null && x.ToLowerInvariant().StartsWith("featureflag:")))
    {
        q.AllKeys
            .Where(x => x != null && x.ToLowerInvariant().StartsWith("featureflag:"))
            .Select(x => SaveFeatureValue(new { key = x.ToLowerInvariant().Replace("featureflag:", ""), value = q[x] }))
            .ToList();
    }
}

private string SaveFeatureValue(dynamic value)
{
    _featureFlagService.SetValue(value.key, value.value == "true");
    return value.key;
}