Add HttpApplication.VaryBy

src/Microsoft.AspNetCore.SystemWebAdapters.CoreServices/HttpApplication/HttpApplicationExtensions.cs

@@ -58,15 +58,6 @@ public static ISystemWebAdapterBuilder AddHttpApplication<TApp>(this ISystemWebA
         return builder;
     }
 
-    internal static void UseHttpApplication(this IApplicationBuilder app)
-    {
-        if (app.AreHttpApplicationEventsRequired())
-        {
-            app.UseMiddleware<HttpApplicationMiddleware>();
-            app.UseHttpApplicationEvent(ApplicationEvent.BeginRequest);
-        }
-    }
-
     internal static void UseHttpApplicationEvent(this IApplicationBuilder app, params ApplicationEvent[] preEvents)
         => app.UseHttpApplicationEvent(preEvents, Array.Empty<ApplicationEvent>());
 

src/Microsoft.AspNetCore.SystemWebAdapters.CoreServices/HttpApplication/HttpApplicationOptions.cs

@@ -5,7 +5,6 @@
 using System.Collections;
 using System.Collections.Generic;
 using System.Diagnostics.CodeAnalysis;
-using System.Globalization;
 using System.Web;
 
 using static System.FormattableString;

src/Microsoft.AspNetCore.SystemWebAdapters.CoreServices/SystemWebAdaptersExtensions.cs

@@ -44,15 +44,11 @@ internal static void UseSystemWebAdapterFeatures(this IApplicationBuilder app)
             return;
         }
 
-        app.UseMiddleware<RegisterAdapterFeaturesMiddleware>();
         app.UseMiddleware<PreBufferRequestStreamMiddleware>();
         app.UseMiddleware<BufferResponseStreamMiddleware>();
-
         app.UseMiddleware<SetDefaultResponseHeadersMiddleware>();
         app.UseMiddleware<SingleThreadedRequestMiddleware>();
         app.UseMiddleware<CurrentPrincipalMiddleware>();
-
-        app.UseHttpApplication();
     }
 
     public static void UseSystemWebAdapters(this IApplicationBuilder app)
@@ -122,6 +118,13 @@ public Action<IApplicationBuilder> Configure(Action<IApplicationBuilder> next)
             => builder =>
             {
                 builder.UseMiddleware<SetHttpContextTimestampMiddleware>();
+                builder.UseMiddleware<RegisterAdapterFeaturesMiddleware>();
+
+                if (builder.AreHttpApplicationEventsRequired())
+                {
+                    builder.UseMiddleware<HttpApplicationMiddleware>();
+                    builder.UseHttpApplicationEvent(ApplicationEvent.BeginRequest);
+                }
 
                 next(builder);
             };

src/Microsoft.AspNetCore.SystemWebAdapters/Generated/Ref.Standard.cs

@@ -56,6 +56,7 @@ public event System.EventHandler ResolveRequestCache { add { } remove { } }
         public event System.EventHandler UpdateRequestCache { add { } remove { } }
         public void CompleteRequest() { throw new System.PlatformNotSupportedException("Only supported when running on ASP.NET Core or System.Web");}
         public void Dispose() { }
+        public virtual string GetVaryByCustomString(System.Web.HttpContext context, string custom) { throw new System.PlatformNotSupportedException("Only supported when running on ASP.NET Core or System.Web");}
     }
     public sealed partial class HttpApplicationState : System.Collections.Specialized.NameObjectCollectionBase
     {

src/Microsoft.AspNetCore.SystemWebAdapters/HttpApplication.cs

@@ -62,6 +62,16 @@ public HttpContext Context
 
     public void CompleteRequest() => Context.Response.End();
 
+    public virtual string? GetVaryByCustomString(HttpContext context, string custom)
+    {
+        if (string.Equals(custom, "browser", StringComparison.OrdinalIgnoreCase))
+        {
+            return context?.Request.Browser.Type;
+        }
+
+        return null;
+    }
+
     public event EventHandler? BeginRequest
     {
         add => AddEvent(value);

test/Microsoft.AspNetCore.SystemWebAdapters.CoreServices.Tests/Modules/ModuleTests.cs

@@ -125,6 +125,7 @@ private static async Task<List<string>> RunAsync(string action, string eventName
                     })
                     .ConfigureServices(services =>
                     {
+                        services.AddSingleton<IStartupFilter>(new ModuleTestStartup(notifier, action));
                         services.AddRouting();
                         services.AddSystemWebAdapters()
                             .AddHttpApplication(options =>
@@ -137,17 +138,6 @@ private static async Task<List<string>> RunAsync(string action, string eventName
                     {
                         app.UseRouting();
 
-                        app.Use(async (ctx, next) =>
-                        {
-                            ctx.Features.Set(notifier);
-                            try
-                            {
-                                await next(ctx);
-                            }
-                            catch (InvalidOperationException) when (action == ModuleTestModule.Throw)
-                            {
-                            }
-                        });
                         app.UseAuthenticationEvents();
                         app.UseAuthorizationEvents();
                         app.UseSystemWebAdapters();
@@ -168,6 +158,37 @@ private sealed class NotificationCollection : List<string>
     {
     }
 
+    private sealed class ModuleTestStartup : IStartupFilter
+    {
+        private readonly NotificationCollection _collection;
+        private readonly string _action;
+
+        public ModuleTestStartup(NotificationCollection collection, string action)
+        {
+            _collection = collection;
+            _action = action;
+        }
+
+        public Action<IApplicationBuilder> Configure(Action<IApplicationBuilder> next)
+            => builder =>
+            {
+                builder.Use(async (ctx, next) =>
+                {
+                    ctx.Features.Set(_collection);
+
+                    try
+                    {
+                        await next(ctx);
+                    }
+                    catch (InvalidOperationException) when (_action == ModuleTestModule.Throw)
+                    {
+                    }
+                });
+
+                next(builder);
+            };
+    }
+
     private sealed class ModuleTestModule : EventsModule
     {
         protected override void InvokeEvent(HttpContext context, string name)