Use assembly name of global.asax type as default IHostEnvironment.ApplicationName

src/Microsoft.AspNetCore.SystemWebAdapters.FrameworkServices/Hosting/ConfigurationManagerConfigExtensions.cs

@@ -1,9 +1,12 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using System.CodeDom.Compiler;
 using System.Collections;
 using System.Configuration;
 using System.Diagnostics;
+using System.Reflection;
+using System.Web;
 using System.Web.Hosting;
 using Microsoft.Extensions.Configuration;
 using Microsoft.Extensions.Configuration.Memory;
@@ -50,33 +53,65 @@ public static IConfigurationManager UseHostingEnvironmentFallback(this IConfigur
         if (configuration[HostDefaults.ApplicationKey] is null)
         {
             optionList = new();
-            optionList.Add(new KeyValuePair<string, string?>(HostDefaults.ApplicationKey, HostingEnvironment.SiteName));
+            optionList.Add(new(HostDefaults.ApplicationKey, GetDefaultApplicationName()));
         }
 
         if (configuration[HostDefaults.EnvironmentKey] is null && (HostingEnvironment.IsDevelopmentEnvironment || IsIISExpress()))
         {
             optionList ??= new();
-            optionList.Add(new KeyValuePair<string, string?>(HostDefaults.EnvironmentKey, Environments.Development));
+            optionList.Add(new(HostDefaults.EnvironmentKey, Environments.Development));
         }
 
         if (configuration[HostDefaults.ContentRootKey] is null)
         {
             optionList ??= new();
-            optionList.Add(new KeyValuePair<string, string?>(HostDefaults.ContentRootKey, HostingEnvironment.ApplicationPhysicalPath));
+            optionList.Add(new(HostDefaults.ContentRootKey, HostingEnvironment.ApplicationPhysicalPath));
         }
 
         if (optionList is not null)
         {
             configuration.AddInMemoryCollection(optionList);
         }
 
+        return configuration;
+
+        /// <summary>
+        /// Does a best effort to get the name of the assembly that was launched by trying to find it via the active <see cref="HttpApplication"/>.
+        /// By using the name of the assembly, it matches ASP.NET Core behavior and allows things like UserSecrets to work which use that name to
+        /// identify where the secrets are.
+        /// </summary>
+        static string GetDefaultApplicationName()
+        {
+            // When called in HttpApplication.Application_Start, there is an initial HttpContext.Current that is set even though we're not in an actual request
+            if (HttpContext.Current is { ApplicationInstance: { } app })
+            {
+                var type = app.GetType();
+
+                // The ApplicationInstance is generally a derived one generated by ASP.NET, so we need to check base types until we find the real one.
+                // The generated ones are marked with the GeneratedCodeAttribute, so we can check for that and skip them.
+                while (type.Assembly.GetCustomAttribute<GeneratedCodeAttribute>() is { } && type.BaseType.GetType() != typeof(HttpApplication))
+                {
+                    type = type.BaseType;
+                }
+
+                var name = type.Assembly.GetName().Name;
+
+                if (type != typeof(HttpApplication) && !string.IsNullOrEmpty(name))
+                {
+                    return name;
+                }
+            }
+
+            // Fallback in case HttpContext.Current is not available; most likely this will be the case in the test environment where the HttpApplication
+            // is not starting up
+            return HostingEnvironment.SiteName ?? AppDomain.CurrentDomain.FriendlyName;
+        }
+
         static bool IsIISExpress()
         {
             using var currentProcess = Process.GetCurrentProcess();
 
             return string.Equals(currentProcess.ProcessName, "iisexpress", StringComparison.Ordinal);
         }
-
-        return configuration;
     }
 }