Add support for application-specific settings files in Host defaults

src/libraries/Microsoft.Extensions.Hosting/src/HostApplicationBuilder.cs

@@ -42,6 +42,7 @@ public sealed class HostApplicationBuilder : IHostApplicationBuilder
         ///     <item><description>load host <see cref="IConfiguration"/> from "DOTNET_" prefixed environment variables</description></item>
         ///     <item><description>load host <see cref="IConfiguration"/> from supplied command line args</description></item>
         ///     <item><description>load app <see cref="IConfiguration"/> from 'appsettings.json' and 'appsettings.[<see cref="IHostEnvironment.EnvironmentName"/>].json'</description></item>
+        ///     <item><description>load app <see cref="IConfiguration"/> from '[<see cref="IHostEnvironment.ApplicationName"/>].settings.json' and '[<see cref="IHostEnvironment.ApplicationName"/>].settings.[<see cref="IHostEnvironment.EnvironmentName"/>].json' when <see cref="IHostEnvironment.ApplicationName"/> is not empty</description></item>
         ///     <item><description>load app <see cref="IConfiguration"/> from User Secrets when <see cref="IHostEnvironment.EnvironmentName"/> is 'Development' using the entry assembly</description></item>
         ///     <item><description>load app <see cref="IConfiguration"/> from environment variables</description></item>
         ///     <item><description>load app <see cref="IConfiguration"/> from supplied command line args</description></item>
@@ -64,6 +65,7 @@ public HostApplicationBuilder()
         ///     <item><description>load host <see cref="IConfiguration"/> from "DOTNET_" prefixed environment variables</description></item>
         ///     <item><description>load host <see cref="IConfiguration"/> from supplied command line args</description></item>
         ///     <item><description>load app <see cref="IConfiguration"/> from 'appsettings.json' and 'appsettings.[<see cref="IHostEnvironment.EnvironmentName"/>].json'</description></item>
+        ///     <item><description>load app <see cref="IConfiguration"/> from '[<see cref="IHostEnvironment.ApplicationName"/>].settings.json' and '[<see cref="IHostEnvironment.ApplicationName"/>].settings.[<see cref="IHostEnvironment.EnvironmentName"/>].json' when <see cref="IHostEnvironment.ApplicationName"/> is not empty</description></item>
         ///     <item><description>load app <see cref="IConfiguration"/> from User Secrets when <see cref="IHostEnvironment.EnvironmentName"/> is 'Development' using the entry assembly</description></item>
         ///     <item><description>load app <see cref="IConfiguration"/> from environment variables</description></item>
         ///     <item><description>load app <see cref="IConfiguration"/> from supplied command line args</description></item>

src/libraries/Microsoft.Extensions.Hosting/src/HostingHostBuilderExtensions.cs

@@ -182,6 +182,7 @@ public static IHostBuilder ConfigureContainer<TContainerBuilder>(this IHostBuild
         ///     * load host <see cref="IConfiguration"/> from "DOTNET_" prefixed environment variables
         ///     * load host <see cref="IConfiguration"/> from supplied command line args
         ///     * load app <see cref="IConfiguration"/> from 'appsettings.json' and 'appsettings.[<see cref="IHostEnvironment.EnvironmentName"/>].json'
+        ///     * load app <see cref="IConfiguration"/> from '[<see cref="IHostEnvironment.ApplicationName"/>].settings.json' and '[<see cref="IHostEnvironment.ApplicationName"/>].settings.[<see cref="IHostEnvironment.EnvironmentName"/>].json' when <see cref="IHostEnvironment.ApplicationName"/> is not empty
         ///     * load app <see cref="IConfiguration"/> from User Secrets when <see cref="IHostEnvironment.EnvironmentName"/> is 'Development' using the entry assembly
         ///     * load app <see cref="IConfiguration"/> from environment variables
         ///     * load app <see cref="IConfiguration"/> from supplied command line args
@@ -241,6 +242,14 @@ internal static void ApplyDefaultAppConfiguration(HostBuilderContext hostingCont
             appConfigBuilder.AddJsonFile("appsettings.json", optional: true, reloadOnChange: reloadOnChange)
                     .AddJsonFile($"appsettings.{env.EnvironmentName}.json", optional: true, reloadOnChange: reloadOnChange);
 
+            if (env.ApplicationName is { Length: > 0 })
+            {
+                string sanitizedApplicationName = env.ApplicationName.Replace(Path.DirectorySeparatorChar, '_')
+                                                                    .Replace(Path.AltDirectorySeparatorChar, '_');
+                appConfigBuilder.AddJsonFile($"{sanitizedApplicationName}.settings.json", optional: true, reloadOnChange: reloadOnChange)
+                        .AddJsonFile($"{sanitizedApplicationName}.settings.{env.EnvironmentName}.json", optional: true, reloadOnChange: reloadOnChange);
+            }
+
             if (env.IsDevelopment() && env.ApplicationName is { Length: > 0 })
             {
                 try

src/libraries/Microsoft.Extensions.Hosting/tests/UnitTests/HostBuilderTests.cs

@@ -826,6 +826,160 @@ private class ServiceC
             public ServiceC(ServiceD serviceD) { }
         }
 
+        [Fact]
+        public void ConfigureDefaults_LoadsApplicationSpecificSettings()
+        {
+            using TempDirectory tempDir = new();
+            string appSettingsPath = Path.Combine(tempDir.Path, "testapp.settings.json");
+            string appSettingsEnvPath = Path.Combine(tempDir.Path, "testapp.settings.Production.json");
+
+            // Create test configuration files
+            File.WriteAllText(appSettingsPath, """{"TestKey": "AppValue"}""");
+            File.WriteAllText(appSettingsEnvPath, """{"TestKey": "AppEnvValue", "EnvKey": "EnvValue"}""");
+
+            using var host = new HostBuilder()
+                .ConfigureDefaults(args: null)
+                .UseContentRoot(tempDir.Path)
+                .ConfigureHostConfiguration(config =>
+                {
+                    config.AddInMemoryCollection(new[]
+                    {
+                        new KeyValuePair<string, string>(HostDefaults.ApplicationKey, "testapp"),
+                        new KeyValuePair<string, string>(HostDefaults.EnvironmentKey, "Production")
+                    });
+                })
+                .Build();
+
+            var configuration = host.Services.GetRequiredService<IConfiguration>();
+
+            // Verify that app-specific environment settings override app-specific settings
+            Assert.Equal("AppEnvValue", configuration["TestKey"]);
+            Assert.Equal("EnvValue", configuration["EnvKey"]);
+        }
+
+        [Fact]
+        public void ConfigureDefaults_LoadsApplicationSpecificSettings_WithDevelopmentEnvironment()
+        {
+            using TempDirectory tempDir = new();
+            string appSettingsPath = Path.Combine(tempDir.Path, "testapp.settings.json");
+            string appSettingsEnvPath = Path.Combine(tempDir.Path, "testapp.settings.Production.json");
+
+            // Create test configuration files
+            File.WriteAllText(appSettingsPath, """{"TestKey": "AppValue"}""");
+            File.WriteAllText(appSettingsEnvPath, """{"TestKey": "ProductionValue", "ProductionKey": "ProductionValue"}""");
+
+            using var host = new HostBuilder()
+                .ConfigureDefaults(args: null)
+                .UseContentRoot(tempDir.Path)
+                .ConfigureHostConfiguration(config =>
+                {
+                    config.AddInMemoryCollection(new[]
+                    {
+                        new KeyValuePair<string, string>(HostDefaults.ApplicationKey, "testapp"),
+                        new KeyValuePair<string, string>(HostDefaults.EnvironmentKey, "Development")
+                    });
+                })
+                .Build();
+
+            var configuration = host.Services.GetRequiredService<IConfiguration>();
+
+            // Verify that Production-specific file is not loaded when running in Development environment
+            Assert.Equal("AppValue", configuration["TestKey"]); // Should come from base settings, not Production
+            Assert.Null(configuration["ProductionKey"]); // Should not be loaded from Production file
+        }
+
+        [Fact]
+        public void ConfigureDefaults_DoesNotLoadApplicationSpecificSettings_WhenApplicationNameIsEmpty()
+        {
+            using TempDirectory tempDir = new();
+            string appSettingsPath = Path.Combine(tempDir.Path, ".settings.json");
+
+            // Create test configuration file that should NOT be loaded
+            File.WriteAllText(appSettingsPath, """{"TestKey": "ShouldNotBeLoaded"}""");
+
+            using var host = new HostBuilder()
+                .ConfigureDefaults(args: null)
+                .UseContentRoot(tempDir.Path)
+                .ConfigureHostConfiguration(config =>
+                {
+                    config.AddInMemoryCollection(new[]
+                    {
+                        new KeyValuePair<string, string>(HostDefaults.ApplicationKey, ""),
+                        new KeyValuePair<string, string>(HostDefaults.EnvironmentKey, "Production")
+                    });
+                })
+                .Build();
+
+            var configuration = host.Services.GetRequiredService<IConfiguration>();
+
+            // Verify that app-specific settings are not loaded when ApplicationName is empty
+            Assert.Null(configuration["TestKey"]);
+        }
+
+        [Fact]
+        public void ConfigureDefaults_ReplacesPathSeparatorsInApplicationName()
+        {
+            using TempDirectory tempDir = new();
+            string appSettingsPath = Path.Combine(tempDir.Path, "my_app.settings.json");
+
+            // Create test configuration file with path separators replaced by underscores
+            File.WriteAllText(appSettingsPath, """{"TestKey": "PathSeparatorValue"}""");
+
+            using var host = new HostBuilder()
+                .ConfigureDefaults(args: null)
+                .UseContentRoot(tempDir.Path)
+                .ConfigureHostConfiguration(config =>
+                {
+                    config.AddInMemoryCollection(new[]
+                    {
+                        new KeyValuePair<string, string>(HostDefaults.ApplicationKey, "my/app"),
+                        new KeyValuePair<string, string>(HostDefaults.EnvironmentKey, "Production")
+                    });
+                })
+                .Build();
+
+            var configuration = host.Services.GetRequiredService<IConfiguration>();
+            var hostEnvironment = host.Services.GetRequiredService<IHostEnvironment>();
+
+            // Verify that ApplicationName retains original value (not sanitized)
+            Assert.Equal("my/app", hostEnvironment.ApplicationName);
+
+            // Verify that path separators are replaced with underscores for file loading
+            Assert.Equal("PathSeparatorValue", configuration["TestKey"]);
+        }
+
+        [Fact]
+        public void ConfigureDefaults_ApplicationSpecificSettingsOverrideAppSettings()
+        {
+            using TempDirectory tempDir = new();
+            string appSettingsPath = Path.Combine(tempDir.Path, "appsettings.json");
+            string appSpecificSettingsPath = Path.Combine(tempDir.Path, "myapp.settings.json");
+
+            // Create test configuration files
+            File.WriteAllText(appSettingsPath, """{"SharedKey": "AppSettingsValue", "AppKey": "AppSettingsOnly"}""");
+            File.WriteAllText(appSpecificSettingsPath, """{"SharedKey": "AppSpecificValue", "SpecificKey": "AppSpecificOnly"}""");
+
+            using var host = new HostBuilder()
+                .ConfigureDefaults(args: null)
+                .UseContentRoot(tempDir.Path)
+                .ConfigureHostConfiguration(config =>
+                {
+                    config.AddInMemoryCollection(new[]
+                    {
+                        new KeyValuePair<string, string>(HostDefaults.ApplicationKey, "myapp"),
+                        new KeyValuePair<string, string>(HostDefaults.EnvironmentKey, "Development")
+                    });
+                })
+                .Build();
+
+            var configuration = host.Services.GetRequiredService<IConfiguration>();
+
+            // Verify that app-specific settings override general appsettings
+            Assert.Equal("AppSpecificValue", configuration["SharedKey"]);
+            Assert.Equal("AppSettingsOnly", configuration["AppKey"]);
+            Assert.Equal("AppSpecificOnly", configuration["SpecificKey"]);
+        }
+
         internal class ServiceD { }
 
         internal class ServiceA { }

src/libraries/Microsoft.Extensions.Hosting/tests/UnitTests/Microsoft.Extensions.Hosting.Unit.Tests.csproj

@@ -9,6 +9,7 @@
   </PropertyGroup>
 
   <ItemGroup>
+    <Compile Include="$(CommonTestPath)System\IO\TempDirectory.cs" Link="Common\System\IO\TempDirectory.cs" />
     <Content Include="testroot\**\*" CopyToOutputDirectory="PreserveNewest" />
     <Content Include="appSettings.json" CopyToOutputDirectory="PreserveNewest" />
   </ItemGroup>