Make custom ApplicationName work for hosting and user secrets

src/DefaultBuilder/src/WebApplicationBuilder.cs

@@ -275,15 +275,13 @@ private static void ApplyDefaultAppConfigurationSlim(IHostEnvironment env, Confi
 
         if (env.IsDevelopment() && env.ApplicationName is { Length: > 0 })
         {
-            try
+            var appAssembly = GetAppAssembly(env.ApplicationName);
+
+            // If the assembly cannot be found, so just skip it.
+            if (appAssembly is not null)
             {
-                var appAssembly = Assembly.Load(new AssemblyName(env.ApplicationName));
                 configuration.AddUserSecrets(appAssembly, optional: true, reloadOnChange: reloadOnChange);
             }
-            catch (FileNotFoundException)
-            {
-                // The assembly cannot be found, so just skip it.
-            }
         }
 
         configuration.AddEnvironmentVariables();
@@ -306,6 +304,18 @@ static bool GetReloadConfigOnChangeValue(ConfigurationManager configuration)
             }
             return result;
         }
+
+        static Assembly? GetAppAssembly(string applicationName)
+        {
+            try
+            {
+                return Assembly.Load(new AssemblyName(applicationName));
+            }
+            catch
+            {
+                return Assembly.GetEntryAssembly();
+            }
+        }
     }
 
     private static void AddDefaultServicesSlim(ConfigurationManager configuration, IServiceCollection services)

src/Hosting/Hosting/src/Internal/WebHostOptions.cs

@@ -30,7 +30,7 @@ public WebHostOptions(IConfiguration primaryConfiguration, IConfiguration? fallb
         PreferHostingUrls = WebHostUtilities.ParseBool(GetConfig(WebHostDefaults.PreferHostingUrlsKey));
 
         // Search the primary assembly and configured assemblies.
-        HostingStartupAssemblies = Split(ApplicationName, GetConfig(WebHostDefaults.HostingStartupAssembliesKey));
+        HostingStartupAssemblies = Split(GetHostingStartupAssemblyName(ApplicationName), GetConfig(WebHostDefaults.HostingStartupAssembliesKey));
         HostingStartupExcludeAssemblies = Split(GetConfig(WebHostDefaults.HostingStartupExcludeAssembliesKey));
 
         var timeout = GetConfig(WebHostDefaults.ShutdownTimeoutKey);
@@ -77,16 +77,29 @@ public IEnumerable<string> GetFinalHostingStartupAssemblies()
     private static IReadOnlyList<string> Split(string? value)
     {
         return value?.Split(';', StringSplitOptions.TrimEntries | StringSplitOptions.RemoveEmptyEntries)
-            ?? Array.Empty<string>();
+            ?? [];
     }
 
     private static IReadOnlyList<string> Split(string applicationName, string? environment)
     {
         if (string.IsNullOrEmpty(environment))
         {
-            return new[] { applicationName };
+            return [applicationName];
         }
 
         return Split($"{applicationName};{environment}");
     }
+
+    private static string GetHostingStartupAssemblyName(string applicationName)
+    {
+        try
+        {
+            _ = Assembly.Load(new AssemblyName(applicationName));
+            return applicationName;
+        }
+        catch
+        {
+            return Assembly.GetEntryAssembly()?.GetName().Name ?? "";
+        }
+    }
 }

src/Hosting/Hosting/test/WebHostBuilderTests.cs

@@ -801,6 +801,24 @@ public void DefaultApplicationNameWithUseStartupFactory(IWebHostBuilder builder)
         }
     }
 
+    [Theory]
+    [MemberData(nameof(DefaultWebHostBuilders))]
+    public void CustomApplicationName(IWebHostBuilder builder)
+    {
+        const string AppName = "MyApp";
+
+        using (var host = builder
+            .UseServer(new TestServer())
+            .UseStartup<Startup>()
+            .UseSetting(WebHostDefaults.ApplicationKey, AppName)
+            .Build())
+        {
+            var hostingEnv = host.Services.GetService<IHostEnvironment>();
+
+            Assert.Equal(AppName, hostingEnv.ApplicationName);
+        }
+    }
+
     [Theory]
     [MemberData(nameof(DefaultWebHostBuilders))]
     public void Configure_SupportsNonStaticMethodDelegate(IWebHostBuilder builder)

src/Mvc/test/Mvc.FunctionalTests/SimpleWithWebApplicationBuilderTests.cs

@@ -146,6 +146,19 @@ public async Task DefaultEnvironment_Is_Development()
         Assert.Equal(expected, content);
     }
 
+    [Fact]
+    public async Task DefaultApplicationName_Is_Name_Of_EntryAssembly()
+    {
+        // Arrange
+        var expected = typeof(SimpleWebSiteWithWebApplicationBuilder.Program).Assembly.GetName().Name;
+
+        // Act
+        var content = await Client.GetStringAsync("http://localhost/appname");
+
+        // Assert
+        Assert.Equal(expected, content);
+    }
+
     [Fact]
     public async Task Configuration_Can_Be_Overridden()
     {
@@ -192,6 +205,25 @@ public async Task Environment_Can_Be_Overridden()
         Assert.Equal(expected, content);
     }
 
+    [Fact]
+    public async Task ApplicationName_Can_Be_Overridden()
+    {
+        // Arrange
+        var fixture = _fixture.WithWebHostBuilder(builder =>
+        {
+            builder.UseSetting(WebHostDefaults.ApplicationKey, "MyApp");
+        });
+
+        var expected = "MyApp";
+        using var client = fixture.CreateDefaultClient();
+
+        // Act
+        var content = await client.GetStringAsync("http://localhost/appname");
+
+        // Assert
+        Assert.Equal(expected, content);
+    }
+
     [Fact]
     public async Task WebRoot_Can_Be_Overriden()
     {

src/Mvc/test/WebSites/SimpleWebSiteWithWebApplicationBuilder/Program.cs

@@ -39,6 +39,7 @@
 
 app.MapGet("/problem", () => Results.Problem("Some problem"));
 
+app.MapGet("/appname", (IHostEnvironment environment) => environment.ApplicationName);
 app.MapGet("/environment", (IHostEnvironment environment) => environment.EnvironmentName);
 app.MapGet("/webroot", (IWebHostEnvironment environment) => environment.WebRootPath);