Add a way to prevent specific hosting startup from runnning (#1243)

src/Microsoft.AspNetCore.Hosting.Abstractions/WebHostDefaults.cs

@@ -8,6 +8,7 @@ namespace Microsoft.AspNetCore.Hosting
         public static readonly string ApplicationKey = "applicationName";
         public static readonly string StartupAssemblyKey = "startupAssembly";
         public static readonly string HostingStartupAssembliesKey = "hostingStartupAssemblies";
+        public static readonly string HostingStartupExcludeAssembliesKey = "hostingStartupExcludeAssemblies";
 
         public static readonly string DetailedErrorsKey = "detailedErrors";
         public static readonly string EnvironmentKey = "environment";

src/Microsoft.AspNetCore.Hosting/Internal/WebHost.cs

@@ -135,7 +135,7 @@ namespace Microsoft.AspNetCore.Hosting.Internal
             // Log the fact that we did load hosting startup assemblies.
             if (_logger.IsEnabled(LogLevel.Debug))
             {
-                foreach (var assembly in _options.HostingStartupAssemblies)
+                foreach (var assembly in _options.GetFinalHostingStartupAssemblies())
                 {
                     _logger.LogDebug("Loaded hosting startup assembly {assemblyName}", assembly);
                 }

src/Microsoft.AspNetCore.Hosting/Internal/WebHostOptions.cs

@@ -4,6 +4,7 @@
 using System;
 using System.Collections.Generic;
 using System.Globalization;
+using System.Linq;
 using Microsoft.Extensions.Configuration;
 
 namespace Microsoft.AspNetCore.Hosting.Internal
@@ -33,8 +34,8 @@ namespace Microsoft.AspNetCore.Hosting.Internal
             SuppressStatusMessages = WebHostUtilities.ParseBool(configuration, WebHostDefaults.SuppressStatusMessagesKey);
 
             // Search the primary assembly and configured assemblies.
-            HostingStartupAssemblies = $"{ApplicationName};{configuration[WebHostDefaults.HostingStartupAssembliesKey]}"
-                .Split(new[] { ';' }, StringSplitOptions.RemoveEmptyEntries) ?? new string[0];
+            HostingStartupAssemblies = Split($"{ApplicationName};{configuration[WebHostDefaults.HostingStartupAssembliesKey]}");
+            HostingStartupExcludeAssemblies = Split(configuration[WebHostDefaults.HostingStartupExcludeAssembliesKey]);
 
             var timeout = configuration[WebHostDefaults.ShutdownTimeoutKey];
             if (!string.IsNullOrEmpty(timeout)
@@ -52,6 +53,8 @@ namespace Microsoft.AspNetCore.Hosting.Internal
 
         public IReadOnlyList<string> HostingStartupAssemblies { get; set; }
 
+        public IReadOnlyList<string> HostingStartupExcludeAssemblies { get; set; }
+
         public bool DetailedErrors { get; set; }
 
         public bool CaptureStartupErrors { get; set; }
@@ -66,5 +69,28 @@ namespace Microsoft.AspNetCore.Hosting.Internal
 
         public TimeSpan ShutdownTimeout { get; set; } = TimeSpan.FromSeconds(5);
 
+        public IEnumerable<string> GetFinalHostingStartupAssemblies()
+        {
+            return HostingStartupAssemblies.Except(HostingStartupExcludeAssemblies, StringComparer.OrdinalIgnoreCase);
+        }
+
+        private IReadOnlyList<string> Split(string value)
+        {
+            if (string.IsNullOrWhiteSpace(value))
+            {
+                return Array.Empty<string>();
+            }
+
+            var list = new List<string>();
+            foreach (var part in value.Split(new[] { ';' }, StringSplitOptions.RemoveEmptyEntries))
+            {
+                var trimmedPart = part;
+                if (!string.IsNullOrEmpty(trimmedPart))
+                {
+                    list.Add(trimmedPart);
+                }
+            }
+            return list;
+        }
     }
 }

src/Microsoft.AspNetCore.Hosting/WebHostBuilder.cs

@@ -176,7 +176,7 @@ namespace Microsoft.AspNetCore.Hosting
 
             var logger = hostingServiceProvider.GetRequiredService<ILogger<WebHost>>();
             // Warn about duplicate HostingStartupAssemblies
-            foreach (var assemblyName in _options.HostingStartupAssemblies.GroupBy(a => a, StringComparer.OrdinalIgnoreCase).Where(g => g.Count() > 1))
+            foreach (var assemblyName in _options.GetFinalHostingStartupAssemblies().GroupBy(a => a, StringComparer.OrdinalIgnoreCase).Where(g => g.Count() > 1))
             {
                 logger.LogWarning($"The assembly {assemblyName} was specified multiple times. Hosting startup assemblies should only be specified once.");
             }
@@ -206,7 +206,7 @@ namespace Microsoft.AspNetCore.Hosting
                 var exceptions = new List<Exception>();
 
                 // Execute the hosting startup assemblies
-                foreach (var assemblyName in _options.HostingStartupAssemblies.Distinct(StringComparer.OrdinalIgnoreCase))
+                foreach (var assemblyName in _options.GetFinalHostingStartupAssemblies().Distinct(StringComparer.OrdinalIgnoreCase))
                 {
                     try
                     {

test/Microsoft.AspNetCore.Hosting.Tests/WebHostBuilderTests.cs

@@ -779,6 +779,23 @@ namespace Microsoft.AspNetCore.Hosting
             }
         }
 
+        [Fact]
+        public void Build_HostingStartupAssemblyCanBeExcluded()
+        {
+            var builder = CreateWebHostBuilder()
+                .CaptureStartupErrors(false)
+                .UseSetting(WebHostDefaults.HostingStartupAssembliesKey, typeof(TestStartupAssembly1.TestHostingStartup1).GetTypeInfo().Assembly.FullName)
+                .UseSetting(WebHostDefaults.HostingStartupExcludeAssembliesKey, typeof(TestStartupAssembly1.TestHostingStartup1).GetTypeInfo().Assembly.FullName)
+                .Configure(app => { })
+                .UseServer(new TestServer());
+
+            using (var host = builder.Build())
+            {
+                Assert.Null(builder.GetSetting("testhostingstartup1"));
+                Assert.Equal("0", builder.GetSetting("testhostingstartup_chain"));
+            }
+        }
+
         [Fact]
         public void Build_ConfigureLoggingInHostingStartupWorks()
         {