Support hosting at non-root URL. Prove it by updating BasicTestApp to serve from non-root location.

src/Microsoft.AspNetCore.Blazor.Browser.JS/src/Boot.ts

@@ -20,7 +20,7 @@ async function boot() {
   // Determine the URLs of the assemblies we want to load
   const loadAssemblyUrls = [entryPointDll]
     .concat(referenceAssemblies)
-    .map(filename => `/_framework/_bin/${filename}`);
+    .map(filename => `_framework/_bin/${filename}`);
 
   try {
     await platform.start(loadAssemblyUrls);

src/Microsoft.AspNetCore.Blazor.Browser.JS/src/Platform/Mono/MonoPlatform.ts

@@ -48,9 +48,9 @@ export const monoPlatform: Platform = {
   callEntryPoint: function callEntryPoint(assemblyName: string, entrypointMethod: string, args: System_Object[]): void {
     // Parse the entrypointMethod, which is of the form MyApp.MyNamespace.MyTypeName::MyMethodName
     // Note that we don't support specifying a method overload, so it has to be unique
-    const entrypointSegments = entrypointMethod.split("::");
+    const entrypointSegments = entrypointMethod.split('::');
     if (entrypointSegments.length != 2) {
-      throw new Error("malformed entry point method name; could not resolve class name and method name");
+      throw new Error('Malformed entry point method name; could not resolve class name and method name.');
     }
     const typeFullName = entrypointSegments[0];
     const methodName = entrypointSegments[1];
@@ -146,7 +146,7 @@ export const monoPlatform: Platform = {
 function addScriptTagsToDocument() {
   // Load either the wasm or asm.js version of the Mono runtime
   const browserSupportsNativeWebAssembly = typeof WebAssembly !== 'undefined' && WebAssembly.validate;
-  const monoRuntimeUrlBase = '/_framework/' + (browserSupportsNativeWebAssembly ? 'wasm' : 'asmjs');
+  const monoRuntimeUrlBase = '_framework/' + (browserSupportsNativeWebAssembly ? 'wasm' : 'asmjs');
   const monoRuntimeScriptUrl = `${monoRuntimeUrlBase}/mono.js`;
 
   if (!browserSupportsNativeWebAssembly) {
@@ -165,8 +165,8 @@ function createEmscriptenModuleInstance(loadAssemblyUrls: string[], onReady: ()
 
   module.print = line => console.log(`WASM: ${line}`);
   module.printErr = line => console.error(`WASM: ${line}`);
-  module.wasmBinaryFile = '/_framework/wasm/mono.wasm';
+  module.wasmBinaryFile = '_framework/wasm/mono.wasm';
-  module.asmjsCodeFile = '/_framework/asmjs/mono.asm.js';
+  module.asmjsCodeFile = '_framework/asmjs/mono.asm.js';
   module.preRun = [];
   module.postRun = [];
   module.preloadPlugins = [];

src/Microsoft.AspNetCore.Blazor.DevHost/Server/Startup.cs

@@ -2,11 +2,12 @@
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
 using System;
-using System.Reflection;
 using Microsoft.AspNetCore.Builder;
 using Microsoft.AspNetCore.Hosting;
 using Microsoft.Extensions.DependencyInjection;
 using System.IO;
+using Microsoft.Extensions.Configuration;
+using Microsoft.AspNetCore.Http;
 
 namespace Microsoft.AspNetCore.Blazor.DevHost.Server
 {
@@ -17,14 +18,40 @@ namespace Microsoft.AspNetCore.Blazor.DevHost.Server
             services.AddRouting();
         }
 
-        public void Configure(IApplicationBuilder app)
+        public void Configure(IApplicationBuilder app, IConfiguration configuration)
         {
             app.UseDeveloperExceptionPage();
+            EnableConfiguredPathbase(app, configuration);
 
             var clientAssemblyPath = FindClientAssemblyPath(app);
             app.UseBlazor(new BlazorOptions { ClientAssemblyPath = clientAssemblyPath });
         }
 
+        private static void EnableConfiguredPathbase(IApplicationBuilder app, IConfiguration configuration)
+        {
+            var pathBase = configuration.GetValue<string>("pathbase");
+            if (!string.IsNullOrEmpty(pathBase))
+            {
+                app.UsePathBase(pathBase);
+
+                // To ensure consistency with a production environment, only handle requests
+                // that match the specified pathbase.
+                app.Use((context, next) =>
+                {
+                    if (context.Request.PathBase == pathBase)
+                    {
+                        return next();
+                    }
+                    else
+                    {
+                        context.Response.StatusCode = 404;
+                        return context.Response.WriteAsync($"The server is configured only to " +
+                            $"handle request URIs within the PathBase '{pathBase}'.");
+                    }
+                });
+            }
+        }
+
         private static string FindClientAssemblyPath(IApplicationBuilder app)
         {
             var env = app.ApplicationServices.GetRequiredService<IHostingEnvironment>();

test/Microsoft.AspNetCore.Blazor.E2ETest/Infrastructure/ServerFixtures/DevHostServerFixture.cs

@@ -1,7 +1,6 @@
 // Copyright (c) .NET Foundation. All rights reserved.
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
-using System.IO;
 using Microsoft.AspNetCore.Hosting;
 using DevHostServerProgram = Microsoft.AspNetCore.Blazor.DevHost.Server.Program;
 
@@ -9,6 +8,8 @@ namespace Microsoft.AspNetCore.Blazor.E2ETest.Infrastructure.ServerFixtures
 {
     public class DevHostServerFixture<TProgram> : WebHostServerFixture
     {
+        public string PathBase { get; set; }
+
         protected override IWebHost CreateWebHost()
         {
             var sampleSitePath = FindSampleOrTestSitePath(
@@ -17,7 +18,8 @@ namespace Microsoft.AspNetCore.Blazor.E2ETest.Infrastructure.ServerFixtures
             return DevHostServerProgram.BuildWebHost(new string[]
             {
                 "--urls", "http://127.0.0.1:0",
-                "--contentroot", sampleSitePath
+                "--contentroot", sampleSitePath,
+                "--pathbase", PathBase
             });
         }
     }

test/Microsoft.AspNetCore.Blazor.E2ETest/Tests/ComponentRenderingTest.cs

@@ -23,7 +23,8 @@ namespace Microsoft.AspNetCore.Blazor.E2ETest.Tests
         public ComponentRenderingTest(BrowserFixture browserFixture, DevHostServerFixture<Program> serverFixture)
             : base(browserFixture, serverFixture)
         {
-            Navigate("/", noReload: true);
+            serverFixture.PathBase = "/subdir";
+            Navigate("/subdir", noReload: true);
         }
 
         [Fact]

test/testapps/BasicTestApp/BasicTestApp.csproj

@@ -5,7 +5,7 @@
 
     <!-- Local alternative to <RunArguments>blazor serve</RunArguments> -->
     <RunCommand>dotnet</RunCommand>
-    <RunArguments>run --project ..\..\..\src\Microsoft.AspNetCore.Blazor.DevHost serve</RunArguments>
+    <RunArguments>run --project ..\..\..\src\Microsoft.AspNetCore.Blazor.DevHost serve --pathbase /subdir</RunArguments>
   </PropertyGroup>
 
   <!-- Local alternative to <PackageReference Include="Microsoft.AspNetCore.Blazor.Build" /> -->

test/testapps/BasicTestApp/wwwroot/index.html

@@ -3,6 +3,7 @@
 <head>
     <meta charset="utf-8" />
     <title>Basic test app</title>
+    <base href="/subdir/" />
 </head>
 <body>
   <app>Loading...</app>