Update CreateSlimBuilder to use Host.CreateEmptyApplicationBuilder. (#46579)

* Update CreateSlimBuilder to use Host.CreateEmptyApplicationBuilder.

Also fix 2 existing bugs:
1. ContentRootPath is set to the app's BaseDirectory, and not CurrentWorkingDirectory. Default to CWD by using the same logic as HostApplicationBuilder.
2. Add un-prefixed environment variables to the configuration, and ensure the configuration order is the same as WebApplication.CreateBuilder.

Fix #46293

Author: eerhardt

src/DefaultBuilder/src/WebApplicationBuilder.cs

@@ -94,25 +94,26 @@ internal WebApplicationBuilder(WebApplicationOptions options, bool slim, Action<
 
         configuration.AddEnvironmentVariables(prefix: "ASPNETCORE_");
 
-        // add the default host configuration sources, so they are picked up by the HostApplicationBuilder constructor.
-        // These won't be added by HostApplicationBuilder since DisableDefaults = true.
+        // SetDefaultContentRoot needs to be added between 'ASPNETCORE_' and 'DOTNET_' in order to match behavior of the non-slim WebApplicationBuilder.
+        SetDefaultContentRoot(options, configuration);
+
+        // Add the default host environment variable configuration source.
+        // This won't be added by CreateEmptyApplicationBuilder.
         configuration.AddEnvironmentVariables(prefix: "DOTNET_");
-        if (options.Args is { Length: > 0 } args)
-        {
-            configuration.AddCommandLine(args);
-        }
 
-        _hostApplicationBuilder = new HostApplicationBuilder(new HostApplicationBuilderSettings
+        _hostApplicationBuilder = Microsoft.Extensions.Hosting.Host.CreateEmptyApplicationBuilder(new HostApplicationBuilderSettings
         {
-            DisableDefaults = true,
             Args = options.Args,
             ApplicationName = options.ApplicationName,
             EnvironmentName = options.EnvironmentName,
             ContentRootPath = options.ContentRootPath,
             Configuration = configuration,
         });
 
-        // configure the ServiceProviderOptions here since DisableDefaults = true means HostApplicationBuilder won't.
+        // Ensure the same behavior of the non-slim WebApplicationBuilder by adding the default "app" Configuration sources
+        ApplyDefaultAppConfiguration(options, configuration);
+
+        // configure the ServiceProviderOptions here since CreateEmptyApplicationBuilder won't.
         var serviceProviderFactory = GetServiceProviderFactory(_hostApplicationBuilder);
         _hostApplicationBuilder.ConfigureContainer(serviceProviderFactory);
 
@@ -177,6 +178,41 @@ private static DefaultServiceProviderFactory GetServiceProviderFactory(HostAppli
         return new DefaultServiceProviderFactory();
     }
 
+    private static void SetDefaultContentRoot(WebApplicationOptions options, ConfigurationManager configuration)
+    {
+        if (options.ContentRootPath is null && configuration[HostDefaults.ContentRootKey] is null)
+        {
+            // Logic taken from https://github.com/dotnet/runtime/blob/78ed4438a42acab80541e9bde1910abaa8841db2/src/libraries/Microsoft.Extensions.Hosting/src/HostingHostBuilderExtensions.cs#L209-L227
+
+            // If we're running anywhere other than C:\Windows\system32, we default to using the CWD for the ContentRoot.
+            // However, since many things like Windows services and MSIX installers have C:\Windows\system32 as there CWD which is not likely
+            // to really be the home for things like appsettings.json, we skip changing the ContentRoot in that case. The non-"default" initial
+            // value for ContentRoot is AppContext.BaseDirectory (e.g. the executable path) which probably makes more sense than the system32.
+
+            // In my testing, both Environment.CurrentDirectory and Environment.GetFolderPath(Environment.SpecialFolder.System) return the path without
+            // any trailing directory separator characters. I'm not even sure the casing can ever be different from these APIs, but I think it makes sense to
+            // ignore case for Windows path comparisons given the file system is usually (always?) going to be case insensitive for the system path.
+            string cwd = System.Environment.CurrentDirectory;
+            if (!OperatingSystem.IsWindows() || !string.Equals(cwd, System.Environment.GetFolderPath(System.Environment.SpecialFolder.System), StringComparison.OrdinalIgnoreCase))
+            {
+                configuration.AddInMemoryCollection(new[]
+                {
+                    new KeyValuePair<string, string?>(HostDefaults.ContentRootKey, cwd),
+                });
+            }
+        }
+    }
+
+    private static void ApplyDefaultAppConfiguration(WebApplicationOptions options, ConfigurationManager configuration)
+    {
+        configuration.AddEnvironmentVariables();
+
+        if (options.Args is { Length: > 0 } args)
+        {
+            configuration.AddCommandLine(args);
+        }
+    }
+
     /// <summary>
     /// Provides information about the web hosting environment an application is running.
     /// </summary>

src/DefaultBuilder/test/Microsoft.AspNetCore.Tests/WebApplicationTests.cs

@@ -5,13 +5,11 @@
 using System.Diagnostics;
 using System.Diagnostics.Tracing;
 using System.Net;
-using System.Net.Http;
 using System.Reflection;
 using System.Security.Claims;
 using System.Text;
 using System.Text.Encodings.Web;
 using Microsoft.AspNetCore.Authentication;
-using Microsoft.AspNetCore.Authorization;
 using Microsoft.AspNetCore.Builder;
 using Microsoft.AspNetCore.HostFiltering;
 using Microsoft.AspNetCore.Hosting;
@@ -23,6 +21,7 @@
 using Microsoft.AspNetCore.TestHost;
 using Microsoft.AspNetCore.Testing;
 using Microsoft.AspNetCore.Tests;
+using Microsoft.DotNet.RemoteExecutor;
 using Microsoft.Extensions.Configuration;
 using Microsoft.Extensions.DependencyInjection;
 using Microsoft.Extensions.DependencyInjection.Extensions;
@@ -556,9 +555,57 @@ public void SettingContentRootToRelativePathUsesAppContextBaseDirectoryAsPathBas
         builder.WebHost.UseContentRoot("");
 
         Assert.Equal(NormalizePath(AppContext.BaseDirectory), NormalizePath(builder.Environment.ContentRootPath));
+    }
+
+    private static string NormalizePath(string unnormalizedPath) =>
+        Path.TrimEndingDirectorySeparator(Path.GetFullPath(unnormalizedPath));
+
+    [ConditionalFact]
+    [RemoteExecutionSupported]
+    public void ContentRootIsDefaultedToCurrentDirectory()
+    {
+        var tmpDir = Directory.CreateTempSubdirectory();
+
+        try
+        {
+            var options = new RemoteInvokeOptions();
+            options.StartInfo.WorkingDirectory = tmpDir.FullName;
+
+            using var remoteHandle = RemoteExecutor.Invoke(static () =>
+            {
+                foreach (object[] data in CreateBuilderFuncs)
+                {
+                    var createBuilder = (CreateBuilderFunc)data[0];
+                    var builder = createBuilder();
 
-        static string NormalizePath(string unnormalizedPath) =>
-            Path.TrimEndingDirectorySeparator(Path.GetFullPath(unnormalizedPath));
+                    Assert.Equal(NormalizePath(Environment.CurrentDirectory), NormalizePath(builder.Environment.ContentRootPath));
+                }
+            }, options);
+        }
+        finally
+        {
+            tmpDir.Delete(recursive: true);
+        }
+    }
+
+    [ConditionalFact]
+    [OSSkipCondition(OperatingSystems.Linux | OperatingSystems.MacOSX)]
+    [RemoteExecutionSupported]
+    public void ContentRootIsBaseDirectoryWhenCurrentIsSpecialFolderSystem()
+    {
+        var options = new RemoteInvokeOptions();
+        options.StartInfo.WorkingDirectory = Environment.GetFolderPath(Environment.SpecialFolder.System);
+
+        using var remoteHandle = RemoteExecutor.Invoke(static () =>
+        {
+            foreach (object[] data in CreateBuilderFuncs)
+            {
+                var createBuilder = (CreateBuilderFunc)data[0];
+                var builder = createBuilder();
+
+                Assert.Equal(NormalizePath(AppContext.BaseDirectory), NormalizePath(builder.Environment.ContentRootPath));
+            }
+        }, options);
     }
 
     [Theory]
@@ -795,6 +842,43 @@ public void WebApplicationBuilderApplicationNameCanBeOverridden(WebApplicationBu
         Assert.Equal(assemblyName, webHostEnv.ApplicationName);
     }
 
+    [ConditionalFact]
+    [RemoteExecutionSupported]
+    public void WebApplicationBuilderConfigurationSourcesOrderedCorrectly()
+    {
+        // all WebApplicationBuilders have the following configuration sources ordered highest to lowest priority:
+        // 1. Command-line arguments
+        // 2. Non-prefixed environment variables
+        // 3. DOTNET_-prefixed environment variables
+        // 4. ASPNETCORE_-prefixed environment variables
+
+        var options = new RemoteInvokeOptions();
+        options.StartInfo.EnvironmentVariables.Add("one", "unprefixed_one");
+        options.StartInfo.EnvironmentVariables.Add("two", "unprefixed_two");
+        options.StartInfo.EnvironmentVariables.Add("DOTNET_one", "DOTNET_one");
+        options.StartInfo.EnvironmentVariables.Add("DOTNET_two", "DOTNET_two");
+        options.StartInfo.EnvironmentVariables.Add("DOTNET_three", "DOTNET_three");
+        options.StartInfo.EnvironmentVariables.Add("ASPNETCORE_one", "ASPNETCORE_one");
+        options.StartInfo.EnvironmentVariables.Add("ASPNETCORE_two", "ASPNETCORE_two");
+        options.StartInfo.EnvironmentVariables.Add("ASPNETCORE_three", "ASPNETCORE_three");
+        options.StartInfo.EnvironmentVariables.Add("ASPNETCORE_four", "ASPNETCORE_four");
+
+        using var remoteHandle = RemoteExecutor.Invoke(static () =>
+        {
+            var args = new[] { "--one=command_line_one" };
+            foreach (object[] data in CreateBuilderArgsFuncs)
+            {
+                var createBuilder = (CreateBuilderArgsFunc)data[0];
+                var builder = createBuilder(args);
+
+                Assert.Equal("command_line_one", builder.Configuration["one"]);
+                Assert.Equal("unprefixed_two", builder.Configuration["two"]);
+                Assert.Equal("DOTNET_three", builder.Configuration["three"]);
+                Assert.Equal("ASPNETCORE_four", builder.Configuration["four"]);
+            }
+        }, options);
+    }
+
     [Theory]
     [MemberData(nameof(CreateBuilderArgsFuncs))]
     public void WebApplicationBuilderCanFlowCommandLineConfigurationToApplication(CreateBuilderArgsFunc createBuilder)
@@ -976,7 +1060,7 @@ public async Task WebApplicationCanObserveSourcesClearedInBuild(CreateBuilderFun
 
     [Theory]
     [MemberData(nameof(CreateBuilderOptionsFuncs))]
-    public async Task WebApplicationCanObserveSourcesClearedInConfiguratHostConfiguration(CreateBuilderOptionsFunc createBuilder)
+    public async Task WebApplicationCanObserveSourcesClearedInHostConfiguration(CreateBuilderOptionsFunc createBuilder)
     {
         // This mimics what WebApplicationFactory<T> does and runs configure
         // services callbacks