Fix host builder case sensitivity and add tests.

src/DefaultBuilder/src/ConfigureWebHostBuilder.cs

@@ -19,7 +19,7 @@ public sealed class ConfigureWebHostBuilder : IWebHostBuilder
 
         private readonly WebHostEnvironment _environment;
         private readonly Configuration _configuration;
-        private readonly Dictionary<string, string?> _settings = new Dictionary<string, string?>();
+        private readonly Dictionary<string, string?> _settings = new(StringComparer.OrdinalIgnoreCase);
         private readonly IServiceCollection _services;
 
         internal ConfigureWebHostBuilder(Configuration configuration, WebHostEnvironment environment, IServiceCollection services)
@@ -78,22 +78,22 @@ public IWebHostBuilder UseSetting(string key, string? value)
                 return this;
             }
 
-            if (key == WebHostDefaults.ApplicationKey)
+            if (string.Equals(key, WebHostDefaults.ApplicationKey, StringComparison.OrdinalIgnoreCase))
             {
                 _environment.ApplicationName = value;
             }
-            else if (key == WebHostDefaults.ContentRootKey)
+            else if (string.Equals(key, WebHostDefaults.ContentRootKey, StringComparison.OrdinalIgnoreCase))
             {
                 _environment.ContentRootPath = value;
                 _environment.ResolveFileProviders(_configuration);
 
                 _configuration.ChangeBasePath(value);
             }
-            else if (key == WebHostDefaults.EnvironmentKey)
+            else if (string.Equals(key, WebHostDefaults.EnvironmentKey, StringComparison.OrdinalIgnoreCase))
             {
                 _environment.EnvironmentName = value;
             }
-            else if (key == WebHostDefaults.WebRootKey)
+            else if (string.Equals(key, WebHostDefaults.WebRootKey, StringComparison.OrdinalIgnoreCase))
             {
                 _environment.WebRootPath = value;
                 _environment.ResolveFileProviders(_configuration);

src/DefaultBuilder/src/WebApplication.cs

@@ -22,7 +22,7 @@ namespace Microsoft.AspNetCore.Builder
     /// <summary>
     /// The web application used to configure the http pipeline, and routes.
     /// </summary>
-    public sealed class WebApplication : IHost, IDisposable, IApplicationBuilder, IEndpointRouteBuilder, IAsyncDisposable
+    public sealed class WebApplication : IHost, IApplicationBuilder, IEndpointRouteBuilder, IAsyncDisposable
     {
         internal const string EndpointRouteBuilder = "__EndpointRouteBuilder";
 

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

@@ -5,6 +5,7 @@
 using System.Collections.Concurrent;
 using System.Collections.Generic;
 using System.Diagnostics.Tracing;
+using System.IO;
 using System.Linq;
 using System.Threading;
 using System.Threading.Tasks;
@@ -65,6 +66,44 @@ public void WebApplicationBuilderWebHost_ThrowsWhenBuiltDirectly()
             Assert.Throws<NotSupportedException>(() => ((IWebHostBuilder)WebApplication.CreateBuilder().WebHost).Build());
         }
 
+        [Fact]
+        public void WebApplicationBuilderWebHostUseSettings_IsCaseInsensitive()
+        {
+            var builder = WebApplication.CreateBuilder();
+
+            var contentRoot = Path.GetTempPath().ToString();
+            var webRoot = Path.GetTempPath().ToString();
+            var envName = $"{nameof(WebApplicationTests)}_ENV";
+
+            builder.WebHost.UseSetting("applicationname", nameof(WebApplicationTests));
+            builder.WebHost.UseSetting("ENVIRONMENT", envName);
+            builder.WebHost.UseSetting("CONTENTROOT", contentRoot);
+            builder.WebHost.UseSetting("WEBROOT", webRoot);
+
+            Assert.Equal(nameof(WebApplicationTests), builder.WebHost.GetSetting("APPLICATIONNAME"));
+            Assert.Equal(envName, builder.WebHost.GetSetting("environment"));
+            Assert.Equal(contentRoot, builder.WebHost.GetSetting("contentroot"));
+            Assert.Equal(webRoot, builder.WebHost.GetSetting("webroot"));
+
+            var app = builder.Build();
+
+            Assert.Equal(nameof(WebApplicationTests), app.Environment.ApplicationName);
+            Assert.Equal(envName, app.Environment.EnvironmentName);
+            Assert.Equal(contentRoot, app.Environment.ContentRootPath);
+            Assert.Equal(webRoot, app.Environment.WebRootPath);
+        }
+
+        [Fact]
+        public void WebApplicationBuilderHostProperties_IsCaseSensitive()
+        {
+            var builder = WebApplication.CreateBuilder();
+
+            builder.Host.Properties["lowercase"] = nameof(WebApplicationTests);
+
+            Assert.Equal(nameof(WebApplicationTests), builder.Host.Properties["lowercase"]);
+            Assert.False(builder.Host.Properties.ContainsKey("Lowercase"));
+        }
+
         [Fact]
         public async Task WebApplicationConfiguration_HostFilterOptionsAreReloadable()
         {