Set BrowserHttpMessageHandler as the default handler when running on Mono WebAssembly

src/Microsoft.AspNetCore.Blazor.Browser/Hosting/WebAssemblyHost.cs

@@ -2,8 +2,11 @@
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
 using System;
+using System.Net.Http;
+using System.Reflection;
 using System.Threading;
 using System.Threading.Tasks;
+using Microsoft.AspNetCore.Blazor.Browser.Http;
 using Microsoft.AspNetCore.Blazor.Browser.Rendering;
 using Microsoft.Extensions.DependencyInjection;
 using Microsoft.JSInterop;
@@ -36,6 +39,7 @@ namespace Microsoft.AspNetCore.Blazor.Hosting
             // JSRuntime in the 'root' execution context which implies that we want to do as part of a direct
             // call from Program.Main, and before any 'awaits'.
             JSRuntime.SetCurrentJSRuntime(_runtime);
+            SetBrowserHttpMessageHandlerAsDefault();
 
             var scopeFactory = Services.GetRequiredService<IServiceScopeFactory>();
             _scope = scopeFactory.CreateScope();
@@ -97,5 +101,35 @@ namespace Microsoft.AspNetCore.Blazor.Hosting
         {
             (Services as IDisposable)?.Dispose();
         }
+
+        private static void SetBrowserHttpMessageHandlerAsDefault()
+        {
+            // Within the Mono WebAssembly BCL, this is a special private static field
+            // that can be assigned to override the default handler
+            const string getHttpMessageHandlerFieldName = "GetHttpMessageHandler";
+            var getHttpMessageHandlerField = typeof(HttpClient).GetField(
+                getHttpMessageHandlerFieldName,
+                BindingFlags.Static | BindingFlags.NonPublic);
+
+            // getHttpMessageHandlerField will be null in tests, but nonnull when actually
+            // running under Mono WebAssembly
+            if (getHttpMessageHandlerField != null)
+            {
+                // Just in case you're not actually using HttpClient, defer the construction
+                // of the BrowserHttpMessageHandler
+                var handlerSingleton = new Lazy<HttpMessageHandler>(
+                    () => new BrowserHttpMessageHandler());
+                Func<HttpMessageHandler> handlerFactory = () => handlerSingleton.Value;
+                getHttpMessageHandlerField.SetValue(null, handlerFactory);
+            }
+            else
+            {
+                // We log a warning in case this ever happens at runtime (even though there's
+                // no obvious way it could be possible), but don't actually throw because that
+                // would break unit tests
+                Console.WriteLine("WARNING: Could not set default HttpMessageHandler because " +
+                    $"'{getHttpMessageHandlerFieldName}' was not found on '{typeof(HttpClient).FullName}'.");
+            }
+        }
     }
 }

src/Microsoft.AspNetCore.Blazor.Browser/Hosting/WebAssemblyHostBuilder.cs

@@ -4,7 +4,6 @@
 using System;
 using System.Collections.Generic;
 using System.Net.Http;
-using Microsoft.AspNetCore.Blazor.Browser.Http;
 using Microsoft.AspNetCore.Blazor.Browser.Services;
 using Microsoft.AspNetCore.Blazor.Services;
 using Microsoft.Extensions.DependencyInjection;
@@ -75,7 +74,7 @@ namespace Microsoft.AspNetCore.Blazor.Hosting
             {
                 // Creating the URI helper needs to wait until the JS Runtime is initialized, so defer it.
                 var uriHelper = s.GetRequiredService<IUriHelper>();
-                return new HttpClient(new BrowserHttpMessageHandler())
+                return new HttpClient
                 {
                     BaseAddress = new Uri(BrowserUriHelper.Instance.GetBaseUri())
                 };

src/Microsoft.AspNetCore.Blazor.Build/targets/BuiltInBclLinkerDescriptor.xml

@@ -16,11 +16,10 @@
       <method signature="System.String CreateString(System.Char[],System.Int32,System.Int32)" />
       <method signature="System.String CreateString(System.Char*)" />
       <method signature="System.String CreateString(System.Char*,System.Int32,System.Int32)" />
-      <method signature="System.String CreateString(System.Byte*)" />
-      <method signature="System.String CreateString(System.Byte*,System.Int32,System.Int32)" />
-      <method signature="System.String CreateString(System.Byte*,System.Int32,System.Int32,System.Text.Encoding)" />
       <method signature="System.String CreateString(System.SByte*)" />
+      <method signature="System.String CreateString(System.SByte*,System.Int32,System.Int32)" />
       <method signature="System.String CreateString(System.SByte*,System.Int32,System.Int32,System.Text.Encoding)" />
+      <method signature="System.String CreateString(System.ReadOnlySpan`1&lt;System.Char&gt;)" />
     </type>
   </assembly>
 </linker>