Fallback for Components router (imported from Blazor PR 1534) (#4794)

Author: SteveSandersonMS

src/Components/src/Microsoft.AspNetCore.Components/Routing/Router.cs

@@ -4,7 +4,6 @@
 using System;
 using System.Collections.Generic;
 using System.Reflection;
-using Microsoft.AspNetCore.Components;
 using Microsoft.AspNetCore.Components.Layouts;
 using Microsoft.AspNetCore.Components.RenderTree;
 using Microsoft.AspNetCore.Components.Services;
@@ -30,6 +29,11 @@ public class Router : IComponent, IDisposable
         /// assemblies, for components matching the URI.
         /// </summary>
         [Parameter] private Assembly AppAssembly { get; set; }
+        
+        /// <summary>
+        /// Gets or sets the type of the component that should be used as a fallback when no match is found for the requested route.
+        /// </summary>
+        [Parameter] private Type FallbackComponent { get; set; }
 
         private RouteTable Routes { get; set; }
 
@@ -80,9 +84,17 @@ private void Refresh()
             locationPath = StringUntilAny(locationPath, _queryOrHashStartChar);
             var context = new RouteContext(locationPath);
             Routes.Route(context);
+
             if (context.Handler == null)
             {
-                throw new InvalidOperationException($"'{nameof(Router)}' cannot find any component with a route for '/{locationPath}'.");
+                if (FallbackComponent != null)
+                {
+                    context.Handler = FallbackComponent;
+                }
+                else
+                {
+                    throw new InvalidOperationException($"'{nameof(Router)}' cannot find any component with a route for '/{locationPath}', and no fallback is defined.");
+                }
             }
 
             if (!typeof(IComponent).IsAssignableFrom(context.Handler))

src/Components/test/Microsoft.AspNetCore.Components.E2ETest/Tests/RoutingTest.cs

@@ -88,6 +88,15 @@ public void CanArriveAtNonDefaultPage()
             AssertHighlightedLinks("Other", "Other with base-relative URL (matches all)");
         }
 
+        [Fact]
+        public void CanArriveAtFallbackPageFromBadURI()
+        {
+            SetUrlViaPushState("/Oopsie_Daisies%20%This_Aint_A_Real_Page"); 
+
+            var app = MountTestComponent<TestRouter>();
+            Assert.Equal("Oops, that component wasn't found!", app.FindElement(By.Id("test-info")).Text);
+        }
+
         [Fact]
         public void CanFollowLinkToOtherPage()
         {

src/Components/test/testapps/BasicTestApp/RouterTest/Error404.cshtml

@@ -0,0 +1 @@
+<div id="test-info">Oops, that component wasn't found!</div>

src/Components/test/testapps/BasicTestApp/RouterTest/TestRouter.cshtml

@@ -1 +1 @@
-<Router AppAssembly=typeof(BasicTestApp.Program).Assembly />
+<Router AppAssembly=typeof(BasicTestApp.Program).Assembly FallbackComponent="typeof(Error404)" />