Add validation to Router component to prevent both NotFound and NotFoundPage being set

src/Components/Components/src/PublicAPI.Unshipped.txt

@@ -1,7 +1,7 @@
 #nullable enable
 *REMOVED*Microsoft.AspNetCore.Components.ResourceAsset.ResourceAsset(string! url, System.Collections.Generic.IReadOnlyList<Microsoft.AspNetCore.Components.ResourceAssetProperty!>? properties) -> void
 Microsoft.AspNetCore.Components.ResourceAsset.ResourceAsset(string! url, System.Collections.Generic.IReadOnlyList<Microsoft.AspNetCore.Components.ResourceAssetProperty!>? properties = null) -> void
-Microsoft.AspNetCore.Components.Routing.Router.NotFoundPage.get -> System.Type!
+Microsoft.AspNetCore.Components.Routing.Router.NotFoundPage.get -> System.Type?
 Microsoft.AspNetCore.Components.Routing.Router.NotFoundPage.set -> void
 Microsoft.AspNetCore.Components.Infrastructure.ComponentsMetricsServiceCollectionExtensions
 Microsoft.AspNetCore.Components.NavigationManager.OnNotFound -> System.EventHandler<Microsoft.AspNetCore.Components.Routing.NotFoundEventArgs!>!

src/Components/Components/src/Routing/Router.cs

@@ -70,14 +70,15 @@ static readonly IReadOnlyDictionary<string, object> _emptyParametersDictionary
     /// Gets or sets the content to display when no match is found for the requested route.
     /// </summary>
     [Parameter]
+    [Obsolete("NotFound is deprecated. Use NotFoundPage instead.")]
     public RenderFragment NotFound { get; set; }
 
     /// <summary>
     /// Gets or sets the page content to display when no match is found for the requested route.
     /// </summary>
     [Parameter]
     [DynamicallyAccessedMembers(LinkerFlags.Component)]
-    public Type NotFoundPage { get; set; } = default!;
+    public Type? NotFoundPage { get; set; }
 
     /// <summary>
     /// Gets or sets the content to display when a match is found for the requested route.
@@ -143,6 +144,12 @@ public async Task SetParametersAsync(ParameterView parameters)
 
         if (NotFoundPage != null)
         {
+#pragma warning disable CS0618 // Type or member is obsolete
+            if (NotFound != null)
+            {
+                throw new InvalidOperationException("Both NotFound and NotFoundPage parameters are set on Router component. NotFoundPage is preferred and NotFound will be deprecated. Consider using only NotFoundPage.");
+            }
+#pragma warning restore CS0618 // Type or member is obsolete
             if (!typeof(IComponent).IsAssignableFrom(NotFoundPage))
             {
                 throw new InvalidOperationException($"The type {NotFoundPage.FullName} " +
@@ -401,10 +408,12 @@ private void RenderNotFound()
                     new RouteData(NotFoundPage, _emptyParametersDictionary));
                 builder.CloseComponent();
             }
+#pragma warning disable CS0618 // Type or member is obsolete
             else if (NotFound != null)
             {
                 NotFound(builder);
             }
+#pragma warning restore CS0618 // Type or member is obsolete
             else
             {
                 DefaultNotFoundContent(builder);
@@ -429,6 +438,7 @@ async Task IHandleAfterRender.OnAfterRenderAsync()
 
     private static partial class Log
     {
+#pragma warning disable CS0618 // Type or member is obsolete
         [LoggerMessage(1, LogLevel.Debug, $"Displaying {nameof(NotFound)} because path '{{Path}}' with base URI '{{BaseUri}}' does not match any component route", EventName = "DisplayingNotFound")]
         internal static partial void DisplayingNotFound(ILogger logger, string path, string baseUri);
 
@@ -440,5 +450,6 @@ private static partial class Log
 
         [LoggerMessage(4, LogLevel.Debug, $"Displaying {nameof(NotFound)} on request", EventName = "DisplayingNotFoundOnRequest")]
         internal static partial void DisplayingNotFound(ILogger logger);
+#pragma warning restore CS0618 // Type or member is obsolete
     }
 }

src/Components/Components/test/Routing/RouterTest.cs

@@ -1,6 +1,8 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+#pragma warning disable CS0618 // Type or member is obsolete
+
 using System.Reflection;
 using Microsoft.AspNetCore.Components.RenderTree;
 using Microsoft.AspNetCore.Components.Test.Helpers;
@@ -265,6 +267,40 @@ await _renderer.Dispatcher.InvokeAsync(() =>
         Assert.Equal("Not found", renderedFrame.TextContent);
     }
 
+    [Fact]
+    public async Task ThrowsExceptionWhenBothNotFoundAndNotFoundPageAreSet()
+    {
+        // Arrange
+        var services = new ServiceCollection();
+        services.AddSingleton<ILoggerFactory>(NullLoggerFactory.Instance);
+        services.AddSingleton<NavigationManager>(_navigationManager);
+        services.AddSingleton<INavigationInterception, TestNavigationInterception>();
+        services.AddSingleton<IScrollToLocationHash, TestScrollToLocationHash>();
+        var serviceProvider = services.BuildServiceProvider();
+
+        var renderer = new TestRenderer(serviceProvider);
+        renderer.ShouldHandleExceptions = true;
+        var router = (Router)renderer.InstantiateComponent<Router>();
+        router.AppAssembly = Assembly.GetExecutingAssembly();
+        router.Found = routeData => (builder) => builder.AddContent(0, $"Rendering route matching {routeData.PageType}");
+        renderer.AssignRootComponentId(router);
+
+        var parameters = new Dictionary<string, object>
+        {
+            { nameof(Router.AppAssembly), typeof(RouterTest).Assembly },
+            { nameof(Router.NotFound), (RenderFragment)(builder => builder.AddContent(0, "Custom not found")) },
+            { nameof(Router.NotFoundPage), typeof(NotFoundTestComponent) }
+        };
+
+        // Act & Assert
+        var exception = await Assert.ThrowsAsync<InvalidOperationException>(async () =>
+            await renderer.Dispatcher.InvokeAsync(() =>
+                router.SetParametersAsync(ParameterView.FromDictionary(parameters))));
+
+        Assert.Contains("Both NotFound and NotFoundPage parameters are set on Router component", exception.Message);
+        Assert.Contains("NotFoundPage is preferred and NotFound will be deprecated", exception.Message);
+    }
+
     internal class TestNavigationManager : NavigationManager
     {
         public TestNavigationManager() =>
@@ -306,4 +342,51 @@ public class MatchAnythingComponent : ComponentBase { }
 
     [Route("a/b/c")]
     public class MultiSegmentRouteComponent : ComponentBase { }
+
+    [Route("not-found")]
+    public class NotFoundTestComponent : ComponentBase { }
+
+    public class TestLogger<T> : ILogger<T>
+    {
+        public List<LogEntry> LogEntries { get; } = new List<LogEntry>();
+
+        public IDisposable BeginScope<TState>(TState state) => null;
+
+        public bool IsEnabled(LogLevel logLevel) => true;
+
+        public void Log<TState>(LogLevel logLevel, EventId eventId, TState state, Exception exception, Func<TState, Exception, string> formatter)
+        {
+            LogEntries.Add(new LogEntry
+            {
+                LogLevel = logLevel,
+                EventId = eventId,
+                Message = formatter(state, exception),
+                Exception = exception
+            });
+        }
+    }
+
+    public class LogEntry
+    {
+        public LogLevel LogLevel { get; set; }
+        public EventId EventId { get; set; }
+        public string Message { get; set; }
+        public Exception Exception { get; set; }
+    }
+
+    public class TestLoggerFactory : ILoggerFactory
+    {
+        private readonly ILogger _logger;
+
+        public TestLoggerFactory(ILogger logger)
+        {
+            _logger = logger;
+        }
+
+        public void AddProvider(ILoggerProvider provider) { }
+
+        public ILogger CreateLogger(string categoryName) => _logger;
+
+        public void Dispose() { }
+    }
 }