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

@@ -77,7 +77,7 @@ static readonly IReadOnlyDictionary<string, object> _emptyParametersDictionary
     /// </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.
@@ -141,6 +141,11 @@ public async Task SetParametersAsync(ParameterView parameters)
             throw new InvalidOperationException($"The {nameof(Router)} component requires a value for the parameter {nameof(Found)}.");
         }
 
+        if (NotFound != null && NotFoundPage != null)
+        {
+            throw new InvalidOperationException($"The {nameof(Router)} component cannot have both {nameof(NotFound)} and {nameof(NotFoundPage)} parameters set. Use either {nameof(NotFound)} render fragment or {nameof(NotFoundPage)} component type, but not both.");
+        }
+
         if (NotFoundPage != null)
         {
             if (!typeof(IComponent).IsAssignableFrom(NotFoundPage))

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

@@ -265,6 +265,73 @@ await _renderer.Dispatcher.InvokeAsync(() =>
         Assert.Equal("Not found", renderedFrame.TextContent);
     }
 
+    [Fact]
+    public async Task ThrowsIfBothNotFoundAndNotFoundPageAreSet()
+    {
+        // Arrange
+        var parameters = new Dictionary<string, object>
+        {
+            { nameof(Router.AppAssembly), typeof(RouterTest).Assembly },
+            { nameof(Router.NotFound), (RenderFragment)(builder => builder.AddContent(0, "Custom content")) },
+            { nameof(Router.NotFoundPage), typeof(TestNotFoundPageComponent) },
+        };
+
+        // Act & Assert
+        var exception = await Assert.ThrowsAsync<InvalidOperationException>(async () =>
+            await _renderer.Dispatcher.InvokeAsync(() =>
+                _router.SetParametersAsync(ParameterView.FromDictionary(parameters))));
+
+        Assert.Contains("cannot have both NotFound and NotFoundPage parameters set", exception.Message);
+    }
+
+    [Fact]
+    public async Task UsesNotFoundPageIfSpecified()
+    {
+        // Arrange
+        _navigationManager.NotifyLocationChanged("https://www.example.com/subdir/nonexistent", false);
+        var parameters = new Dictionary<string, object>
+        {
+            { nameof(Router.AppAssembly), typeof(RouterTest).Assembly },
+            { nameof(Router.NotFoundPage), typeof(TestNotFoundPageComponent) },
+        };
+
+        // Act
+        await _renderer.Dispatcher.InvokeAsync(() =>
+            _router.SetParametersAsync(ParameterView.FromDictionary(parameters)));
+
+        // Assert
+        var batch = _renderer.Batches.First();
+        var renderedFrame = batch.ReferenceFrames.Skip(1).First(); // Skip the RouteView frame
+        Assert.Equal(RenderTreeFrameType.Attribute, renderedFrame.FrameType);
+        Assert.Equal(nameof(RouteView.RouteData), renderedFrame.AttributeName);
+        var routeData = (RouteData)renderedFrame.AttributeValue;
+        Assert.Equal(typeof(TestNotFoundPageComponent), routeData.PageType);
+    }
+
+    [Fact]
+    public async Task UsesNotFoundPageOverNotFound()
+    {
+        // Arrange - This test verifies that if only NotFoundPage is set (not both), it takes precedence over default
+        _navigationManager.NotifyLocationChanged("https://www.example.com/subdir/nonexistent", false);
+        var parameters = new Dictionary<string, object>
+        {
+            { nameof(Router.AppAssembly), typeof(RouterTest).Assembly },
+            { nameof(Router.NotFoundPage), typeof(TestNotFoundPageComponent) },
+        };
+
+        // Act
+        await _renderer.Dispatcher.InvokeAsync(() =>
+            _router.SetParametersAsync(ParameterView.FromDictionary(parameters)));
+
+        // Assert
+        var batch = _renderer.Batches.First();
+        var renderedFrame = batch.ReferenceFrames.Skip(1).First(); // Skip the RouteView frame
+        Assert.Equal(RenderTreeFrameType.Attribute, renderedFrame.FrameType);
+        Assert.Equal(nameof(RouteView.RouteData), renderedFrame.AttributeName);
+        var routeData = (RouteData)renderedFrame.AttributeValue;
+        Assert.Equal(typeof(TestNotFoundPageComponent), routeData.PageType);
+    }
+
     internal class TestNavigationManager : NavigationManager
     {
         public TestNavigationManager() =>
@@ -306,4 +373,7 @@ public class MatchAnythingComponent : ComponentBase { }
 
     [Route("a/b/c")]
     public class MultiSegmentRouteComponent : ComponentBase { }
+
+    [Route("/not-found")]
+    public class TestNotFoundPageComponent : ComponentBase { }
 }