Throw if a component produces an invalid render tree. Fixes #24579 (#24650)

src/Components/Components/src/Rendering/ComponentState.cs

@@ -69,6 +69,8 @@ namespace Microsoft.AspNetCore.Components.Rendering
             CurrentRenderTree.Clear();
             renderFragment(CurrentRenderTree);
 
+            CurrentRenderTree.AssertTreeIsValid(Component);
+
             var diff = RenderTreeDiffBuilder.ComputeDiff(
                 _renderer,
                 batchBuilder,

src/Components/Components/src/Rendering/RenderTreeBuilder.cs

@@ -685,6 +685,17 @@ namespace Microsoft.AspNetCore.Components.Rendering
         public ArrayRange<RenderTreeFrame> GetFrames() =>
             _entries.ToRange();
 
+        internal void AssertTreeIsValid(IComponent component)
+        {
+            if (_openElementIndices.Count > 0)
+            {
+                // It's never valid to leave an element/component/region unclosed. Doing so
+                // could cause undefined behavior in diffing.
+                ref var invalidFrame = ref _entries.Buffer[_openElementIndices.Peek()];
+                throw new InvalidOperationException($"Render output is invalid for component of type '{component.GetType().FullName}'. A frame of type '{invalidFrame.FrameType}' was left unclosed. Do not use try/catch inside rendering logic, because partial output cannot be undone.");
+            }
+        }
+
         // Internal for testing
         internal void ProcessDuplicateAttributes(int first)
         {

src/Components/Components/test/RendererTest.cs

@@ -4001,6 +4001,22 @@ namespace Microsoft.AspNetCore.Components.Test
                 requestedType => Assert.Equal(typeof(TestComponent), requestedType));
         }
 
+        [Fact]
+        public async Task ThrowsIfComponentProducesInvalidRenderTree()
+        {
+            // Arrange
+            var renderer = new TestRenderer();
+            var component = new TestComponent(builder =>
+            {
+                builder.OpenElement(0, "myElem");
+            });
+            var rootComponentId = renderer.AssignRootComponentId(component);
+
+            // Act/Assert
+            var ex = await Assert.ThrowsAsync<InvalidOperationException>(() => renderer.RenderRootComponentAsync(rootComponentId));
+            Assert.StartsWith($"Render output is invalid for component of type '{typeof(TestComponent).FullName}'. A frame of type 'Element' was left unclosed.", ex.Message);
+        }
+
         private class TestComponentActivator<TResult> : IComponentActivator where TResult : IComponent, new()
         {
             public List<Type> RequestedComponentTypes { get; } = new List<Type>();

src/Components/Components/test/Rendering/RenderTreeBuilderTest.cs

@@ -1831,6 +1831,69 @@ namespace Microsoft.AspNetCore.Components.Rendering
                 f => AssertFrame.Attribute(f, "3", "see ya"));
         }
 
+        [Fact]
+        public void AcceptsClosedFramesAsValid()
+        {
+            // Arrange
+            var builder = new RenderTreeBuilder();
+            var component = new TestComponent();
+            builder.OpenElement(0, "myElem");
+            builder.OpenRegion(1);
+            builder.OpenComponent<OtherComponent>(2);
+            builder.CloseComponent();
+            builder.CloseRegion();
+            builder.CloseElement();
+
+            // Act/Assert
+            // Lack of exception is success
+            builder.AssertTreeIsValid(component);
+        }
+
+        [Fact]
+        public void ReportsUnclosedElementAsInvalid()
+        {
+            // Arrange
+            var builder = new RenderTreeBuilder();
+            var component = new TestComponent();
+            builder.OpenElement(0, "outerElem");
+            builder.OpenElement(1, "innerElem");
+            builder.CloseElement();
+
+            // Act/Assert
+            var ex = Assert.Throws<InvalidOperationException>(() => builder.AssertTreeIsValid(component));
+            Assert.StartsWith($"Render output is invalid for component of type '{typeof(TestComponent).FullName}'. A frame of type 'Element' was left unclosed.", ex.Message);
+        }
+
+        [Fact]
+        public void ReportsUnclosedComponentAsInvalid()
+        {
+            // Arrange
+            var builder = new RenderTreeBuilder();
+            var component = new TestComponent();
+            builder.OpenComponent<OtherComponent>(0);
+            builder.OpenComponent<OtherComponent>(1);
+            builder.CloseComponent();
+
+            // Act/Assert
+            var ex = Assert.Throws<InvalidOperationException>(() => builder.AssertTreeIsValid(component));
+            Assert.StartsWith($"Render output is invalid for component of type '{typeof(TestComponent).FullName}'. A frame of type 'Component' was left unclosed.", ex.Message);
+        }
+
+        [Fact]
+        public void ReportsUnclosedRegionAsInvalid()
+        {
+            // Arrange
+            var builder = new RenderTreeBuilder();
+            var component = new TestComponent();
+            builder.OpenRegion(0);
+            builder.OpenRegion(1);
+            builder.CloseRegion();
+
+            // Act/Assert
+            var ex = Assert.Throws<InvalidOperationException>(() => builder.AssertTreeIsValid(component));
+            Assert.StartsWith($"Render output is invalid for component of type '{typeof(TestComponent).FullName}'. A frame of type 'Region' was left unclosed.", ex.Message);
+        }
+
         private class TestComponent : IComponent
         {
             public void Attach(RenderHandle renderHandle) { }
@@ -1839,6 +1902,10 @@ namespace Microsoft.AspNetCore.Components.Rendering
                 => throw new NotImplementedException();
         }
 
+        private class OtherComponent : TestComponent
+        {
+        }
+
         private class TestRenderer : Renderer
         {
             public TestRenderer() : base(new TestServiceProvider(), NullLoggerFactory.Instance)