From 9f7dab309684cc84a0f7ca1f5804fef5cd19f87e Mon Sep 17 00:00:00 2001
From: Steve Sanderson <SteveSandersonMS@users.noreply.github.com>
Date: Fri, 26 Jan 2018 10:10:06 -0800
Subject: [PATCH] Better error handling when setting properties on child
 components

---
 .../Rendering/Renderer.cs                     | 20 +++++++++-
 .../RenderTreeDiffComputerTest.cs             | 39 ++++++++++++++++++-
 2 files changed, 56 insertions(+), 3 deletions(-)

diff --git a/src/Microsoft.AspNetCore.Blazor/Rendering/Renderer.cs b/src/Microsoft.AspNetCore.Blazor/Rendering/Renderer.cs
index 98372bcf48..7dfdc43153 100644
--- a/src/Microsoft.AspNetCore.Blazor/Rendering/Renderer.cs
+++ b/src/Microsoft.AspNetCore.Blazor/Rendering/Renderer.cs
@@ -104,9 +104,25 @@ namespace Microsoft.AspNetCore.Blazor.Rendering
 
         private void SetPropertyOnComponent(IComponent component, in RenderTreeNode attributeNode)
         {
-            // TODO: Cache the reflection
+            // TODO: Is it beneficial to cache the reflection?
             var property = component.GetType().GetProperty(attributeNode.AttributeName);
-            property.SetValue(component, attributeNode.AttributeValue);
+            if (property == null)
+            {
+                throw new InvalidOperationException(
+                    $"Component of type '{component.GetType().FullName}' does not have a property " +
+                    $"matching the name '{attributeNode.AttributeName}'.");
+            }
+
+            try
+            {
+                property.SetValue(component, attributeNode.AttributeValue);
+            }
+            catch (Exception ex)
+            {
+                throw new InvalidOperationException(
+                    $"Unable to set property '{attributeNode.AttributeName}' on component of " +
+                    $"type '{component.GetType().FullName}'. The error was: {ex.Message}", ex);
+            }
         }
     }
 }
diff --git a/test/Microsoft.AspNetCore.Blazor.Test/RenderTreeDiffComputerTest.cs b/test/Microsoft.AspNetCore.Blazor.Test/RenderTreeDiffComputerTest.cs
index f341aa7e07..5f791a9127 100644
--- a/test/Microsoft.AspNetCore.Blazor.Test/RenderTreeDiffComputerTest.cs
+++ b/test/Microsoft.AspNetCore.Blazor.Test/RenderTreeDiffComputerTest.cs
@@ -702,9 +702,46 @@ namespace Microsoft.AspNetCore.Blazor.Test
         }
 
         [Fact]
-        public void RejectsUnknownPropertiesOnChildComponents()
+        public void ThrowsIfAssigningUnknownPropertiesToChildComponents()
         {
+            // Arrange
+            var renderer = new FakeRenderer();
+            var oldTree = new RenderTreeBuilder(renderer);
+            var newTree = new RenderTreeBuilder(renderer);
+            var diff = new RenderTreeDiffComputer(renderer);
+            var testObject = new object();
+            newTree.OpenComponentElement<FakeComponent>(0);
+            newTree.AddAttribute(1, "SomeUnknownProperty", 123);
+            newTree.CloseElement();
 
+            // Act/Assert
+            var ex = Assert.Throws<InvalidOperationException>(() =>
+            {
+                diff.ApplyNewRenderTreeVersion(oldTree.GetNodes(), newTree.GetNodes());
+            });
+            Assert.Equal($"Component of type '{typeof(FakeComponent).FullName}' does not have a property matching the name 'SomeUnknownProperty'.", ex.Message);
+        }
+
+        [Fact]
+        public void ThrowsIfAssigningReadOnlyPropertiesToChildComponents()
+        {
+            // Arrange
+            var renderer = new FakeRenderer();
+            var oldTree = new RenderTreeBuilder(renderer);
+            var newTree = new RenderTreeBuilder(renderer);
+            var diff = new RenderTreeDiffComputer(renderer);
+            var testObject = new object();
+            newTree.OpenComponentElement<FakeComponent>(0);
+            newTree.AddAttribute(1, nameof(FakeComponent.ReadonlyProperty), 123);
+            newTree.CloseElement();
+
+            // Act/Assert
+            var ex = Assert.Throws<InvalidOperationException>(() =>
+            {
+                diff.ApplyNewRenderTreeVersion(oldTree.GetNodes(), newTree.GetNodes());
+            });
+            Assert.StartsWith($"Unable to set property '{nameof(FakeComponent.ReadonlyProperty)}' on " +
+                $"component of type '{typeof(FakeComponent).FullName}'.", ex.Message);
         }
 
         [Fact]