From 2d4b110b94ff406d1bc458992070f27d8f73b048 Mon Sep 17 00:00:00 2001
From: Steve Sanderson <SteveSandersonMS@users.noreply.github.com>
Date: Tue, 29 Oct 2019 17:16:58 +0000
Subject: [PATCH] Correctly handle AddMultipleAttributes terminated by
 OpenRegion. Fixes #16570

---
 .../src/Rendering/RenderTreeBuilder.cs        |  8 +++++
 .../test/Rendering/RenderTreeBuilderTest.cs   | 33 +++++++++++++++++++
 2 files changed, 41 insertions(+)

diff --git a/src/Components/Components/src/Rendering/RenderTreeBuilder.cs b/src/Components/Components/src/Rendering/RenderTreeBuilder.cs
index 6876c97f0d..f8a28273e4 100644
--- a/src/Components/Components/src/Rendering/RenderTreeBuilder.cs
+++ b/src/Components/Components/src/Rendering/RenderTreeBuilder.cs
@@ -581,6 +581,14 @@ namespace Microsoft.AspNetCore.Components.Rendering
         /// <param name="sequence">An integer that represents the position of the instruction in the source code.</param>
         public void OpenRegion(int sequence)
         {
+            // We are entering a new scope, since we track the "duplicate attributes" per
+            // element/component we might need to clean them up now.
+            if (_hasSeenAddMultipleAttributes)
+            {
+                var indexOfLastElementOrComponent = _openElementIndices.Peek();
+                ProcessDuplicateAttributes(first: indexOfLastElementOrComponent + 1);
+            }
+
             _openElementIndices.Push(_entries.Count);
             Append(RenderTreeFrame.Region(sequence));
         }
diff --git a/src/Components/Components/test/Rendering/RenderTreeBuilderTest.cs b/src/Components/Components/test/Rendering/RenderTreeBuilderTest.cs
index 7dfce8a79c..31364dca00 100644
--- a/src/Components/Components/test/Rendering/RenderTreeBuilderTest.cs
+++ b/src/Components/Components/test/Rendering/RenderTreeBuilderTest.cs
@@ -298,6 +298,39 @@ namespace Microsoft.AspNetCore.Components.Rendering
                 frame => AssertFrame.Attribute(frame, "attribute7", "the end"));
         }
 
+        [Fact]
+        public void CanAddMultipleAttributes_WithChildRegion()
+        {
+            // This represents bug https://github.com/aspnet/AspNetCore/issues/16570
+            // If a sequence of attributes is terminated by a call to builder.OpenRegion,
+            // then the attribute deduplication logic wasn't working correctly
+
+            // Arrange
+            var builder = new RenderTreeBuilder();
+
+            // Act
+            builder.OpenElement(0, "myelement");
+            builder.AddAttribute(0, "attribute1", "value1");
+            builder.AddMultipleAttributes(1, new Dictionary<string, object>()
+            {
+                { "attribute1", "value2" },
+            });
+            builder.OpenRegion(2);
+            builder.OpenElement(3, "child");
+            builder.CloseElement();
+            builder.CloseRegion();
+            builder.CloseElement();
+
+            // Assert
+            var frames = builder.GetFrames().AsEnumerable().ToArray();
+            Assert.Collection(
+                frames,
+                frame => AssertFrame.Element(frame, "myelement", 4),
+                frame => AssertFrame.Attribute(frame, "attribute1", "value2"),
+                frame => AssertFrame.Region(frame, 2, 2),
+                frame => AssertFrame.Element(frame, "child", 1, 3));
+        }
+
         [Fact]
         public void CanAddMultipleAttributes_DictionaryObject()
         {