From 2e8bb07dfceafc897355e5453a2c80401fdacd58 Mon Sep 17 00:00:00 2001
From: jacalvar <jacalvar@microsoft.com>
Date: Sun, 27 Mar 2016 18:26:57 -0700
Subject: [PATCH] [Fixes #4317] UrlResolutionTagHelper doesn't work with
 TagHelpers that run before and supress output

---
 .../TagHelpers/UrlResolutionTagHelper.cs      |  5 ++++
 .../TagHelpers/UrlResolutionTagHelperTest.cs  | 29 +++++++++++++++++++
 2 files changed, 34 insertions(+)

diff --git a/src/Microsoft.AspNetCore.Mvc.Razor/TagHelpers/UrlResolutionTagHelper.cs b/src/Microsoft.AspNetCore.Mvc.Razor/TagHelpers/UrlResolutionTagHelper.cs
index 880438f107..956d5cb0fa 100644
--- a/src/Microsoft.AspNetCore.Mvc.Razor/TagHelpers/UrlResolutionTagHelper.cs
+++ b/src/Microsoft.AspNetCore.Mvc.Razor/TagHelpers/UrlResolutionTagHelper.cs
@@ -125,6 +125,11 @@ public override void Process(TagHelperContext context, TagHelperOutput output)
                 throw new ArgumentNullException(nameof(output));
             }
 
+            if (output.TagName == null)
+            {
+                return;
+            }
+
             string[] attributeNames;
             if (ElementAttributeLookups.TryGetValue(output.TagName, out attributeNames))
             {
diff --git a/test/Microsoft.AspNetCore.Mvc.Razor.Test/TagHelpers/UrlResolutionTagHelperTest.cs b/test/Microsoft.AspNetCore.Mvc.Razor.Test/TagHelpers/UrlResolutionTagHelperTest.cs
index 669dab0cd3..0e2e10d047 100644
--- a/test/Microsoft.AspNetCore.Mvc.Razor.Test/TagHelpers/UrlResolutionTagHelperTest.cs
+++ b/test/Microsoft.AspNetCore.Mvc.Razor.Test/TagHelpers/UrlResolutionTagHelperTest.cs
@@ -36,6 +36,35 @@ public static TheoryData ResolvableUrlData
             }
         }
 
+        [Fact]
+        public void Process_DoesNothingIfTagNameIsNull()
+        {
+            // Arrange
+            var tagHelperOutput = new TagHelperOutput(
+                tagName: null,
+                attributes: new TagHelperAttributeList
+                {
+                    { "href", "~/home/index.html" }
+                },
+                getChildContentAsync: (useCachedResult, encoder) => Task.FromResult<TagHelperContent>(null));
+
+            var tagHelper = new UrlResolutionTagHelper(Mock.Of<IUrlHelperFactory>(), new HtmlTestEncoder());
+            var context = new TagHelperContext(
+                allAttributes: new TagHelperAttributeList(
+                    Enumerable.Empty<TagHelperAttribute>()),
+                items: new Dictionary<object, object>(),
+                uniqueId: "test");
+
+            // Act
+            tagHelper.Process(context, tagHelperOutput);
+
+            // Assert
+            var attribute = Assert.Single(tagHelperOutput.Attributes);
+            Assert.Equal("href", attribute.Name, StringComparer.Ordinal);
+            var attributeValue = Assert.IsType<string>(attribute.Value);
+            Assert.Equal("~/home/index.html", attributeValue, StringComparer.Ordinal);
+        }
+
         [Theory]
         [MemberData(nameof(ResolvableUrlData))]
         public void Process_ResolvesTildeSlashValues(string url, string expectedHref)