From 42dd4499e0ba395d1ec45ddc549402c9c199b59f Mon Sep 17 00:00:00 2001
From: Shahriar Gholami <shrgholami@gmail.com>
Date: Mon, 23 May 2016 22:18:20 +0430
Subject: [PATCH] Fixed a bug in TagBuilder when attribute value is null

 #4710
---
 .../Rendering/TagBuilder.cs                   |  2 +-
 .../Rendering/TagBuilderTest.cs               | 20 +++++++++++++++++++
 2 files changed, 21 insertions(+), 1 deletion(-)

diff --git a/src/Microsoft.AspNetCore.Mvc.ViewFeatures/Rendering/TagBuilder.cs b/src/Microsoft.AspNetCore.Mvc.ViewFeatures/Rendering/TagBuilder.cs
index 1af2ee2c6f..80eb96b782 100644
--- a/src/Microsoft.AspNetCore.Mvc.ViewFeatures/Rendering/TagBuilder.cs
+++ b/src/Microsoft.AspNetCore.Mvc.ViewFeatures/Rendering/TagBuilder.cs
@@ -205,7 +205,7 @@ namespace Microsoft.AspNetCore.Mvc.Rendering
                     writer.Write(" ");
                     writer.Write(key);
                     writer.Write("=\"");
-                    encoder.Encode(writer, attribute.Value);
+                    encoder.Encode(writer, attribute.Value ?? string.Empty);
                     writer.Write("\"");
                 }
             }
diff --git a/test/Microsoft.AspNetCore.Mvc.ViewFeatures.Test/Rendering/TagBuilderTest.cs b/test/Microsoft.AspNetCore.Mvc.ViewFeatures.Test/Rendering/TagBuilderTest.cs
index bba75f5904..87d0f7ff0e 100644
--- a/test/Microsoft.AspNetCore.Mvc.ViewFeatures.Test/Rendering/TagBuilderTest.cs
+++ b/test/Microsoft.AspNetCore.Mvc.ViewFeatures.Test/Rendering/TagBuilderTest.cs
@@ -113,6 +113,26 @@ namespace Microsoft.AspNetCore.Mvc.Core.Rendering
             Assert.Equal(output, result);
         }
 
+        [Theory]
+        [InlineData("attribute", "value", "<p attribute=\"HtmlEncode[[value]]\"></p>")]
+        [InlineData("attribute", null, "<p attribute=\"\"></p>")]
+        [InlineData("attribute", "", "<p attribute=\"\"></p>")]
+        public void WriteTo_WriteEmptyAttribute_WhenValueIsNullOrEmpty(string attributeKey, string attributeValue, string expectedOutput)
+        {
+            // Arrange
+            var tagBuilder = new TagBuilder("p");
+
+            // Act
+            tagBuilder.Attributes.Add(attributeKey, attributeValue);
+
+            // Assert
+            using (var writer = new StringWriter())
+            {
+                tagBuilder.WriteTo(writer, new HtmlTestEncoder());
+                Assert.Equal(expectedOutput, writer.ToString());
+            }
+        }
+
         [Fact]
         public void WriteTo_IncludesInnerHtml()
         {