Add TagBuilder copy constructor (#10955) (#22729)

Co-authored-by: Filip Staffa <git@filipstaffa.net>

src/Mvc/Mvc.ViewFeatures/ref/Microsoft.AspNetCore.Mvc.ViewFeatures.netcoreapp.cs

@@ -652,6 +652,7 @@ namespace Microsoft.AspNetCore.Mvc.Rendering
     [System.Diagnostics.DebuggerDisplayAttribute("{DebuggerToString()}")]
     public partial class TagBuilder : Microsoft.AspNetCore.Html.IHtmlContent
     {
+        public TagBuilder(Microsoft.AspNetCore.Mvc.Rendering.TagBuilder tagBuilder) { }
         public TagBuilder(string tagName) { }
         public Microsoft.AspNetCore.Mvc.ViewFeatures.AttributeDictionary Attributes { get { throw null; } }
         public bool HasInnerHtml { get { throw null; } }

src/Mvc/Mvc.ViewFeatures/src/Rendering/TagBuilder.cs

@@ -37,6 +37,34 @@ namespace Microsoft.AspNetCore.Mvc.Rendering
             TagName = tagName;
         }
 
+        /// <summary>
+        /// Creates a copy of the HTML tag passed as <paramref name="tagBuilder"/>.
+        /// </summary>
+        /// <param name="tagBuilder">Tag to copy.</param>
+        public TagBuilder(TagBuilder tagBuilder)
+        {
+            if (tagBuilder == null)
+            {
+                throw new ArgumentException(Resources.ArgumentCannotBeNullOrEmpty, nameof(tagBuilder));
+            }
+
+            if (tagBuilder._attributes != null)
+            {
+                foreach (var tag in tagBuilder._attributes)
+                {
+                    Attributes.Add(tag);
+                }
+            }
+
+            if (tagBuilder._innerHtml != null)
+            {
+                tagBuilder.InnerHtml.CopyTo(InnerHtml);
+            }
+
+            TagName = tagBuilder.TagName;
+            TagRenderMode = tagBuilder.TagRenderMode;
+        }
+
         /// <summary>
         /// Gets the set of attributes that will be written to the tag.
         /// </summary>

src/Mvc/Mvc.ViewFeatures/test/Rendering/TagBuilderTest.cs

@@ -253,5 +253,64 @@ namespace Microsoft.AspNetCore.Mvc.Core.Rendering
             // Assert
             Assert.Equal("<span>Hello</span>", HtmlContentUtilities.HtmlContentToString(tag));
         }
+
+        [Fact]
+        public void Constructor_Copy_CopiesTagRenderMode()
+        {
+            // Arrange
+            var originalTagBuilder = new TagBuilder("p");
+            originalTagBuilder.TagRenderMode = TagRenderMode.SelfClosing;
+
+            // Act
+            var clonedTagBuilder = new TagBuilder(originalTagBuilder);
+
+
+            // Assert
+            Assert.Equal(originalTagBuilder.TagRenderMode, clonedTagBuilder.TagRenderMode);
+        }
+
+        [Fact]
+        public void Constructor_Copy_DoesShallowCopyOfInnerHtml()
+        {
+            // Arrange
+            var originalTagBuilder = new TagBuilder("p");
+            originalTagBuilder.InnerHtml.AppendHtml("<span>Hello</span>");
+
+            // Act
+            var clonedTagBuilder = new TagBuilder(originalTagBuilder);
+
+            // Assert
+            Assert.NotEqual(originalTagBuilder.InnerHtml, clonedTagBuilder.InnerHtml);
+            Assert.Equal(HtmlContentUtilities.HtmlContentToString(originalTagBuilder.RenderBody()), HtmlContentUtilities.HtmlContentToString(clonedTagBuilder.RenderBody()));
+        }
+
+        [Fact]
+        public void Constructor_Copy_DoesShallowCopyOfAttributes()
+        {
+            // Arrange
+            var originalTagBuilder = new TagBuilder("p");
+            originalTagBuilder.AddCssClass("class1");
+
+            // Act
+            var clonedTagBuilder = new TagBuilder(originalTagBuilder);
+
+            // Assert
+            Assert.NotSame(originalTagBuilder.Attributes, clonedTagBuilder.Attributes);
+            Assert.Equal(originalTagBuilder.Attributes, clonedTagBuilder.Attributes);
+        }
+
+        [Fact]
+        public void Constructor_Copy_CopiesTagName()
+        {
+            // Arrange
+            var originalTagBuilder = new TagBuilder("p");
+
+            // Act
+
+            var clonedTagBuilder = new TagBuilder(originalTagBuilder);
+
+            // Assert
+            Assert.Equal(originalTagBuilder.TagName, clonedTagBuilder.TagName);
+        }
     }
 }