Sanitize "id" attributes for HTML 4.0.1

- #704 part 2 of 2
- change `@Html.Id()` to sanitize return value; was identical to `@Html.Name()`

Copied `TagBuilder.CreateSanitizedId()` and `TagBuilder.Html401IdUtil` from MVC 5.2
- except this `CreateSanitizedId()` returns a valid identifier if first `char` is not a letter
 - e.g. "[0].Name"

nits:
- expand variable names, use lots of `var`, put `public` members first
- add doc comments for `CreateSanitizedId()`

Note users will be able to apply different sanitization once we fix #1188.

src/Microsoft.AspNet.Mvc.Core/Rendering/Html/HtmlHelper.cs

@@ -679,7 +679,9 @@ namespace Microsoft.AspNet.Mvc.Rendering
         protected virtual string GenerateId(string expression)
         {
             var fullName = DefaultHtmlGenerator.GetFullHtmlFieldName(ViewContext, name: expression);
-            return fullName;
+            var id = TagBuilder.CreateSanitizedId(fullName, IdAttributeDotReplacement);
+
+            return id;
         }
 
         protected virtual HtmlString GenerateLabel([NotNull] ModelMetadata metadata,

src/Microsoft.AspNet.Mvc.Core/Rendering/Html/TagBuilder.cs

@@ -49,32 +49,48 @@ namespace Microsoft.AspNet.Mvc.Rendering
             }
         }
 
-        public static string CreateSanitizedId(string originalId, [NotNull] string invalidCharReplacement)
+        /// <summary>
+        /// Return valid HTML 4.01 "id" attribute for an element with the given <paramref name="name"/>.
+        /// </summary>
+        /// <param name="name">The original element name.</param>
+        /// <param name="invalidCharReplacement">
+        /// The <see cref="string"/> (normally a single <see cref="char"/>) to substitute for invalid characters in
+        /// <paramref name="name"/>.
+        /// </param>
+        /// <returns>
+        /// Valid HTML 4.01 "id" attribute for an element with the given <paramref name="name"/>.
+        /// </returns>
+        /// <remarks>Valid "id" attributes are defined in http://www.w3.org/TR/html401/types.html#type-id</remarks>
+        public static string CreateSanitizedId(string name, [NotNull] string invalidCharReplacement)
         {
-            if (string.IsNullOrEmpty(originalId))
+            if (string.IsNullOrEmpty(name))
             {
                 return string.Empty;
             }
 
-            var firstChar = originalId[0];
+            var firstChar = name[0];
-
+            if (!Html401IdUtil.IsAsciiLetter(firstChar))
-            var sb = new StringBuilder(originalId.Length);
-            sb.Append(firstChar);
-
-            for (var i = 1; i < originalId.Length; i++)
             {
-                var thisChar = originalId[i];
+                // The first character must be a letter according to the HTML 4.01 specification.
-                if (!char.IsWhiteSpace(thisChar))
+                firstChar = 'z';
+            }
+
+            var stringBuffer = new StringBuilder(name.Length);
+            stringBuffer.Append(firstChar);
+            for (var index = 1; index < name.Length; index++)
+            {
+                var thisChar = name[index];
+                if (Html401IdUtil.IsValidIdCharacter(thisChar))
                 {
-                    sb.Append(thisChar);
+                    stringBuffer.Append(thisChar);
                 }
                 else
                 {
-                    sb.Append(invalidCharReplacement);
+                    stringBuffer.Append(invalidCharReplacement);
                 }
             }
 
-            return sb.ToString();
+            return stringBuffer.ToString();
         }
 
         public void GenerateId(string name, [NotNull] string idAttributeDotReplacement)
@@ -195,5 +211,39 @@ namespace Microsoft.AspNet.Mvc.Rendering
 
             return sb.ToString();
         }
+
+        private static class Html401IdUtil
+        {
+            public static bool IsAsciiLetter(char testChar)
+            {
+                return (('A' <= testChar && testChar <= 'Z') || ('a' <= testChar && testChar <= 'z'));
+            }
+
+            public static bool IsValidIdCharacter(char testChar)
+            {
+                return (IsAsciiLetter(testChar) || IsAsciiDigit(testChar) || IsAllowableSpecialCharacter(testChar));
+            }
+
+            private static bool IsAsciiDigit(char testChar)
+            {
+                return ('0' <= testChar && testChar <= '9');
+            }
+
+            private static bool IsAllowableSpecialCharacter(char testChar)
+            {
+                switch (testChar)
+                {
+                case '-':
+                case '_':
+                case ':':
+                    // Note '.' is valid according to the HTML 4.01 specification. Disallowed here to avoid confusion
+                    // with CSS class selectors or when using jQuery.
+                    return true;
+
+                default:
+                    return false;
+                }
+            }
+        }
     }
 }

test/Microsoft.AspNet.Mvc.Core.Test/Rendering/HtmlHelperLabelExtensionsTest.cs

@@ -60,8 +60,8 @@ namespace Microsoft.AspNet.Mvc.Core
             var labelForResult = helper.LabelFor(m => m.Inner.Id);
 
             // Assert
-            Assert.Equal("<label for=\"Inner.Id\">Id</label>", labelResult.ToString());
+            Assert.Equal("<label for=\"Inner_Id\">Id</label>", labelResult.ToString());
-            Assert.Equal("<label for=\"Inner.Id\">Id</label>", labelForResult.ToString());
+            Assert.Equal("<label for=\"Inner_Id\">Id</label>", labelForResult.ToString());
         }
 
         [Fact]
@@ -229,13 +229,14 @@ namespace Microsoft.AspNet.Mvc.Core
         }
 
         [Theory]
-        [InlineData("A", "A")]
+        [InlineData("A", "A", "A")]
-        [InlineData("A[23]", "A[23]")]
+        [InlineData("A[23]", "A[23]", "A_23_")]
-        [InlineData("A[0].B", "B")]
+        [InlineData("A[0].B", "B", "A_0__B")]
-        [InlineData("A.B.C.D", "D")]
+        [InlineData("A.B.C.D", "D", "A_B_C_D")]
         public void Label_DisplaysRightmostExpressionSegment_IfPropertiesNotFound(
             string expression,
-            string expectedResult)
+            string expectedText,
+            string expectedId)
         {
             // Arrange
             var metadataHelper = new MetadataHelper();
@@ -246,7 +247,7 @@ namespace Microsoft.AspNet.Mvc.Core
 
             // Assert
             // Label() falls back to expression name when DisplayName and PropertyName are null.
-            Assert.Equal("<label for=\"" + expression + "\">" + expectedResult + "</label>", result.ToString());
+            Assert.Equal("<label for=\"" + expectedId + "\">" + expectedText + "</label>", result.ToString());
         }
 
         [Fact]

test/Microsoft.AspNet.Mvc.Core.Test/Rendering/HtmlHelperNameExtensionsTest.cs

@@ -46,12 +46,12 @@ namespace Microsoft.AspNet.Mvc.Core
         }
 
         [Theory]
-        [InlineData("")]
+        [InlineData("", "")]
-        [InlineData("A")]
+        [InlineData("A", "A")]
-        [InlineData("A[23]")]
+        [InlineData("A[23]", "A_23_")]
-        [InlineData("A[0].B")]
+        [InlineData("A[0].B", "A_0__B")]
-        [InlineData("A.B.C.D")]
+        [InlineData("A.B.C.D", "A_B_C_D")]
-        public void IdAndNameHelpers_ReturnPrefixForModel(string prefix)
+        public void IdAndNameHelpers_ReturnPrefixForModel(string prefix, string expectedId)
         {
             // Arrange
             var helper = DefaultTemplatesUtilities.GetHtmlHelper();
@@ -66,9 +66,9 @@ namespace Microsoft.AspNet.Mvc.Core
             var nameForModelResult = helper.NameForModel();
 
             // Assert
-            Assert.Equal(prefix, idResult);
+            Assert.Equal(expectedId, idResult);
-            Assert.Equal(prefix, idForResult);
+            Assert.Equal(expectedId, idForResult);
-            Assert.Equal(prefix, idForModelResult);
+            Assert.Equal(expectedId, idForModelResult);
             Assert.Equal(prefix, nameResult);
             Assert.Equal(prefix, nameForResult);
             Assert.Equal(prefix, nameForModelResult);
@@ -94,16 +94,17 @@ namespace Microsoft.AspNet.Mvc.Core
         }
 
         [Theory]
-        [InlineData(null, "Property1")]
+        [InlineData(null, "Property1", "Property1")]
-        [InlineData("", "Property1")]
+        [InlineData("", "Property1", "Property1")]
-        [InlineData("A", "A.Property1")]
+        [InlineData("A", "A.Property1", "A_Property1")]
-        [InlineData("A[23]", "A[23].Property1")]
+        [InlineData("A[23]", "A[23].Property1", "A_23__Property1")]
-        [InlineData("A[0].B", "A[0].B.Property1")]
+        [InlineData("A[0].B", "A[0].B.Property1", "A_0__B_Property1")]
-        [InlineData("A.B.C.D", "A.B.C.D.Property1")]
+        [InlineData("A.B.C.D", "A.B.C.D.Property1", "A_B_C_D_Property1")]
-        public void IdAndNameHelpers_ReturnPrefixAndPropertyName(string prefix, string expectedResult)
+        public void IdAndNameHelpers_ReturnPrefixAndPropertyName(string prefix, string expectedName, string expectedId)
         {
             // Arrange
             var helper = DefaultTemplatesUtilities.GetHtmlHelper();
+            helper.ViewData.TemplateInfo.HtmlFieldPrefix = prefix;
 
             // Act
             var idResult = helper.Id("Property1");
@@ -112,10 +113,10 @@ namespace Microsoft.AspNet.Mvc.Core
             var nameForResult = helper.NameFor(m => m.Property1);
 
             // Assert
-            Assert.Equal("Property1", idResult);
+            Assert.Equal(expectedId, idResult);
-            Assert.Equal("Property1", idForResult);
+            Assert.Equal(expectedId, idForResult);
-            Assert.Equal("Property1", nameResult);
+            Assert.Equal(expectedName, nameResult);
-            Assert.Equal("Property1", nameForResult);
+            Assert.Equal(expectedName, nameForResult);
         }
 
         [Fact]
@@ -131,8 +132,8 @@ namespace Microsoft.AspNet.Mvc.Core
             var nameForResult = helper.NameFor(m => m.Inner.Id);
 
             // Assert
-            Assert.Equal("Inner.Id", idResult);
+            Assert.Equal("Inner_Id", idResult);
-            Assert.Equal("Inner.Id", idForResult);
+            Assert.Equal("Inner_Id", idForResult);
             Assert.Equal("Inner.Id", nameResult);
             Assert.Equal("Inner.Id", nameForResult);
         }
@@ -194,10 +195,10 @@ namespace Microsoft.AspNet.Mvc.Core
         }
 
         [Theory]
-        [InlineData("A")]
+        [InlineData("A", "A")]
-        [InlineData("A[0].B")]
+        [InlineData("A[0].B", "A_0__B")]
-        [InlineData("A.B.C.D")]
+        [InlineData("A.B.C.D", "A_B_C_D")]
-        public void IdAndName_ReturnExpression_EvenIfExpressionNotFound(string expression)
+        public void IdAndName_ReturnExpression_EvenIfExpressionNotFound(string expression, string expectedId)
         {
             // Arrange
             var helper = DefaultTemplatesUtilities.GetHtmlHelper();
@@ -207,7 +208,7 @@ namespace Microsoft.AspNet.Mvc.Core
             var nameResult = helper.Name(expression);
 
             // Assert
-            Assert.Equal(expression, idResult);
+            Assert.Equal(expectedId, idResult);
             Assert.Equal(expression, nameResult);
         }
 

test/Microsoft.AspNet.Mvc.TagHelpers.Test/InputTagHelperTest.cs

@@ -14,8 +14,8 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
     public class InputTagHelperTest
     {
         // Model (List<Model> or Model instance), container type (Model or NestModel), model accessor,
-        // property path, expected value.
+        // property path / id, expected value.
-        public static TheoryData<object, Type, Func<object>, string, string> TestDataSet
+        public static TheoryData<object, Type, Func<object>, NameAndId, string> TestDataSet
         {
             get
             {
@@ -41,32 +41,32 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                     modelWithText,
                 };
 
-                return new TheoryData<object, Type, Func<object>, string, string>
+                return new TheoryData<object, Type, Func<object>, NameAndId, string>
                 {
-                    { null, typeof(Model), () => null, "Text",
+                    { null, typeof(Model), () => null, new NameAndId("Text", "Text"),
                         string.Empty },
 
-                    { modelWithNull, typeof(Model), () => modelWithNull.Text, "Text",
+                    { modelWithNull, typeof(Model), () => modelWithNull.Text, new NameAndId("Text", "Text"),
                         string.Empty },
-                    { modelWithText, typeof(Model), () => modelWithText.Text, "Text",
+                    { modelWithText, typeof(Model), () => modelWithText.Text, new NameAndId("Text", "Text"),
                         "outer text" },
 
-                    { modelWithNull, typeof(NestedModel), () => modelWithNull.NestedModel.Text, "NestedModel.Text",
+                    { modelWithNull, typeof(NestedModel), () => modelWithNull.NestedModel.Text,
-                        string.Empty },
+                        new NameAndId("NestedModel.Text", "NestedModel_Text"), string.Empty },
-                    { modelWithText, typeof(NestedModel), () => modelWithText.NestedModel.Text, "NestedModel.Text",
+                    { modelWithText, typeof(NestedModel), () => modelWithText.NestedModel.Text,
-                        "inner text" },
+                        new NameAndId("NestedModel.Text", "NestedModel_Text"), "inner text" },
 
                     // Top-level indexing does not work end-to-end due to code generation issue #1345.
                     // TODO: Remove above comment when #1345 is fixed.
-                    { models, typeof(Model), () => models[0].Text, "[0].Text",
+                    { models, typeof(Model), () => models[0].Text,
-                        string.Empty },
+                        new NameAndId("[0].Text", "z0__Text"), string.Empty },
-                    { models, typeof(Model), () => models[1].Text, "[1].Text",
+                    { models, typeof(Model), () => models[1].Text,
-                        "outer text" },
+                        new NameAndId("[1].Text", "z1__Text"), "outer text" },
 
-                    { models, typeof(NestedModel), () => models[0].NestedModel.Text, "[0].NestedModel.Text",
+                    { models, typeof(NestedModel), () => models[0].NestedModel.Text,
-                        string.Empty },
+                        new NameAndId("[0].NestedModel.Text", "z0__NestedModel_Text"), string.Empty },
-                    { models, typeof(NestedModel), () => models[1].NestedModel.Text, "[1].NestedModel.Text",
+                    { models, typeof(NestedModel), () => models[1].NestedModel.Text,
-                        "inner text" },
+                        new NameAndId("[1].NestedModel.Text", "z1__NestedModel_Text"), "inner text" },
                 };
             }
         }
@@ -77,7 +77,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             object model,
             Type containerType,
             Func<object> modelAccessor,
-            string propertyPath,
+            NameAndId nameAndId,
             string expectedValue)
         {
             // Arrange
@@ -85,8 +85,8 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             {
                 { "class", "form-control" },
                 { "type", "text" },
-                { "id", propertyPath },
+                { "id", nameAndId.Id },
-                { "name", propertyPath },
+                { "name", nameAndId.Name },
                 { "valid", "from validation attributes" },
                 { "value", expectedValue },
             };
@@ -97,7 +97,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
 
             // Property name is either nameof(Model.Text) or nameof(NestedModel.Text).
             var metadata = metadataProvider.GetMetadataForProperty(modelAccessor, containerType, propertyName: "Text");
-            var modelExpression = new ModelExpression(propertyPath, metadata);
+            var modelExpression = new ModelExpression(nameAndId.Name, metadata);
 
             var tagHelperContext = new TagHelperContext(new Dictionary<string, object>());
             var htmlAttributes = new Dictionary<string, string>
@@ -185,6 +185,19 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             Assert.Equal(expectedTagName, output.TagName);
         }
 
+        public class NameAndId
+        {
+            public NameAndId(string name, string id)
+            {
+                Name = name;
+                Id = id;
+            }
+
+            public string Name { get; private set; }
+
+            public string Id { get; private set; }
+        }
+
         private class Model
         {
             public string Text { get; set; }

test/Microsoft.AspNet.Mvc.TagHelpers.Test/LabelTagHelperTest.cs

@@ -44,45 +44,45 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                 return new TheoryData<object, Type, Func<object>, string, TagHelperOutputContent>
                 {
                     { null, typeof(Model), () => null, "Text",
-                        new TagHelperOutputContent(Environment.NewLine, "Text") },
+                        new TagHelperOutputContent(Environment.NewLine, "Text", "Text") },
 
                     { modelWithNull, typeof(Model), () => modelWithNull.Text, "Text",
-                        new TagHelperOutputContent(Environment.NewLine, "Text") },
+                        new TagHelperOutputContent(Environment.NewLine, "Text", "Text") },
                     { modelWithText, typeof(Model), () => modelWithText.Text, "Text",
-                        new TagHelperOutputContent(Environment.NewLine, "Text") },
+                        new TagHelperOutputContent(Environment.NewLine, "Text", "Text") },
                     { modelWithText, typeof(Model), () => modelWithNull.Text, "Text",
-                        new TagHelperOutputContent("Hello World", "Hello World") },
+                        new TagHelperOutputContent("Hello World", "Hello World", "Text") },
                     { modelWithText, typeof(Model), () => modelWithText.Text, "Text",
-                        new TagHelperOutputContent("Hello World", "Hello World") },
+                        new TagHelperOutputContent("Hello World", "Hello World", "Text") },
 
                     { modelWithNull, typeof(NestedModel), () => modelWithNull.NestedModel.Text, "NestedModel.Text",
-                        new TagHelperOutputContent(Environment.NewLine, "Text") },
+                        new TagHelperOutputContent(Environment.NewLine, "Text", "NestedModel_Text") },
                     { modelWithText, typeof(NestedModel), () => modelWithText.NestedModel.Text, "NestedModel.Text",
-                        new TagHelperOutputContent(Environment.NewLine, "Text") },
+                        new TagHelperOutputContent(Environment.NewLine, "Text", "NestedModel_Text") },
                     { modelWithNull, typeof(NestedModel), () => modelWithNull.NestedModel.Text, "NestedModel.Text",
-                        new TagHelperOutputContent("Hello World", "Hello World") },
+                        new TagHelperOutputContent("Hello World", "Hello World", "NestedModel_Text") },
                     { modelWithText, typeof(NestedModel), () => modelWithText.NestedModel.Text, "NestedModel.Text",
-                        new TagHelperOutputContent("Hello World", "Hello World") },
+                        new TagHelperOutputContent("Hello World", "Hello World", "NestedModel_Text") },
 
                     // Note: Tests cases below here will not work in practice due to current limitations on indexing 
                     // into ModelExpressions. Will be fixed in https://github.com/aspnet/Mvc/issues/1345.
                     { models, typeof(Model), () => models[0].Text, "[0].Text",
-                        new TagHelperOutputContent(Environment.NewLine, "Text") },
+                        new TagHelperOutputContent(Environment.NewLine, "Text", "z0__Text") },
                     { models, typeof(Model), () => models[1].Text, "[1].Text",
-                        new TagHelperOutputContent(Environment.NewLine, "Text") },
+                        new TagHelperOutputContent(Environment.NewLine, "Text", "z1__Text") },
                     { models, typeof(Model), () => models[0].Text, "[0].Text",
-                        new TagHelperOutputContent("Hello World", "Hello World") },
+                        new TagHelperOutputContent("Hello World", "Hello World", "z0__Text") },
                     { models, typeof(Model), () => models[1].Text, "[1].Text",
-                        new TagHelperOutputContent("Hello World", "Hello World") },
+                        new TagHelperOutputContent("Hello World", "Hello World", "z1__Text") },
 
                     { models, typeof(NestedModel), () => models[0].NestedModel.Text, "[0].NestedModel.Text",
-                        new TagHelperOutputContent(Environment.NewLine, "Text") },
+                        new TagHelperOutputContent(Environment.NewLine, "Text", "z0__NestedModel_Text") },
                     { models, typeof(NestedModel), () => models[1].NestedModel.Text, "[1].NestedModel.Text",
-                        new TagHelperOutputContent(Environment.NewLine, "Text") },
+                        new TagHelperOutputContent(Environment.NewLine, "Text", "z1__NestedModel_Text") },
                     { models, typeof(NestedModel), () => models[0].NestedModel.Text, "[0].NestedModel.Text",
-                        new TagHelperOutputContent("Hello World", "Hello World") },
+                        new TagHelperOutputContent("Hello World", "Hello World", "z0__NestedModel_Text") },
                     { models, typeof(NestedModel), () => models[1].NestedModel.Text, "[1].NestedModel.Text",
-                        new TagHelperOutputContent("Hello World", "Hello World") },
+                        new TagHelperOutputContent("Hello World", "Hello World", "z1__NestedModel_Text") },
                 };
             }
         }
@@ -100,7 +100,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             var expectedAttributes = new Dictionary<string, string>
             {
                 { "class", "form-control" },
-                { "for", propertyPath }
+                { "for", tagHelperOutputContent.ExpectedId }
             };
             var metadataProvider = new DataAnnotationsModelMetadataProvider();
 
@@ -173,14 +173,18 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
 
         public class TagHelperOutputContent
         {
-            public TagHelperOutputContent(string outputContent, string expectedContent)
+            public TagHelperOutputContent(string outputContent, string expectedContent, string expectedId)
             {
                 OriginalContent = outputContent;
                 ExpectedContent = expectedContent;
+                ExpectedId = expectedId;
             }
 
             public string OriginalContent { get; set; }
+
             public string ExpectedContent { get; set; }
+
+            public string ExpectedId { get; set; }
         }
 
         private class Model

test/Microsoft.AspNet.Mvc.TagHelpers.Test/TextAreaTagHelperTest.cs

@@ -14,8 +14,8 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
     public class TextAreaTagHelperTest
     {
         // Model (List<Model> or Model instance), container type (Model or NestModel), model accessor,
-        // property path, expected content.
+        // property path / id, expected content.
-        public static TheoryData<object, Type, Func<object>, string, string> TestDataSet
+        public static TheoryData<object, Type, Func<object>, NameAndId, string> TestDataSet
         {
             get
             {
@@ -41,31 +41,40 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                     modelWithText,
                 };
 
-                return new TheoryData<object, Type, Func<object>, string, string>
+                return new TheoryData<object, Type, Func<object>, NameAndId, string>
                 {
-                    { null, typeof(Model), () => null, "Text",
+                    { null, typeof(Model), () => null,
+                        new NameAndId("Text", "Text"),
                         Environment.NewLine },
 
-                    { modelWithNull, typeof(Model), () => modelWithNull.Text, "Text",
+                    { modelWithNull, typeof(Model), () => modelWithNull.Text,
+                        new NameAndId("Text", "Text"),
                         Environment.NewLine },
-                    { modelWithText, typeof(Model), () => modelWithText.Text, "Text",
+                    { modelWithText, typeof(Model), () => modelWithText.Text,
+                        new NameAndId("Text", "Text"),
                         Environment.NewLine + "outer text" },
 
-                    { modelWithNull, typeof(NestedModel), () => modelWithNull.NestedModel.Text, "NestedModel.Text",
+                    { modelWithNull, typeof(NestedModel), () => modelWithNull.NestedModel.Text,
+                        new NameAndId("NestedModel.Text", "NestedModel_Text"),
                         Environment.NewLine },
-                    { modelWithText, typeof(NestedModel), () => modelWithText.NestedModel.Text, "NestedModel.Text",
+                    { modelWithText, typeof(NestedModel), () => modelWithText.NestedModel.Text,
+                        new NameAndId("NestedModel.Text", "NestedModel_Text"),
                         Environment.NewLine + "inner text" },
 
                     // Top-level indexing does not work end-to-end due to code generation issue #1345.
                     // TODO: Remove above comment when #1345 is fixed.
-                    { models, typeof(Model), () => models[0].Text, "[0].Text",
+                    { models, typeof(Model), () => models[0].Text,
+                        new NameAndId("[0].Text", "z0__Text"),
                         Environment.NewLine },
-                    { models, typeof(Model), () => models[1].Text, "[1].Text",
+                    { models, typeof(Model), () => models[1].Text,
+                        new NameAndId("[1].Text", "z1__Text"),
                         Environment.NewLine + "outer text" },
 
-                    { models, typeof(NestedModel), () => models[0].NestedModel.Text, "[0].NestedModel.Text",
+                    { models, typeof(NestedModel), () => models[0].NestedModel.Text,
+                        new NameAndId("[0].NestedModel.Text", "z0__NestedModel_Text"),
                         Environment.NewLine },
-                    { models, typeof(NestedModel), () => models[1].NestedModel.Text, "[1].NestedModel.Text",
+                    { models, typeof(NestedModel), () => models[1].NestedModel.Text,
+                        new NameAndId("[1].NestedModel.Text", "z1__NestedModel_Text"),
                         Environment.NewLine + "inner text" },
                 };
             }
@@ -77,15 +86,15 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             object model,
             Type containerType,
             Func<object> modelAccessor,
-            string propertyPath,
+            NameAndId nameAndId,
             string expectedContent)
         {
             // Arrange
             var expectedAttributes = new Dictionary<string, string>
             {
                 { "class", "form-control" },
-                { "id", propertyPath },
+                { "id", nameAndId.Id },
-                { "name", propertyPath },
+                { "name", nameAndId.Name },
                 {  "valid", "from validation attributes" },
             };
             var expectedTagName = "textarea";
@@ -94,7 +103,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
 
             // Property name is either nameof(Model.Text) or nameof(NestedModel.Text).
             var metadata = metadataProvider.GetMetadataForProperty(modelAccessor, containerType, propertyName: "Text");
-            var modelExpression = new ModelExpression(propertyPath, metadata);
+            var modelExpression = new ModelExpression(nameAndId.Name, metadata);
             var tagHelper = new TextAreaTagHelper
             {
                 For = modelExpression,
@@ -178,6 +187,19 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             Assert.Equal(expectedTagName, output.TagName);
         }
 
+        public class NameAndId
+        {
+            public NameAndId(string name, string id)
+            {
+                Name = name;
+                Id = id;
+            }
+
+            public string Name { get; private set; }
+
+            public string Id { get; private set; }
+        }
+
         private class Model
         {
             public string Text { get; set; }