Change `RazorError`s to consistently provide error lengths.

- Updated `TagHelper` errors to no longer highlight the entire tag as an error, instead just the tag name is marked as an error. This is now consistent with nested tags in `@{ ... }` errors.
- Updated `RazorError` and corresponding error logging constructs to disallow creation without providing lengths.
- Updated `TagHelperDescriptorResolver` and related classes to properly determine assembly locations within directives. This allows for exact error locations in the `@addTagHelper` directive.

#386

src/Microsoft.AspNet.Razor.Runtime/TagHelpers/TagHelperDescriptorFactory.cs

@@ -315,7 +315,7 @@ namespace Microsoft.AspNet.Razor.Runtime.TagHelpers
 
             if (string.IsNullOrWhiteSpace(name))
             {
-                errorSink.OnError(SourceLocation.Zero, whitespaceError);
+                errorSink.OnError(SourceLocation.Zero, whitespaceError, length: 0);
 
                 validName = false;
             }
@@ -327,7 +327,7 @@ namespace Microsoft.AspNet.Razor.Runtime.TagHelpers
                         InvalidNonWhitespaceNameCharacters.Contains(character))
                     {
                         var error = characterErrorBuilder(character);
-                        errorSink.OnError(SourceLocation.Zero, error);
+                        errorSink.OnError(SourceLocation.Zero, error, length: 0);
 
                         validName = false;
                     }
@@ -379,7 +379,8 @@ namespace Microsoft.AspNet.Razor.Runtime.TagHelpers
                             type.FullName,
                             property.Name,
                             typeof(HtmlAttributeNameAttribute).FullName,
-                            nameof(HtmlAttributeNameAttribute.Name)));
+                            nameof(HtmlAttributeNameAttribute.Name)),
+                        length: 0);
                     continue;
                 }
 
@@ -438,7 +439,8 @@ namespace Microsoft.AspNet.Razor.Runtime.TagHelpers
                     SourceLocation.Zero,
                     Resources.FormatTagHelperDescriptorFactory_InvalidAttributeNameNullOrEmpty(
                         parentType.FullName,
-                        attributeDescriptor.PropertyName));
+                        attributeDescriptor.PropertyName),
+                    length: 0);
 
                 return false;
             }
@@ -492,7 +494,8 @@ namespace Microsoft.AspNet.Razor.Runtime.TagHelpers
                     Resources.FormatTagHelperDescriptorFactory_InvalidAttributeNameOrPrefixWhitespace(
                         parentType.FullName,
                         propertyName,
-                        nameOrPrefix));
+                        nameOrPrefix),
+                    length: 0);
 
                 return false;
             }
@@ -508,7 +511,8 @@ namespace Microsoft.AspNet.Razor.Runtime.TagHelpers
                         propertyName,
                         nameOrPrefix,
                         attributeNameOrPrefix,
-                        DataDashPrefix));
+                        DataDashPrefix),
+                    length: 0);
 
                 return false;
             }
@@ -525,7 +529,8 @@ namespace Microsoft.AspNet.Razor.Runtime.TagHelpers
                             propertyName,
                             nameOrPrefix,
                             attributeNameOrPrefix,
-                            character));
+                            character),
+                    length: 0);
 
                     isValid = false;
                 }
@@ -576,7 +581,8 @@ namespace Microsoft.AspNet.Razor.Runtime.TagHelpers
                             property.Name,
                             nameof(HtmlAttributeNameAttribute),
                             nameof(HtmlAttributeNameAttribute.DictionaryAttributePrefix),
-                            "IDictionary<string, TValue>"));
+                            "IDictionary<string, TValue>"),
+                        length: 0);
                 }
                 else if (attributeNameAttribute != null && !hasPublicSetter)
                 {
@@ -589,7 +595,8 @@ namespace Microsoft.AspNet.Razor.Runtime.TagHelpers
                             parentType.FullName,
                             property.Name,
                             nameof(HtmlAttributeNameAttribute),
-                            "IDictionary<string, TValue>"));
+                            "IDictionary<string, TValue>"),
+                        length: 0);
                 }
 
                 return null;
@@ -608,7 +615,8 @@ namespace Microsoft.AspNet.Razor.Runtime.TagHelpers
                         property.Name,
                         nameof(HtmlAttributeNameAttribute),
                         nameof(HtmlAttributeNameAttribute.DictionaryAttributePrefix),
-                        "IDictionary<string, TValue>"));
+                        "IDictionary<string, TValue>"),
+                    length: 0);
 
                 return null;
             }

src/Microsoft.AspNet.Razor.Runtime/TagHelpers/TagHelperDescriptorResolver.cs

@@ -81,7 +81,7 @@ namespace Microsoft.AspNet.Razor.Runtime.TagHelpers
                     {
                         var descriptors = ResolveDescriptorsInAssembly(
                             lookupInfo.AssemblyName,
-                            directiveDescriptor.Location,
+                            lookupInfo.AssemblyNameLocation,
                             context.ErrorSink);
 
                         // Only use descriptors that match our lookup info
@@ -101,7 +101,8 @@ namespace Microsoft.AspNet.Razor.Runtime.TagHelpers
                         Resources.FormatTagHelperDescriptorResolver_EncounteredUnexpectedError(
                             "@" + directiveName,
                             directiveDescriptor.DirectiveText,
-                            ex.Message));
+                            ex.Message),
+                        GetErrorLength(directiveDescriptor.DirectiveText));
                 }
             }
 
@@ -183,7 +184,8 @@ namespace Microsoft.AspNet.Razor.Runtime.TagHelpers
                     context.ErrorSink.OnError(
                         directive.Location,
                         Resources.FormatTagHelperDescriptorResolver_InvalidTagHelperDirective(
-                            SyntaxConstants.CSharp.TagHelperPrefixKeyword));
+                            SyntaxConstants.CSharp.TagHelperPrefixKeyword),
+                        GetErrorLength(directive.DirectiveText));
                 }
             }
 
@@ -213,7 +215,8 @@ namespace Microsoft.AspNet.Razor.Runtime.TagHelpers
                         Resources.FormatTagHelperDescriptorResolver_InvalidTagHelperPrefixValue(
                             SyntaxConstants.CSharp.TagHelperPrefixKeyword,
                             character,
-                            prefix));
+                            prefix),
+                        prefix.Length);
 
                     return false;
                 }
@@ -241,8 +244,9 @@ namespace Microsoft.AspNet.Razor.Runtime.TagHelpers
             return regex.IsMatch(descriptor.TypeName);
         }
 
-        private static LookupInfo GetLookupInfo(TagHelperDirectiveDescriptor directiveDescriptor,
+        private static LookupInfo GetLookupInfo(
-                                                ErrorSink errorSink)
+            TagHelperDirectiveDescriptor directiveDescriptor,
+            ErrorSink errorSink)
         {
             var lookupText = directiveDescriptor.DirectiveText;
             var lookupStrings = lookupText?.Split(new[] { ',' });
@@ -256,23 +260,42 @@ namespace Microsoft.AspNet.Razor.Runtime.TagHelpers
             {
                 errorSink.OnError(
                     directiveDescriptor.Location,
-                    Resources.FormatTagHelperDescriptorResolver_InvalidTagHelperLookupText(lookupText));
+                    Resources.FormatTagHelperDescriptorResolver_InvalidTagHelperLookupText(lookupText),
+                    GetErrorLength(lookupText));
 
                 return null;
             }
 
+            var trimmedAssemblyName = lookupStrings[1].Trim();
+
+            // + 1 is for the comma separator in the lookup text.
+            var assemblyNameIndex = lookupStrings[0].Length + 1 + lookupStrings[1].IndexOf(trimmedAssemblyName);
+            var assemblyNamePrefix = directiveDescriptor.DirectiveText.Substring(0, assemblyNameIndex);
+            var assemblyNameLocation = SourceLocation.Advance(directiveDescriptor.Location, assemblyNamePrefix);
+
             return new LookupInfo
             {
                 TypePattern = lookupStrings[0].Trim(),
-                AssemblyName = lookupStrings[1].Trim()
+                AssemblyName = trimmedAssemblyName,
+                AssemblyNameLocation = assemblyNameLocation,
             };
         }
 
+        private static int GetErrorLength(string directiveText)
+        {
+            var nonNullLength = directiveText == null ? 1 : directiveText.Length;
+            var normalizeEmptyStringLength = Math.Max(nonNullLength, 1);
+
+            return normalizeEmptyStringLength;
+        }
+
         private class LookupInfo
         {
             public string AssemblyName { get; set; }
 
             public string TypePattern { get; set; }
+
+            public SourceLocation AssemblyNameLocation { get; set; }
         }
     }
 }

src/Microsoft.AspNet.Razor.Runtime/TagHelpers/TagHelperTypeResolver.cs

@@ -35,14 +35,18 @@ namespace Microsoft.AspNet.Razor.Runtime.TagHelpers
         /// <see cref="ITagHelper"/> <see cref="Type"/>s.</param>
         /// <returns>An <see cref="IEnumerable{Type}"/> of valid <see cref="ITagHelper"/> <see cref="Type"/>s.
         /// </returns>
-        public IEnumerable<Type> Resolve(string name, 
+        public IEnumerable<Type> Resolve(
-                                         SourceLocation documentLocation, 
+            string name,
-                                         [NotNull] ErrorSink errorSink)
+            SourceLocation documentLocation,
+            [NotNull] ErrorSink errorSink)
         {
             if (string.IsNullOrEmpty(name))
             {
-                errorSink.OnError(documentLocation,
+                var errorLength = name == null ? 1 : Math.Max(name.Length, 1);
-                                  Resources.TagHelperTypeResolver_TagHelperAssemblyNameCannotBeEmptyOrNull);
+                errorSink.OnError(
+                    documentLocation,
+                    Resources.TagHelperTypeResolver_TagHelperAssemblyNameCannotBeEmptyOrNull,
+                    errorLength);
 
                 return Type.EmptyTypes;
             }
@@ -60,7 +64,8 @@ namespace Microsoft.AspNet.Razor.Runtime.TagHelpers
                     documentLocation,
                     Resources.FormatTagHelperTypeResolver_CannotResolveTagHelperAssembly(
                         assemblyName.Name,
-                        ex.Message));
+                        ex.Message),
+                    name.Length);
 
                 return Type.EmptyTypes;
             }

src/Microsoft.AspNet.Razor/ErrorSink.cs

@@ -40,16 +40,6 @@ namespace Microsoft.AspNet.Razor
             _errors.Add(error);
         }
 
-        /// <summary>
-        /// Creates and tracks a new <see cref="RazorError"/>.
-        /// </summary>
-        /// <param name="location"><see cref="SourceLocation"/> of the error.</param>
-        /// <param name="message">A message describing the error.</param>
-        public void OnError(SourceLocation location, string message)
-        {
-            OnError(location, message, RazorError.DefaultErrorLength);
-        }
-
         /// <summary>
         /// Creates and tracks a new <see cref="RazorError"/>.
         /// </summary>

src/Microsoft.AspNet.Razor/Parser/CSharpCodeParser.Directives.cs

@@ -102,7 +102,10 @@ namespace Microsoft.AspNet.Razor.Parser
                 if (!errorReported)
                 {
                     errorReported = true;
-                    Context.OnError(errorLocation, RazorResources.ParseError_MissingOpenBraceAfterSection);
+                    Context.OnError(
+                        errorLocation,
+                        RazorResources.ParseError_MissingOpenBraceAfterSection,
+                        length: 1  /* { */);
                 }
 
                 PutCurrentBack();
@@ -133,9 +136,10 @@ namespace Microsoft.AspNet.Razor.Parser
             if (!Optional(CSharpSymbolType.RightBrace))
             {
                 editHandler.AutoCompleteString = "}";
-                Context.OnError(CurrentLocation,
+                Context.OnError(
-                                RazorResources.FormatParseError_Expected_X(
+                    CurrentLocation,
-                                    Language.GetSample(CSharpSymbolType.RightBrace)));
+                    RazorResources.FormatParseError_Expected_X(Language.GetSample(CSharpSymbolType.RightBrace)),
+                    length: 1 /* } */);
             }
             else
             {
@@ -160,8 +164,10 @@ namespace Microsoft.AspNet.Razor.Parser
 
             if (!At(CSharpSymbolType.LeftBrace))
             {
-                Context.OnError(CurrentLocation,
+                Context.OnError(
-                                RazorResources.FormatParseError_Expected_X(Language.GetSample(CSharpSymbolType.LeftBrace)));
+                    CurrentLocation,
+                    RazorResources.FormatParseError_Expected_X(Language.GetSample(CSharpSymbolType.LeftBrace)),
+                    length: 1 /* { */);
                 CompleteBlock();
                 Output(SpanKind.MetaCode);
                 return;
@@ -186,7 +192,10 @@ namespace Microsoft.AspNet.Razor.Parser
             if (!At(CSharpSymbolType.RightBrace))
             {
                 editHandler.AutoCompleteString = "}";
-                Context.OnError(blockStart, RazorResources.FormatParseError_Expected_EndOfBlock_Before_EOF(block.Name, "}", "{"));
+                Context.OnError(
+                    blockStart,
+                    RazorResources.FormatParseError_Expected_EndOfBlock_Before_EOF(block.Name, "}", "{"),
+                    length: 1 /* } */);
                 CompleteBlock();
                 Output(SpanKind.Code);
             }

src/Microsoft.AspNet.Razor/Parser/CSharpCodeParser.Statements.cs

@@ -29,7 +29,10 @@ namespace Microsoft.AspNet.Razor.Parser
 
         protected virtual void ReservedDirective(bool topLevel)
         {
-            Context.OnError(CurrentLocation, RazorResources.FormatParseError_ReservedWord(CurrentSymbol.Content));
+            Context.OnError(
+                CurrentLocation,
+                RazorResources.FormatParseError_ReservedWord(CurrentSymbol.Content),
+                CurrentSymbol.Content.Length);
             AcceptAndMoveNext();
             Span.EditHandler.AcceptedCharacters = AcceptedCharacters.None;
             Span.ChunkGenerator = SpanChunkGenerator.Null;
@@ -109,7 +112,10 @@ namespace Microsoft.AspNet.Razor.Parser
                 // using Identifier ==> Using Declaration
                 if (!topLevel)
                 {
-                    Context.OnError(block.Start, RazorResources.ParseError_NamespaceImportAndTypeAlias_Cannot_Exist_Within_CodeBlock);
+                    Context.OnError(
+                        block.Start,
+                        RazorResources.ParseError_NamespaceImportAndTypeAlias_Cannot_Exist_Within_CodeBlock,
+                        block.Name.Length);
                     StandardStatement();
                 }
                 else
@@ -327,10 +333,12 @@ namespace Microsoft.AspNet.Razor.Parser
                 // Check for "{" to make sure we're at a block
                 if (!At(CSharpSymbolType.LeftBrace))
                 {
-                    Context.OnError(CurrentLocation,
+                    Context.OnError(
-                                    RazorResources.FormatParseError_SingleLine_ControlFlowStatements_Not_Allowed(
+                        CurrentLocation,
-                                        Language.GetSample(CSharpSymbolType.LeftBrace),
+                        RazorResources.FormatParseError_SingleLine_ControlFlowStatements_Not_Allowed(
-                                        CurrentSymbol.Content));
+                            Language.GetSample(CSharpSymbolType.LeftBrace),
+                            CurrentSymbol.Content),
+                        CurrentSymbol.Content.Length);
                 }
 
                 // Parse the statement and then we're done
@@ -480,7 +488,10 @@ namespace Microsoft.AspNet.Razor.Parser
             {
                 if (type == CSharpSymbolType.Transition && !isSingleLineMarkup)
                 {
-                    Context.OnError(loc, RazorResources.ParseError_AtInCode_Must_Be_Followed_By_Colon_Paren_Or_Identifier_Start);
+                    Context.OnError(
+                        loc,
+                        RazorResources.ParseError_AtInCode_Must_Be_Followed_By_Colon_Paren_Or_Identifier_Start,
+                        length: 1 /* @ */);
                 }
 
                 // Markup block
@@ -560,7 +571,10 @@ namespace Microsoft.AspNet.Razor.Parser
                 // Throw errors as necessary, but continue parsing
                 if (At(CSharpSymbolType.LeftBrace))
                 {
-                    Context.OnError(CurrentLocation, RazorResources.ParseError_Unexpected_Nested_CodeBlock);
+                    Context.OnError(
+                        CurrentLocation,
+                        RazorResources.ParseError_Unexpected_Nested_CodeBlock,
+                        length: 1  /* { */);
                 }
 
                 // @( or @foo - Nested expression, parse a child block
@@ -649,7 +663,10 @@ namespace Microsoft.AspNet.Razor.Parser
 
             if (EndOfFile)
             {
-                Context.OnError(block.Start, RazorResources.FormatParseError_Expected_EndOfBlock_Before_EOF(block.Name, '}', '{'));
+                Context.OnError(
+                    block.Start,
+                    RazorResources.FormatParseError_Expected_EndOfBlock_Before_EOF(block.Name, '}', '{'),
+                    length: 1  /* { OR } */);
             }
             else if (acceptTerminatingBrace)
             {

src/Microsoft.AspNet.Razor/Parser/CSharpCodeParser.cs

@@ -256,15 +256,25 @@ namespace Microsoft.AspNet.Razor.Parser
                     };
                     if (At(CSharpSymbolType.WhiteSpace) || At(CSharpSymbolType.NewLine))
                     {
-                        Context.OnError(CurrentLocation, RazorResources.ParseError_Unexpected_WhiteSpace_At_Start_Of_CodeBlock_CS);
+                        Context.OnError(
+                            CurrentLocation,
+                            RazorResources.ParseError_Unexpected_WhiteSpace_At_Start_Of_CodeBlock_CS,
+                            CurrentSymbol.Content.Length);
                     }
                     else if (EndOfFile)
                     {
-                        Context.OnError(CurrentLocation, RazorResources.ParseError_Unexpected_EndOfFile_At_Start_Of_CodeBlock);
+                        Context.OnError(
+                            CurrentLocation,
+                            RazorResources.ParseError_Unexpected_EndOfFile_At_Start_Of_CodeBlock,
+                            length: 1 /* end of file */);
                     }
                     else
                     {
-                        Context.OnError(CurrentLocation, RazorResources.FormatParseError_Unexpected_Character_At_Start_Of_CodeBlock_CS(CurrentSymbol.Content));
+                        Context.OnError(
+                            CurrentLocation,
+                            RazorResources.FormatParseError_Unexpected_Character_At_Start_Of_CodeBlock_CS(
+                                CurrentSymbol.Content),
+                            CurrentSymbol.Content.Length);
                     }
                 }
                 finally
@@ -529,7 +539,10 @@ namespace Microsoft.AspNet.Razor.Parser
                 if (!success)
                 {
                     AcceptUntil(CSharpSymbolType.LessThan);
-                    Context.OnError(block.Start, RazorResources.FormatParseError_Expected_EndOfBlock_Before_EOF(block.Name, ")", "("));
+                    Context.OnError(
+                        block.Start,
+                        RazorResources.FormatParseError_Expected_EndOfBlock_Before_EOF(block.Name, ")", "("),
+                        length: 1 /* ( */);
                 }
 
                 // If necessary, put an empty-content marker symbol here
@@ -556,7 +569,10 @@ namespace Microsoft.AspNet.Razor.Parser
         {
             if (Context.IsWithin(BlockType.Template))
             {
-                Context.OnError(CurrentLocation, RazorResources.ParseError_InlineMarkup_Blocks_Cannot_Be_Nested);
+                Context.OnError(
+                    CurrentLocation,
+                    RazorResources.ParseError_InlineMarkup_Blocks_Cannot_Be_Nested,
+                    length: 1 /* @ */);
             }
             Output(SpanKind.Code);
             using (Context.StartBlock(BlockType.Template))

src/Microsoft.AspNet.Razor/Parser/HtmlMarkupParser.Block.cs

@@ -15,6 +15,8 @@ namespace Microsoft.AspNet.Razor.Parser
 {
     public partial class HtmlMarkupParser
     {
+        private const string ScriptTagName = "script";
+
         private SourceLocation _lastTagStart = SourceLocation.Zero;
         private HtmlSymbol _bufferedOpenAngle;
 
@@ -62,7 +64,10 @@ namespace Microsoft.AspNet.Razor.Parser
                     }
                     else
                     {
-                        Context.OnError(CurrentSymbol.Start, RazorResources.ParseError_MarkupBlock_Must_Start_With_Tag);
+                        Context.OnError(
+                            CurrentSymbol.Start,
+                            RazorResources.ParseError_MarkupBlock_Must_Start_With_Tag,
+                            CurrentSymbol.Content.Length);
                     }
                     Output(SpanKind.Markup);
                 }
@@ -221,7 +226,10 @@ namespace Microsoft.AspNet.Razor.Parser
             }
             if (tags.Count == 0)
             {
-                Context.OnError(CurrentLocation, RazorResources.ParseError_OuterTagMissingName);
+                Context.OnError(
+                    CurrentLocation,
+                    RazorResources.ParseError_OuterTagMissingName,
+                    length: 1  /* end of file */);
             }
             return false;
         }
@@ -346,11 +354,10 @@ namespace Microsoft.AspNet.Razor.Parser
 
         private bool EndTextTag(HtmlSymbol solidus, IDisposable tagBlockWrapper)
         {
-            var start = _bufferedOpenAngle.Start;
-
             Accept(_bufferedOpenAngle);
             Accept(solidus);
 
+            var textLocation = CurrentLocation;
             Assert(HtmlSymbolType.Text);
             AcceptAndMoveNext();
 
@@ -358,7 +365,10 @@ namespace Microsoft.AspNet.Razor.Parser
 
             if (!seenCloseAngle)
             {
-                Context.OnError(start, RazorResources.ParseError_TextTagCannotContainAttributes);
+                Context.OnError(
+                    textLocation,
+                    RazorResources.ParseError_TextTagCannotContainAttributes,
+                    length: 4 /* text */);
 
                 Span.EditHandler.AcceptedCharacters = AcceptedCharacters.Any;
                 RecoverTextTag();
@@ -751,6 +761,7 @@ namespace Microsoft.AspNet.Razor.Parser
                 Span.ChunkGenerator = SpanChunkGenerator.Null;
 
                 Accept(_bufferedOpenAngle);
+                var textLocation = CurrentLocation;
                 Assert(HtmlSymbolType.Text);
 
                 AcceptAndMoveNext();
@@ -771,7 +782,10 @@ namespace Microsoft.AspNet.Razor.Parser
                 {
                     Context.Source.Position = bookmark;
                     NextToken();
-                    Context.OnError(tag.Item2, RazorResources.ParseError_TextTagCannotContainAttributes);
+                    Context.OnError(
+                        textLocation,
+                        RazorResources.ParseError_TextTagCannotContainAttributes,
+                        length: 4 /* text */);
 
                     RecoverTextTag();
                 }
@@ -821,7 +835,10 @@ namespace Microsoft.AspNet.Razor.Parser
             var seenClose = Optional(HtmlSymbolType.CloseAngle);
             if (!seenClose)
             {
-                Context.OnError(tag.Item2, RazorResources.FormatParseError_UnfinishedTag(tag.Item1.Content));
+                Context.OnError(
+                    SourceLocation.Advance(tag.Item2, "<"),
+                    RazorResources.FormatParseError_UnfinishedTag(tag.Item1.Content),
+                    Math.Max(tag.Item1.Content.Length, 1));
             }
             else
             {
@@ -883,7 +900,7 @@ namespace Microsoft.AspNet.Razor.Parser
                         Context.Source.Position = bookmark;
                         NextToken();
                     }
-                    else if (string.Equals(tagName, "script", StringComparison.OrdinalIgnoreCase))
+                    else if (string.Equals(tagName, ScriptTagName, StringComparison.OrdinalIgnoreCase))
                     {
                         CompleteTagBlockWithSpan(tagBlockWrapper, AcceptedCharacters.None, SpanKind.Markup);
 
@@ -916,7 +933,8 @@ namespace Microsoft.AspNet.Razor.Parser
                     var solidus = CurrentSymbol;
                     NextToken(); // Skip over '/', current should be text
 
-                    if (At(HtmlSymbolType.Text) && string.Equals(CurrentSymbol.Content, "script", StringComparison.OrdinalIgnoreCase))
+                    if (At(HtmlSymbolType.Text) &&
+                        string.Equals(CurrentSymbol.Content, ScriptTagName, StringComparison.OrdinalIgnoreCase))
                     {
                         seenEndScript = true;
                     }
@@ -944,7 +962,10 @@ namespace Microsoft.AspNet.Razor.Parser
                         SkipToAndParseCode(HtmlSymbolType.CloseAngle);
                         if (!Optional(HtmlSymbolType.CloseAngle))
                         {
-                            Context.OnError(tagStart, RazorResources.FormatParseError_UnfinishedTag("script"));
+                            Context.OnError(
+                                SourceLocation.Advance(tagStart, "</"),
+                                RazorResources.FormatParseError_UnfinishedTag(ScriptTagName),
+                                ScriptTagName.Length);
                         }
                         Output(SpanKind.Markup);
                     }
@@ -999,11 +1020,17 @@ namespace Microsoft.AspNet.Razor.Parser
             }
             if (currentTag != null)
             {
-                Context.OnError(currentTag.Item2, RazorResources.FormatParseError_MissingEndTag(currentTag.Item1.Content));
+                Context.OnError(
+                    SourceLocation.Advance(currentTag.Item2, "<"),
+                    RazorResources.FormatParseError_MissingEndTag(currentTag.Item1.Content),
+                    currentTag.Item1.Content.Length);
             }
             else
             {
-                Context.OnError(tagStart, RazorResources.FormatParseError_UnexpectedEndTag(tagName));
+                Context.OnError(
+                    SourceLocation.Advance(tagStart, "</"),
+                    RazorResources.FormatParseError_UnexpectedEndTag(tagName),
+                    tagName.Length);
             }
             return false;
         }
@@ -1018,7 +1045,10 @@ namespace Microsoft.AspNet.Razor.Parser
                     tags.Pop();
                 }
                 var tag = tags.Pop();
-                Context.OnError(tag.Item2, RazorResources.FormatParseError_MissingEndTag(tag.Item1.Content));
+                Context.OnError(
+                    SourceLocation.Advance(tag.Item2, "<"),
+                    RazorResources.FormatParseError_MissingEndTag(tag.Item1.Content),
+                    tag.Item1.Content.Length);
             }
             else if (complete)
             {

src/Microsoft.AspNet.Razor/Parser/ParserContext.cs

@@ -200,14 +200,6 @@ namespace Microsoft.AspNet.Razor.Parser
             _errorSink.OnError(error);
         }
 
-        public void OnError(SourceLocation location, string message)
-        {
-            EnusreNotTerminated();
-            AssertOnOwnerTask();
-
-            _errorSink.OnError(location, message);
-        }
-
         public void OnError(SourceLocation location, string message, int length)
         {
             EnusreNotTerminated();
@@ -216,12 +208,12 @@ namespace Microsoft.AspNet.Razor.Parser
             _errorSink.OnError(location, message, length);
         }
 
-        public void OnError(SourceLocation location, string message, params object[] args)
+        public void OnError(SourceLocation location, string message, int length, params object[] args)
         {
             EnusreNotTerminated();
             AssertOnOwnerTask();
 
-            OnError(location, string.Format(CultureInfo.CurrentCulture, message, args));
+            OnError(location, string.Format(CultureInfo.CurrentCulture, message, args), length);
         }
 
         public ParserResults CompleteParse()

src/Microsoft.AspNet.Razor/Parser/TagHelpers/TagHelperBlockRewriter.cs

@@ -296,8 +296,10 @@ namespace Microsoft.AspNet.Razor.Parser.TagHelpers.Internal
 
             if (childSpan == null || childSpan.Kind != SpanKind.Markup)
             {
-                errorSink.OnError(block.Children.First().Start,
+                errorSink.OnError(
-                                  RazorResources.FormatTagHelpers_CannotHaveCSharpInTagDeclaration(tagName));
+                    block.Start,
+                    RazorResources.FormatTagHelpers_CannotHaveCSharpInTagDeclaration(tagName),
+                    block.Length);
 
                 return null;
             }
@@ -316,7 +318,10 @@ namespace Microsoft.AspNet.Razor.Parser.TagHelpers.Internal
 
             if (name == null)
             {
-                errorSink.OnError(childSpan.Start, RazorResources.FormatTagHelpers_AttributesMustHaveAName(tagName));
+                errorSink.OnError(
+                    childSpan.Start,
+                    RazorResources.FormatTagHelpers_AttributesMustHaveAName(tagName),
+                    childSpan.Length);
 
                 return null;
             }

src/Microsoft.AspNet.Razor/Parser/TagHelpers/TagHelperDirectiveSpanVisitor.cs

@@ -67,11 +67,12 @@ namespace Microsoft.AspNet.Razor.Parser.TagHelpers
                     chunkGenerator.RemoveTagHelperDescriptors ?
                     TagHelperDirectiveType.RemoveTagHelper :
                     TagHelperDirectiveType.AddTagHelper;
+                var textLocation = GetSubTextSourceLocation(span, chunkGenerator.LookupText);
 
                 var directiveDescriptor = new TagHelperDirectiveDescriptor
                 {
                     DirectiveText = chunkGenerator.LookupText,
-                    Location = span.Start,
+                    Location = textLocation,
                     DirectiveType = directive
                 };
 
@@ -80,16 +81,26 @@ namespace Microsoft.AspNet.Razor.Parser.TagHelpers
             else if (span.ChunkGenerator is TagHelperPrefixDirectiveChunkGenerator)
             {
                 var chunkGenerator = (TagHelperPrefixDirectiveChunkGenerator)span.ChunkGenerator;
+                var textLocation = GetSubTextSourceLocation(span, chunkGenerator.Prefix);
 
                 var directiveDescriptor = new TagHelperDirectiveDescriptor
                 {
                     DirectiveText = chunkGenerator.Prefix,
-                    Location = span.Start,
+                    Location = textLocation,
                     DirectiveType = TagHelperDirectiveType.TagHelperPrefix
                 };
 
                 _directiveDescriptors.Add(directiveDescriptor);
             }
         }
+
+        private static SourceLocation GetSubTextSourceLocation(Span span, string text)
+        {
+            var startOffset = span.Content.IndexOf(text);
+            var offsetContent = span.Content.Substring(0, startOffset);
+            var offsetTextLocation = SourceLocation.Advance(span.Start, offsetContent);
+
+            return offsetTextLocation;
+        }
     }
 }

src/Microsoft.AspNet.Razor/Parser/TagHelpers/TagHelperParseTreeRewriter.cs

@@ -208,12 +208,12 @@ namespace Microsoft.AspNet.Razor.Parser.TagHelpers.Internal
                     {
                         // End tag TagHelper that states it shouldn't have an end tag.
                         context.ErrorSink.OnError(
-                            tagBlock.Start,
+                            SourceLocation.Advance(tagBlock.Start, "</"),
                             RazorResources.FormatTagHelperParseTreeRewriter_EndTagTagHelperMustNotHaveAnEndTag(
                                 tagName,
                                 invalidDescriptor.TypeName,
                                 invalidDescriptor.TagStructure),
-                            tagBlock.Length);
+                            tagName.Length);
 
                         return false;
                     }
@@ -233,8 +233,9 @@ namespace Microsoft.AspNet.Razor.Parser.TagHelpers.Internal
                         // Could not recover, the end tag helper has no corresponding start tag, create
                         // an error based on the current childBlock.
                         context.ErrorSink.OnError(
-                            tagBlock.Start,
+                            SourceLocation.Advance(tagBlock.Start, "</"),
-                            RazorResources.FormatTagHelpersParseTreeRewriter_FoundMalformedTagHelper(tagName));
+                            RazorResources.FormatTagHelpersParseTreeRewriter_FoundMalformedTagHelper(tagName),
+                            tagName.Length);
 
                         return false;
                     }
@@ -333,8 +334,9 @@ namespace Microsoft.AspNet.Razor.Parser.TagHelpers.Internal
                 tagName,
                 tracker.Builder.TagName,
                 allowedChildrenString);
+            var errorStart = GetTagDeclarationErrorStart(tagBlock);
 
-            errorSink.OnError(tagBlock.Start, errorMessage, tagBlock.Length);
+            errorSink.OnError(errorStart, errorMessage, tagName.Length);
         }
 
         private static void ValidateDescriptors(
@@ -373,9 +375,12 @@ namespace Microsoft.AspNet.Razor.Parser.TagHelpers.Internal
             // We assume an invalid syntax until we verify that the tag meets all of our "valid syntax" criteria.
             if (IsPartialTag(tag))
             {
+                var errorStart = GetTagDeclarationErrorStart(tag);
+
                 context.ErrorSink.OnError(
-                    tag.Start,
+                    errorStart,
-                    RazorResources.FormatTagHelpersParseTreeRewriter_MissingCloseAngle(tagName));
+                    RazorResources.FormatTagHelpersParseTreeRewriter_MissingCloseAngle(tagName),
+                    tagName.Length);
 
                 return false;
             }
@@ -383,6 +388,13 @@ namespace Microsoft.AspNet.Razor.Parser.TagHelpers.Internal
             return true;
         }
 
+        private static SourceLocation GetTagDeclarationErrorStart(Block tagBlock)
+        {
+            var advanceBy = IsEndTag(tagBlock) ? "</" : "<";
+
+            return SourceLocation.Advance(tagBlock.Start, advanceBy);
+        }
+
         private static bool IsPartialTag(Block tagBlock)
         {
             // No need to validate the tag end because in order to be a tag block it must start with '<'.
@@ -501,9 +513,10 @@ namespace Microsoft.AspNet.Razor.Parser.TagHelpers.Internal
                 var malformedTagHelper = _trackerStack.Peek().Builder;
 
                 context.ErrorSink.OnError(
-                    malformedTagHelper.Start,
+                    SourceLocation.Advance(malformedTagHelper.Start, "<"),
                     RazorResources.FormatTagHelpersParseTreeRewriter_FoundMalformedTagHelper(
-                        malformedTagHelper.TagName));
+                        malformedTagHelper.TagName),
+                    malformedTagHelper.TagName.Length);
 
                 BuildCurrentlyTrackedTagHelperBlock(endTag: null);
             }

src/Microsoft.AspNet.Razor/Parser/TokenizerBackedParser.Helpers.cs

@@ -71,10 +71,12 @@ namespace Microsoft.AspNet.Razor.Parser
             AcceptAndMoveNext();
             if (EndOfFile && ((mode & BalancingModes.NoErrorOnFailure) != BalancingModes.NoErrorOnFailure))
             {
-                Context.OnError(start,
+                Context.OnError(
-                                RazorResources.FormatParseError_Expected_CloseBracket_Before_EOF(
+                    start,
-                                    Language.GetSample(left),
+                    RazorResources.FormatParseError_Expected_CloseBracket_Before_EOF(
-                                    Language.GetSample(right)));
+                        Language.GetSample(left),
+                        Language.GetSample(right)),
+                    length: 1 /* { OR } */);
             }
 
             return Balance(mode, left, right, start);
@@ -119,10 +121,12 @@ namespace Microsoft.AspNet.Razor.Parser
                 {
                     if ((mode & BalancingModes.NoErrorOnFailure) != BalancingModes.NoErrorOnFailure)
                     {
-                        Context.OnError(start,
+                        Context.OnError(
-                                        RazorResources.FormatParseError_Expected_CloseBracket_Before_EOF(
+                            start,
-                                            Language.GetSample(left),
+                            RazorResources.FormatParseError_Expected_CloseBracket_Before_EOF(
-                                            Language.GetSample(right)));
+                                Language.GetSample(left),
+                                Language.GetSample(right)),
+                            length: 1 /* { OR } */);
                     }
                     if ((mode & BalancingModes.BacktrackOnFailure) == BalancingModes.BacktrackOnFailure)
                     {
@@ -355,9 +359,17 @@ namespace Microsoft.AspNet.Razor.Parser
                     error = RazorResources.FormatErrorComponent_Character(CurrentSymbol.Content);
                 }
 
-                Context.OnError(
+                int errorLength;
-                    CurrentLocation,
+                if (CurrentSymbol == null || CurrentSymbol.Content == null)
-                    errorBase(error));
+                {
+                    errorLength = 1;
+                }
+                else
+                {
+                    errorLength = Math.Max(CurrentSymbol.Content.Length, 1);
+                }
+
+                Context.OnError(CurrentLocation, errorBase(error), errorLength);
             }
             return found;
         }
@@ -521,7 +533,10 @@ namespace Microsoft.AspNet.Razor.Parser
                     if (!Optional(KnownSymbolType.CommentStar))
                     {
                         errorReported = true;
-                        Context.OnError(start, RazorResources.ParseError_RazorComment_Not_Terminated);
+                        Context.OnError(
+                            start,
+                            RazorResources.ParseError_RazorComment_Not_Terminated,
+                            length: 2 /* @* */);
                     }
                     else
                     {
@@ -533,7 +548,10 @@ namespace Microsoft.AspNet.Razor.Parser
                         if (!errorReported)
                         {
                             errorReported = true;
-                            Context.OnError(start, RazorResources.ParseError_RazorComment_Not_Terminated);
+                            Context.OnError(
+                                start,
+                                RazorResources.ParseError_RazorComment_Not_Terminated,
+                                length: 2 /* @* */);
                         }
                     }
                     else

src/Microsoft.AspNet.Razor/RazorError.cs

@@ -11,18 +11,16 @@ namespace Microsoft.AspNet.Razor
     {
         internal const int DefaultErrorLength = 1;
 
+        /// <summary>
+        /// Used only for deserialization.
+        /// </summary>
         public RazorError()
-            : this(message: string.Empty, location: SourceLocation.Undefined)
+            : this(message: string.Empty, location: SourceLocation.Undefined, length: -1)
         {
         }
 
-        public RazorError(string message, SourceLocation location)
+        public RazorError(string message, int absoluteIndex, int lineIndex, int columnIndex, int length)
-            : this(message, location, DefaultErrorLength)
+            : this(message, new SourceLocation(absoluteIndex, lineIndex, columnIndex), length)
-        {
-        }
-
-        public RazorError(string message, int absoluteIndex, int lineIndex, int columnIndex)
-            : this(message, new SourceLocation(absoluteIndex, lineIndex, columnIndex))
         {
         }
 
@@ -33,11 +31,6 @@ namespace Microsoft.AspNet.Razor
             Length = length;
         }
 
-        public RazorError(string message, int absoluteIndex, int lineIndex, int columnIndex, int length)
-            : this(message, new SourceLocation(absoluteIndex, lineIndex, columnIndex), length)
-        {
-        }
-
         /// <summary>
         /// Gets (or sets) the message describing the error.
         /// </summary>

src/Microsoft.AspNet.Razor/Tokenizer/CSharpTokenizer.cs

@@ -264,7 +264,11 @@ namespace Microsoft.AspNet.Razor.Tokenizer
             }
             else if (EndOfFile)
             {
-                CurrentErrors.Add(new RazorError(RazorResources.ParseError_Unterminated_String_Literal, CurrentStart));
+                CurrentErrors.Add(
+                    new RazorError(
+                        RazorResources.ParseError_Unterminated_String_Literal,
+                        CurrentStart,
+                        length: 1 /* end of file */));
             }
             return Transition(EndSymbol(CSharpSymbolType.StringLiteral), Data);
         }
@@ -285,7 +289,11 @@ namespace Microsoft.AspNet.Razor.Tokenizer
             }
             else if (EndOfFile || ParserHelpers.IsNewLine(CurrentCharacter))
             {
-                CurrentErrors.Add(new RazorError(RazorResources.ParseError_Unterminated_String_Literal, CurrentStart));
+                CurrentErrors.Add(
+                    new RazorError(
+                        RazorResources.ParseError_Unterminated_String_Literal,
+                        CurrentStart,
+                        length: 1 /* " */));
             }
             else
             {
@@ -300,7 +308,11 @@ namespace Microsoft.AspNet.Razor.Tokenizer
             TakeUntil(c => c == '*');
             if (EndOfFile)
             {
-                CurrentErrors.Add(new RazorError(RazorResources.ParseError_BlockComment_Not_Terminated, CurrentStart));
+                CurrentErrors.Add(
+                    new RazorError(
+                        RazorResources.ParseError_BlockComment_Not_Terminated,
+                        CurrentStart,
+                        length: 1 /* end of file */));
                 return Transition(EndSymbol(CSharpSymbolType.Comment), Data);
             }
             if (CurrentCharacter == '*')