diff --git a/src/Microsoft.AspNetCore.Razor.Evolution/DefaultRazorSourceDocument.cs b/src/Microsoft.AspNetCore.Razor.Evolution/DefaultRazorSourceDocument.cs
index f280b62f4f..01b5866d92 100644
--- a/src/Microsoft.AspNetCore.Razor.Evolution/DefaultRazorSourceDocument.cs
+++ b/src/Microsoft.AspNetCore.Razor.Evolution/DefaultRazorSourceDocument.cs
@@ -2,13 +2,17 @@
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
 using System;
+using System.Collections.Generic;
+using System.Diagnostics;
 using System.Text;
+using Microsoft.AspNetCore.Razor.Evolution.Legacy;
 
 namespace Microsoft.AspNetCore.Razor.Evolution
 {
     internal class DefaultRazorSourceDocument : RazorSourceDocument
     {
         private readonly string _content;
+        private readonly RazorSourceLineCollection _lines;
 
         public DefaultRazorSourceDocument(string content, Encoding encoding, string filename)
         {
@@ -25,6 +29,8 @@ namespace Microsoft.AspNetCore.Razor.Evolution
             _content = content;
             Encoding = encoding;
             Filename = filename;
+
+            _lines = new LineCollection(this, LineCollection.GetLineStarts(content));
         }
 
         public override char this[int position] => _content[position];
@@ -35,6 +41,8 @@ namespace Microsoft.AspNetCore.Razor.Evolution
 
         public override int Length => _content.Length;
 
+        public override RazorSourceLineCollection Lines => _lines;
+
         public override void CopyTo(int sourceIndex, char[] destination, int destinationIndex, int count)
         {
             if (destination == null)
@@ -64,5 +72,118 @@ namespace Microsoft.AspNetCore.Razor.Evolution
 
             _content.CopyTo(sourceIndex, destination, destinationIndex, count);
         }
+
+        private class LineCollection : RazorSourceLineCollection
+        {
+            private readonly DefaultRazorSourceDocument _document;
+            private readonly int[] _lineStarts;
+
+            public LineCollection(DefaultRazorSourceDocument document, int[] lineStarts)
+            {
+                _document = document;
+                _lineStarts = lineStarts;
+            }
+
+            public override int Count => _lineStarts.Length;
+
+            public override int GetLineLength(int index)
+            {
+                if (index < 0 || index >= _lineStarts.Length)
+                {
+                    throw new IndexOutOfRangeException(nameof(index));
+                }
+
+                if (index == _lineStarts.Length - 1)
+                {
+                    // Last line is special.
+                    return _document.Length - _lineStarts[index];
+                }
+
+                return _lineStarts[index + 1] - _lineStarts[index];
+            }
+
+            internal override SourceLocation GetLocation(int position)
+            {
+                if (position < 0 || position >= _document.Length)
+                {
+                    throw new IndexOutOfRangeException(nameof(position));
+                }
+
+                var index = Array.BinarySearch<int>(_lineStarts, position);
+                if (index >= 0)
+                {
+                    // We have an exact match for the start of a line.
+                    Debug.Assert(_lineStarts[index] == position);
+
+                    return new SourceLocation(_document.Filename, position, index, characterIndex: 0);
+                }
+
+
+                // Index is the complement of the line *after* the one we want, because BinarySearch tells
+                // us where we'd put position *if* it were the start of a line.
+                index = (~index) - 1;
+                if (index == -1)
+                {
+                    // There's no preceding line, so it's based on the start of the string
+                    return new SourceLocation(_document.Filename, position, 0, position);
+                }
+                else
+                {
+                    var characterIndex = position - _lineStarts[index];
+                    return new SourceLocation(_document.Filename, position, index, characterIndex);
+                }
+            }
+
+            public static int[] GetLineStarts(string text)
+            {
+                var starts = new List<int>();
+
+                // We always consider a document to have at least a 0th line, even if it's empty.
+                starts.Add(0);
+
+                var unprocessedCR = false;
+
+                // Length - 1 because we don't care if there was a linebreak as the last character.
+                for (var i = 0; i < text.Length - 1; i++)
+                {
+                    var c = text[i];
+                    var isLineBreak = false;
+
+                    switch (c)
+                    {
+                        case '\r':
+                            unprocessedCR = true;
+                            continue;
+
+                        case '\n':
+                            unprocessedCR = false;
+                            isLineBreak = true;
+                            break;
+
+                        case '\u0085':
+                        case '\u2028':
+                        case '\u2029':
+                            isLineBreak = true;
+                            break;
+
+                    }
+
+                    if (unprocessedCR)
+                    {
+                        // If we get here it means that we had a CR followed by something other than an LF.
+                        // Add the CR as a line break.
+                        starts.Add(i);
+                        unprocessedCR = false;
+                    }
+
+                    if (isLineBreak)
+                    {
+                        starts.Add(i + 1);
+                    }
+                }
+
+                return starts.ToArray();
+            }
+        }
     }
 }
diff --git a/src/Microsoft.AspNetCore.Razor.Evolution/RazorSourceDocument.cs b/src/Microsoft.AspNetCore.Razor.Evolution/RazorSourceDocument.cs
index 8ac389e999..6ec2a48c6b 100644
--- a/src/Microsoft.AspNetCore.Razor.Evolution/RazorSourceDocument.cs
+++ b/src/Microsoft.AspNetCore.Razor.Evolution/RazorSourceDocument.cs
@@ -17,6 +17,8 @@ namespace Microsoft.AspNetCore.Razor.Evolution
 
         public abstract int Length { get; }
 
+        public abstract RazorSourceLineCollection Lines { get; }
+
         public abstract void CopyTo(int sourceIndex, char[] destination, int destinationIndex, int count);
 
         public static RazorSourceDocument ReadFrom(Stream stream, string filename)
diff --git a/src/Microsoft.AspNetCore.Razor.Evolution/RazorSourceLineCollection.cs b/src/Microsoft.AspNetCore.Razor.Evolution/RazorSourceLineCollection.cs
new file mode 100644
index 0000000000..2a3dc6b358
--- /dev/null
+++ b/src/Microsoft.AspNetCore.Razor.Evolution/RazorSourceLineCollection.cs
@@ -0,0 +1,16 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using Microsoft.AspNetCore.Razor.Evolution.Legacy;
+
+namespace Microsoft.AspNetCore.Razor.Evolution
+{
+    public abstract class RazorSourceLineCollection
+    {
+        public abstract int Count { get; }
+
+        public abstract int GetLineLength(int index);
+
+        internal abstract SourceLocation GetLocation(int position);
+    }
+}
diff --git a/test/Microsoft.AspNetCore.Razor.Evolution.Test/DefaultRazorSourceDocumentTest.cs b/test/Microsoft.AspNetCore.Razor.Evolution.Test/DefaultRazorSourceDocumentTest.cs
index 9d6909c9f7..74ff8bea26 100644
--- a/test/Microsoft.AspNetCore.Razor.Evolution.Test/DefaultRazorSourceDocumentTest.cs
+++ b/test/Microsoft.AspNetCore.Razor.Evolution.Test/DefaultRazorSourceDocumentTest.cs
@@ -151,5 +151,295 @@ namespace Microsoft.AspNetCore.Razor.Evolution
                 Assert.Equal("Hi", copiedContent);
             }
         }
+
+        [Fact]
+        public void Lines_Count_EmptyDocument()
+        {
+            // Arrange
+            var content = string.Empty;
+            var document = new DefaultRazorSourceDocument(content, Encoding.UTF8, filename: null);
+
+            // Act
+            var actual = document.Lines.Count;
+
+            // Assert
+            Assert.Equal(1, actual);
+        }
+
+        [Fact]
+        public void Lines_GetLineLength_EmptyDocument()
+        {
+            // Arrange
+            var content = string.Empty;
+            var document = new DefaultRazorSourceDocument(content, Encoding.UTF8, filename: null);
+
+            // Act
+            var actual = document.Lines.GetLineLength(0);
+
+            // Assert
+            Assert.Equal(0, actual);
+        }
+
+        [Fact]
+        public void Lines_GetLineLength_TrailingNewlineDoesNotStartNewLine()
+        {
+            // Arrange
+            var content = "hello\n";
+            var document = new DefaultRazorSourceDocument(content, Encoding.UTF8, filename: null);
+
+            // Act
+            var actual = document.Lines.GetLineLength(0);
+
+            // Assert
+            Assert.Equal(6, actual);
+        }
+
+        [Fact]
+        public void Lines_GetLineLength_TrailingNewlineDoesNotStartNewLine_CRLF()
+        {
+            // Arrange
+            var content = "hello\r\n";
+            var document = new DefaultRazorSourceDocument(content, Encoding.UTF8, filename: null);
+
+            // Act
+            var actual = document.Lines.GetLineLength(0);
+
+            // Assert
+            Assert.Equal(7, actual);
+        }
+
+        [Fact]
+        public void Lines_Simple_Document()
+        {
+            // Arrange
+            var content = new StringBuilder()
+                .Append("The quick brown").Append('\n')
+                .Append("fox").Append("\r\n")
+                .Append("jumps over the lazy dog.")
+                .ToString();
+
+            var document = new DefaultRazorSourceDocument(content, Encoding.UTF8, filename: null);
+
+            // Act
+            var actual = GetAllLineMappings(document);
+
+            // Assert
+            Assert.Equal(new int[]{ 16, 5, 24 }, actual);
+        }
+
+        [Fact]
+        public void Lines_CRLF_OnlyCountsAsASingleNewLine()
+        {
+            // Arrange
+            var content = "Hello\r\nWorld!";
+
+            var document = new DefaultRazorSourceDocument(content, Encoding.UTF8, filename: null);
+
+            // Act
+            var actual = GetAllLineMappings(document);
+
+            // Assert
+            Assert.Equal(new int[] { 7, 6 }, actual);
+        }
+
+        [Fact]
+        public void Lines_CR_IsNewLine()
+        {
+            // Arrange
+            var content = "Hello\rWorld!";
+
+            var document = new DefaultRazorSourceDocument(content, Encoding.UTF8, filename: null);
+
+            // Act
+            var actual = GetAllLineMappings(document);
+
+            // Assert
+            Assert.Equal(new int[] { 6, 6 }, actual);
+        }
+
+        // CR handling is stateful in the parser, making sure we properly reset the state.
+        [Fact]
+        public void Lines_CR_IsNewLine_MultipleCRs()
+        {
+            // Arrange
+            var content = "Hello\rBig\r\nWorld!";
+
+            var document = new DefaultRazorSourceDocument(content, Encoding.UTF8, filename: null);
+
+            // Act
+            var actual = GetAllLineMappings(document);
+
+            // Assert
+            Assert.Equal(new int[] { 6, 5, 6 }, actual);
+        }
+
+        [Fact]
+        public void Lines_LF_IsNewLine()
+        {
+            // Arrange
+            var content = "Hello\nWorld!";
+
+            var document = new DefaultRazorSourceDocument(content, Encoding.UTF8, filename: null);
+
+            // Act
+            var actual = GetAllLineMappings(document);
+
+            // Assert
+            Assert.Equal(new int[] { 6, 6 }, actual);
+        }
+
+        [Fact]
+        public void Lines_Unicode0085_IsNewLine()
+        {
+            // Arrange
+            var content = "Hello\u0085World!";
+
+            var document = new DefaultRazorSourceDocument(content, Encoding.UTF8, filename: null);
+
+            // Act
+            var actual = GetAllLineMappings(document);
+
+            // Assert
+            Assert.Equal(new int[] { 6, 6 }, actual);
+        }
+
+        [Fact]
+        public void Lines_Unicode2028_IsNewLine()
+        {
+            // Arrange
+            var content = "Hello\u2028World!";
+
+            var document = new DefaultRazorSourceDocument(content, Encoding.UTF8, filename: null);
+
+            // Act
+            var actual = GetAllLineMappings(document);
+
+            // Assert
+            Assert.Equal(new int[] { 6, 6 }, actual);
+        }
+
+        [Fact]
+        public void Lines_Unicode2029_IsNewLine()
+        {
+            // Arrange
+            var content = "Hello\u2029World!";
+
+            var document = new DefaultRazorSourceDocument(content, Encoding.UTF8, filename: null);
+
+            // Act
+            var actual = GetAllLineMappings(document);
+
+            // Assert
+            Assert.Equal(new int[] { 6, 6 }, actual);
+        }
+
+        [Fact]
+        public void Lines_GetLocation_IncludesAbsoluteIndexAndDocument()
+        {
+            // Arrange
+            var content = "Hello, World!";
+
+            var document = new DefaultRazorSourceDocument(content, Encoding.UTF8, filename: "Hi.cshtml");
+
+            // Act
+            var actual = document.Lines.GetLocation(1);
+
+            // Assert
+            Assert.Equal("Hi.cshtml", actual.FilePath);
+            Assert.Equal(1, actual.AbsoluteIndex);
+        }
+
+        // Beginnings of lines are special because the BinarySearch in the implementation
+        // will succeed. It's a different code path.
+        [Fact]
+        public void Lines_GetLocation_FirstCharacter()
+        {
+            // Arrange
+            var content = "Hello\nBig\r\nWorld!";
+
+            var document = new DefaultRazorSourceDocument(content, Encoding.UTF8, filename: null);
+
+            // Act
+            var actual = document.Lines.GetLocation(0);
+
+            // Assert
+            Assert.Equal(0, actual.LineIndex);
+            Assert.Equal(0, actual.CharacterIndex);
+        }
+
+        [Fact]
+        public void Lines_GetLocation_EndOfFirstLine()
+        {
+            // Arrange
+            var content = "Hello\nBig\r\nWorld!";
+
+            var document = new DefaultRazorSourceDocument(content, Encoding.UTF8, filename: null);
+
+            // Act
+            var actual = document.Lines.GetLocation(5);
+
+            // Assert
+            Assert.Equal(0, actual.LineIndex);
+            Assert.Equal(5, actual.CharacterIndex);
+        }
+
+        [Fact]
+        public void Lines_GetLocation_InteriorLine()
+        {
+            // Arrange
+            var content = "Hello\nBig\r\nWorld!";
+
+            var document = new DefaultRazorSourceDocument(content, Encoding.UTF8, filename: null);
+
+            // Act
+            var actual = document.Lines.GetLocation(7);
+
+            // Assert
+            Assert.Equal(1, actual.LineIndex);
+            Assert.Equal(1, actual.CharacterIndex);
+        }
+
+        [Fact]
+        public void Lines_GetLocation_StartOfLastLine()
+        {
+            // Arrange
+            var content = "Hello\nBig\r\nWorld!";
+
+            var document = new DefaultRazorSourceDocument(content, Encoding.UTF8, filename: null);
+
+            // Act
+            var actual = document.Lines.GetLocation(11);
+
+            // Assert
+            Assert.Equal(2, actual.LineIndex);
+            Assert.Equal(0, actual.CharacterIndex);
+        }
+
+        [Fact]
+        public void Lines_GetLocation_EndOfLastLine()
+        {
+            // Arrange
+            var content = "Hello\nBig\r\nWorld!";
+
+            var document = new DefaultRazorSourceDocument(content, Encoding.UTF8, filename: null);
+
+            // Act
+            var actual = document.Lines.GetLocation(16);
+
+            // Assert
+            Assert.Equal(2, actual.LineIndex);
+            Assert.Equal(5, actual.CharacterIndex);
+        }
+
+        private static int[] GetAllLineMappings(RazorSourceDocument source)
+        {
+            var lines = new int[source.Lines.Count];
+            for (var i = 0; i < lines.Length; i++)
+            {
+                lines[i] = source.Lines.GetLineLength(i);
+            }
+
+            return lines;
+        }
     }
 }