Make SourceText.Lines non-virtual and introduce GetLinesCore protected virtual method.

src/Compilers/Core/Portable/PublicAPI.txt

@@ -1327,6 +1327,7 @@ Microsoft.CodeAnalysis.Text.SourceText
 Microsoft.CodeAnalysis.Text.SourceText.ChecksumAlgorithm.get
 Microsoft.CodeAnalysis.Text.SourceText.ContentEquals(Microsoft.CodeAnalysis.Text.SourceText other)
 Microsoft.CodeAnalysis.Text.SourceText.GetSubText(int start)
+Microsoft.CodeAnalysis.Text.SourceText.Lines.get
 Microsoft.CodeAnalysis.Text.SourceText.Replace(Microsoft.CodeAnalysis.Text.TextSpan span, string newText)
 Microsoft.CodeAnalysis.Text.SourceText.Replace(int start, int length, string newText)
 Microsoft.CodeAnalysis.Text.SourceText.SourceText(System.Collections.Immutable.ImmutableArray<byte> checksum = default(System.Collections.Immutable.ImmutableArray<byte>), Microsoft.CodeAnalysis.Text.SourceHashAlgorithm checksumAlgorithm = Microsoft.CodeAnalysis.Text.SourceHashAlgorithm.Sha1, Microsoft.CodeAnalysis.Text.SourceTextContainer container = null)
@@ -2106,9 +2107,9 @@ virtual Microsoft.CodeAnalysis.SyntaxWalker.VisitTrivia(Microsoft.CodeAnalysis.S
 virtual Microsoft.CodeAnalysis.Text.SourceText.Container.get
 virtual Microsoft.CodeAnalysis.Text.SourceText.ContentEqualsImpl(Microsoft.CodeAnalysis.Text.SourceText other)
 virtual Microsoft.CodeAnalysis.Text.SourceText.GetChangeRanges(Microsoft.CodeAnalysis.Text.SourceText oldText)
+virtual Microsoft.CodeAnalysis.Text.SourceText.GetLinesCore()
 virtual Microsoft.CodeAnalysis.Text.SourceText.GetSubText(Microsoft.CodeAnalysis.Text.TextSpan span)
 virtual Microsoft.CodeAnalysis.Text.SourceText.GetTextChanges(Microsoft.CodeAnalysis.Text.SourceText oldText)
-virtual Microsoft.CodeAnalysis.Text.SourceText.Lines.get
 virtual Microsoft.CodeAnalysis.Text.SourceText.ToString(Microsoft.CodeAnalysis.Text.TextSpan span)
 virtual Microsoft.CodeAnalysis.Text.SourceText.WithChanges(System.Collections.Generic.IEnumerable<Microsoft.CodeAnalysis.Text.TextChange> changes)
 virtual Microsoft.CodeAnalysis.Text.SourceText.Write(System.IO.TextWriter writer, Microsoft.CodeAnalysis.Text.TextSpan span, System.Threading.CancellationToken cancellationToken = default(System.Threading.CancellationToken))

src/Compilers/Core/Portable/Text/SourceText.cs

@@ -25,7 +25,7 @@ public abstract class SourceText
 
         private readonly SourceHashAlgorithm _checksumAlgorithm;
         private SourceTextContainer _lazyContainer;
-        private LineInfo _lazyLineInfo;
+        private TextLineCollection _lazyLineInfo;
         private ImmutableArray<byte> _lazyChecksum;
 
         protected SourceText(ImmutableArray<byte> checksum = default(ImmutableArray<byte>), SourceHashAlgorithm checksumAlgorithm = SourceHashAlgorithm.Sha1, SourceTextContainer container = null)
@@ -479,20 +479,20 @@ public virtual IReadOnlyList<TextChange> GetTextChanges(SourceText oldText)
         /// <summary>
         /// The collection of individual text lines.
         /// </summary>
-        public virtual TextLineCollection Lines
+        public TextLineCollection Lines
         {
             get
             {
-                if (_lazyLineInfo == null)
-                {
-                    var info = new LineInfo(this, this.ParseLineStarts());
-                    Interlocked.CompareExchange(ref _lazyLineInfo, info, null);
-                }
-
-                return _lazyLineInfo;
+                var info = _lazyLineInfo;
+                return info ?? Interlocked.CompareExchange(ref _lazyLineInfo, info = GetLinesCore(), null) ?? info;
             }
         }
 
+        protected virtual TextLineCollection GetLinesCore()
+        {
+            return new LineInfo(this, ParseLineStarts());
+        }
+
         private class LineInfo : TextLineCollection
         {
             private readonly SourceText _text;