Tweaks

src/Features/CSharp/Portable/PopulateSwitch/CSharpPopulateSwitchExpressionCodeFixProvider.cs

@@ -31,6 +31,7 @@ protected override SwitchExpressionArmSyntax CreateSwitchArm(SyntaxGenerator gen
             => SwitchExpressionArm(ConstantPattern(caseLabel), Exception(generator, compilation));
 
         protected override SwitchExpressionSyntax InsertSwitchArms(SyntaxGenerator generator, SwitchExpressionSyntax switchNode, int insertLocation, List<SwitchExpressionArmSyntax> newArms)
-            => switchNode.WithArms(switchNode.Arms.InsertRangeWithTrailingSeparator(insertLocation, newArms));
+            => switchNode.WithArms(switchNode.Arms.InsertRangeWithTrailingSeparator(
+                insertLocation, newArms, SyntaxKind.CommaToken));
     }
 }

src/Workspaces/CSharp/Portable/Extensions/SeparatedSyntaxListExtensions.cs

@@ -8,23 +8,19 @@ namespace Microsoft.CodeAnalysis.CSharp.Extensions
 {
     internal static class SeparatedSyntaxListExtensions
     {
-        public static SeparatedSyntaxList<T> InsertRangeWithTrailingSeparator<T>(this SeparatedSyntaxList<T> separatedList, int index, IEnumerable<T> nodes)
+        public static SeparatedSyntaxList<T> InsertRangeWithTrailingSeparator<T>(
+            this SeparatedSyntaxList<T> separatedList, int index, IEnumerable<T> nodes, SyntaxKind separator)
             where T : SyntaxNode
         {
-            // Could be implemented more efficiently by parameterizing the implementation of SeparatedSyntaxList<>.InsertRange
             var newList = separatedList.InsertRange(index, nodes);
+            if (index < separatedList.Count)
+                return newList;
 
-            if (index >= separatedList.Count)
-            {
-                var nodesAndTokens = newList.GetWithSeparators();
+            var nodesAndTokens = newList.GetWithSeparators();
+            if (!nodesAndTokens.Last().IsNode)
+                return newList;
 
-                if (nodesAndTokens.Last().IsNode)
-                {
-                    return SyntaxFactory.SeparatedList<T>(nodesAndTokens.Add(SyntaxFactory.Token(SyntaxKind.CommaToken)));
-                }
-            }
-
-            return newList;
+            return SyntaxFactory.SeparatedList<T>(nodesAndTokens.Add(SyntaxFactory.Token(separator)));
         }
     }
 }