Properly handle adding multiple usings when there is a banner at the top of the file

src/EditorFeatures/CSharpTest/ImplementAbstractClass/ImplementAbstractClassTests.cs

@@ -1544,5 +1544,52 @@ class Derived : Base
     public override int Prop => throw new NotImplementedException();
 }", options: Option(ImplementTypeOptions.InsertionBehavior, ImplementTypeInsertionBehavior.AtTheEnd));
         }
+
+        [WorkItem(17274, "https://github.com/dotnet/roslyn/issues/17274")]
+        [Fact, Trait(Traits.Feature, Traits.Features.CodeActionsImplementAbstractClass)]
+        public async Task TestAddedUsingWithBanner1()
+        {
+            await TestAsync(
+@"// Copyright ...
+
+using Microsoft.Win32;
+
+namespace My
+{
+    public abstract class Foo
+    {
+        public abstract void Bar(System.Collections.Generic.List<object> values);
+    }
+
+    public class [|Foo2|] : Foo // Implement Abstract Class
+    {
+    }
+}",
+@"// Copyright ...
+
+using System;
+using System.Collections.Generic;
+using Microsoft.Win32;
+
+namespace My
+{
+    public abstract class Foo
+    {
+        public abstract void Bar(System.Collections.Generic.List<object> values);
+    }
+
+    public class Foo2 : Foo // Implement Abstract Class
+    {
+        public override void Bar(List<object> values)
+        {
+            throw new NotImplementedException();
+        }
     }
+}", compareTokens: false);
+        }
+                
+#if false
+
+#endif
+        }
 }
\ No newline at end of file

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

@@ -154,15 +154,21 @@ private static TextSpan GetUsingsSpan(CompilationUnitSyntax root, NamespaceDecla
                         firstToken.LeadingTrivia.Where(t => !IsDocCommentOrElastic(t)));
                     usings[0] = newFirstUsing;
                 }
-                else if (usings[0] != root.Usings[0])
+                else
                 {
-                    // We added a new first-using.  Take the trivia on the existing first using
-                    // And move it to the new using.
-                    var newFirstUsing = usings[0].WithLeadingTrivia(usings[1].GetLeadingTrivia())
-                                                 .WithAppendedTrailingTrivia(SyntaxFactory.ElasticCarriageReturnLineFeed);
-
-                    usings[0] = newFirstUsing;
-                    usings[1] = usings[1].WithoutLeadingTrivia();
+                    var originalFirstUsing = root.Usings[0];
+                    if (usings[0] != originalFirstUsing)
+                    {
+                        // We added a new first-using.  Take the trivia on the existing first using
+                        // And move it to the new using.
+                        var originalFirstUsingCurrentIndex = usings.IndexOf(originalFirstUsing);
+
+                        var newFirstUsing = usings[0].WithLeadingTrivia(originalFirstUsing.GetLeadingTrivia())
+                                                     .WithAppendedTrailingTrivia(SyntaxFactory.ElasticCarriageReturnLineFeed);
+
+                        usings[0] = newFirstUsing;
+                        usings[originalFirstUsingCurrentIndex] = originalFirstUsing.WithoutLeadingTrivia();
+                    }
                 }
             }
 
@@ -181,9 +187,9 @@ private static bool IsDocCommentOrElastic(SyntaxTrivia t)
             // We need to try and not place the using inside of a directive if possible.
             var usings = new List<UsingDirectiveSyntax>();
             var endOfList = root.Usings.Count - 1;
-            int startOfLastDirective = -1;
-            int endOfLastDirective = -1;
-            for (int i = 0; i < root.Usings.Count; i++)
+            var startOfLastDirective = -1;
+            var endOfLastDirective = -1;
+            for (var i = 0; i < root.Usings.Count; i++)
             {
                 if (root.Usings[i].GetLeadingTrivia().Any(trivia => trivia.IsKind(SyntaxKind.IfDirectiveTrivia)))
                 {