Keep comments on an 'else' keyword when converting to conditional expressions.

src/Analyzers/CSharp/CodeFixes/UseConditionalExpression/CSharpUseConditionalExpressionForAssignmentCodeFixProvider.cs

@@ -59,4 +59,7 @@ protected override ExpressionSyntax ConvertToExpression(IThrowOperation throwOpe
 
     protected override ISyntaxFormatting SyntaxFormatting
         => CSharpSyntaxFormatting.Instance;
+
+    protected override (ConditionalExpressionSyntax conditional, bool makeMultiLine) UpdateConditionalExpression(IConditionalOperation originalIfStatement, ConditionalExpressionSyntax conditional)
+        => CSharpUseConditionalExpressionHelpers.UpdateConditionalExpression(originalIfStatement, conditional);
 }

src/Analyzers/CSharp/CodeFixes/UseConditionalExpression/CSharpUseConditionalExpressionForReturnCodeFixProvider.cs

@@ -41,7 +41,7 @@ protected override StatementSyntax WrapWithBlockIfAppropriate(
         return statement;
     }
 
-    protected override SyntaxNode WrapIfStatementIfNecessary(IConditionalOperation operation)
+    protected override ExpressionSyntax WrapIfStatementIfNecessary(IConditionalOperation operation)
     {
         if (operation.Syntax is IfStatementSyntax { Condition: CheckedExpressionSyntax exp })
             return exp;
@@ -62,4 +62,7 @@ protected override ExpressionSyntax ConvertToExpression(IThrowOperation throwOpe
 
     protected override ISyntaxFormatting SyntaxFormatting
         => CSharpSyntaxFormatting.Instance;
+
+    protected override (ConditionalExpressionSyntax conditional, bool makeMultiLine) UpdateConditionalExpression(IConditionalOperation originalIfStatement, ConditionalExpressionSyntax conditional)
+        => CSharpUseConditionalExpressionHelpers.UpdateConditionalExpression(originalIfStatement, conditional);
 }

src/Analyzers/CSharp/CodeFixes/UseConditionalExpression/CSharpUseConditionalExpressionHelpers.cs

@@ -2,12 +2,19 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+using System;
+using System.Linq;
+using Microsoft.CodeAnalysis.CSharp.CodeGeneration;
+using Microsoft.CodeAnalysis.CSharp.Extensions;
 using Microsoft.CodeAnalysis.CSharp.Syntax;
 using Microsoft.CodeAnalysis.Operations;
+using Microsoft.CodeAnalysis.Shared.Extensions;
 using Roslyn.Utilities;
 
 namespace Microsoft.CodeAnalysis.CSharp.UseConditionalExpression;
 
+using static CSharpSyntaxTokens;
+
 internal static class CSharpUseConditionalExpressionHelpers
 {
     public static ExpressionSyntax ConvertToExpression(IThrowOperation throwOperation)
@@ -16,4 +23,21 @@ public static ExpressionSyntax ConvertToExpression(IThrowOperation throwOperatio
         RoslynDebug.Assert(throwStatement.Expression != null);
         return SyntaxFactory.ThrowExpression(throwStatement.ThrowKeyword, throwStatement.Expression);
     }
+
+    public static (ConditionalExpressionSyntax conditional, bool makeMultiLine) UpdateConditionalExpression(
+        IConditionalOperation originalIfStatement,
+        ConditionalExpressionSyntax conditional)
+    {
+        var ifStatement = (IfStatementSyntax)originalIfStatement.Syntax;
+
+        // Move any comments on the `else` keyword to then be on the `:` keyword.  In that case, we definitely want to
+        // make this multiline.
+        if (ifStatement.Else is null || !ifStatement.Else.ElseKeyword.LeadingTrivia.Any(t => t.IsSingleOrMultiLineComment()))
+            return (conditional, makeMultiLine: false);
+
+        var finalConditional = conditional
+            .WithColonToken(ColonToken.WithPrependedLeadingTrivia(ifStatement.Else.ElseKeyword.LeadingTrivia))
+            .WithWhenTrue(conditional.WhenTrue.WithAppendedTrailingTrivia(SyntaxFactory.ElasticCarriageReturnLineFeed));
+        return (finalConditional, makeMultiLine: true);
+    }
 }

src/Analyzers/CSharp/Tests/UseConditionalExpression/UseConditionalExpressionForAssignmentTests.cs

@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+using System.Diagnostics.CodeAnalysis;
 using System.Threading.Tasks;
 using Microsoft.CodeAnalysis.CodeStyle;
 using Microsoft.CodeAnalysis.CSharp;
@@ -22,7 +23,7 @@ namespace Microsoft.CodeAnalysis.Editor.CSharp.UnitTests.UseConditionalExpressio
 public sealed partial class UseConditionalExpressionForAssignmentTests
 {
     private static async Task TestMissingAsync(
-        string testCode,
+        [StringSyntax(PredefinedEmbeddedLanguageNames.CSharpTest)] string testCode,
         LanguageVersion languageVersion = LanguageVersion.CSharp8,
         OptionsCollection? options = null)
     {
@@ -37,8 +38,8 @@ private static async Task TestMissingAsync(
     }
 
     private static async Task TestInRegularAndScript1Async(
-        string testCode,
-        string fixedCode,
+        [StringSyntax(PredefinedEmbeddedLanguageNames.CSharpTest)] string testCode,
+        [StringSyntax(PredefinedEmbeddedLanguageNames.CSharpTest)] string fixedCode,
         LanguageVersion languageVersion = LanguageVersion.CSharp8,
         OptionsCollection? options = null,
         string? equivalenceKey = null)
@@ -2144,4 +2145,45 @@ public async Task TestGlobalStatements()
             }
         }.RunAsync();
     }
+
+    [Fact, WorkItem("https://github.com/dotnet/roslyn/issues/58897")]
+    public async Task TestCommentsOnElse()
+    {
+        await TestInRegularAndScript1Async(
+            """
+            using System;
+
+            class C
+            {
+                void M(bool containsHighBits)
+                {
+                    int write;
+
+                    [|if|] (containsHighBits)
+                    {
+                        write = 0;
+                    }
+                    // Comment on else
+                    else
+                    {
+                        write = 1;
+                    }
+                }
+            }
+            """,
+            """
+            using System;
+
+            class C
+            {
+                void M(bool containsHighBits)
+                {
+                    int write = containsHighBits
+                        ? 0
+                        // Comment on else
+                        : 1;
+                }
+            }
+            """, LanguageVersion.CSharp9);
+    }
 }

src/Analyzers/Core/CodeFixes/UseConditionalExpression/AbstractUseConditionalExpressionCodeFixProvider.cs

@@ -96,21 +96,22 @@ protected async Task<TExpressionSyntax> CreateConditionalExpressionAsync(
         {
             return negate
                 ? (TExpressionSyntax)generator.Negate(generatorInternal, condition, semanticModel, cancellationToken).WithoutTrivia()
-                : (TExpressionSyntax)condition.WithoutTrivia();
+                : condition.WithoutTrivia();
         }
 
         var trueExpression = MakeRef(generatorInternal, isRef, CastValueIfNecessary(generator, trueStatement, trueValue));
         var falseExpression = MakeRef(generatorInternal, isRef, CastValueIfNecessary(generator, falseStatement, falseValue));
         trueExpression = WrapReturnExpressionIfNecessary(trueExpression, trueStatement);
         falseExpression = WrapReturnExpressionIfNecessary(falseExpression, falseStatement);
 
-        var conditionalExpression = (TConditionalExpressionSyntax)generator.ConditionalExpression(
+        var initialExpression = (TConditionalExpressionSyntax)generator.ConditionalExpression(
             condition.WithoutTrivia(),
             trueExpression,
             falseExpression);
+        var (conditionalExpression, makeMultiLine) = UpdateConditionalExpression(ifOperation, initialExpression);
 
         conditionalExpression = conditionalExpression.WithAdditionalAnnotations(Simplifier.Annotation);
-        var makeMultiLine = await MakeMultiLineAsync(
+        makeMultiLine = makeMultiLine || await MakeMultiLineAsync(
             document, condition,
             trueValue.Syntax, falseValue.Syntax, formattingOptions, cancellationToken).ConfigureAwait(false);
         if (makeMultiLine)
@@ -122,8 +123,14 @@ protected async Task<TExpressionSyntax> CreateConditionalExpressionAsync(
         return MakeRef(generatorInternal, isRef, conditionalExpression);
     }
 
-    protected virtual SyntaxNode WrapIfStatementIfNecessary(IConditionalOperation operation)
-        => operation.Condition.Syntax;
+    protected virtual (TConditionalExpressionSyntax conditional, bool makeMultiLine) UpdateConditionalExpression(
+        IConditionalOperation originalIfStatement, TConditionalExpressionSyntax conditionalExpression)
+    {
+        return (conditionalExpression, makeMultiLine: false);
+    }
+
+    protected virtual TExpressionSyntax WrapIfStatementIfNecessary(IConditionalOperation operation)
+        => (TExpressionSyntax)operation.Condition.Syntax;
 
     protected virtual TExpressionSyntax WrapReturnExpressionIfNecessary(TExpressionSyntax returnExpression, IOperation returnOperation)
         => returnExpression;

src/Analyzers/VisualBasic/CodeFixes/UseConditionalExpression/VisualBasicUseConditionalExpressionForAssignmentCodeFixProvider.vb

@@ -11,14 +11,14 @@ Imports Microsoft.CodeAnalysis.LanguageService
 Imports Microsoft.CodeAnalysis.Operations
 Imports Microsoft.CodeAnalysis.Simplification
 Imports Microsoft.CodeAnalysis.UseConditionalExpression
+Imports Microsoft.CodeAnalysis.VisualBasic.CodeGeneration
 Imports Microsoft.CodeAnalysis.VisualBasic.Formatting
 Imports Microsoft.CodeAnalysis.VisualBasic.LanguageService
 Imports Microsoft.CodeAnalysis.VisualBasic.Syntax
 
 Namespace Microsoft.CodeAnalysis.VisualBasic.UseConditionalExpression
-
     <ExportCodeFixProvider(LanguageNames.VisualBasic, Name:=PredefinedCodeFixProviderNames.UseConditionalExpressionForAssignment), [Shared]>
-    Friend Class VisualBasicUseConditionalExpressionForAssignmentCodeFixProvider
+    Friend NotInheritable Class VisualBasicUseConditionalExpressionForAssignmentCodeFixProvider
         Inherits AbstractUseConditionalExpressionForAssignmentCodeFixProvider(Of
             StatementSyntax, MultiLineIfBlockSyntax, LocalDeclarationStatementSyntax, VariableDeclaratorSyntax, ExpressionSyntax, TernaryConditionalExpressionSyntax)