Change all comments with ReplaceTrivia

Question

CodeGeneration. Problems calling ReplaceTrivia method on some comments. The issue arises when attempting to replace all comments in a method using the ReplaceTrivia method from the Roslyn API. Although the intention is to change each comment to a new format, only the first comment gets replaced correctly.

Source code

using Microsoft.CodeAnalysis;
using Microsoft.CodeAnalysis.CSharp;
...

 public string Transform(string methodBody)
 {
 var tree = CSharpSyntaxTree.ParseText(methodBody);
 var root = tree.GetRoot();

 var comments = root.DescendantTrivia()
 .Where(trivia => trivia.IsKind(SyntaxKind.SingleLineCommentTrivia))
 .ToList();

 var commentGenerator = new MethodCommentGenerator();
 foreach (var comment in comments)
 {
 var newCommentText = commentGenerator.GenerateCommentName(comments.IndexOf(comment));
 var newTrivia = SyntaxFactory.Comment(newCommentText);
 root = root.ReplaceTrivia(comment, newTrivia);
 }

 return root.ToFullString();
 }

 public class MethodCommentGenerator
 {
 public string GenerateCommentName(int order) => $"// comment{++order}";
 }

I want to change all comments in the method to my own. For some reason, only the first one changes. For example,

    public string TestFunc(string key, string value) 
{ 
// set val1 
var val1 = key;
 // set val2 
var val2 = value; 
// return result 
return $"({val1}{ val2})"; 
}

turns into

public string TestFunc(string key, string value)
{
// comment1
var val1 = key;
// set val2
var val2 = value;
// return result
return $"({val1}{ val2})";
}

I want it to be

public string TestFunc(string key, string value)
{
// comment1
var val1 = key;
// comment2
var val2 = value;
// comment3
 return $"({val1}{ val2})";
 }

PhiseySt · Accepted Answer · 2025-04-06 08:05:36Z

//We need regenerate tree, root and comments
public string Transform(string methodBody)
    {
        if (!CurrentSettingsState.SingleCommentNameSelected) return methodBody;

        var tree = CSharpSyntaxTree.ParseText(methodBody);
        var root = tree.GetRoot();

        var comments = root.DescendantTrivia()
            .Where(trivia => trivia.IsKind(SyntaxKind.SingleLineCommentTrivia))
            .ToList();

        var commentGenerator = new MethodSingleCommentGenerator();

        for (var i = 0; i < comments.Count; i++)
        {
            var newCommentText = commentGenerator.GenerateSingleCommentName(i);
            var newTrivia = SyntaxFactory.Comment(newCommentText);
            root = root.ReplaceTrivia(comments[i], newTrivia);
            methodBody = root.ToFullString();
            tree = CSharpSyntaxTree.ParseText(methodBody);
            root = tree.GetRoot();
            comments = root.DescendantTrivia()
                .Where(trivia => trivia.IsKind(SyntaxKind.SingleLineCommentTrivia))
                .ToList();
        }

        return root.ToFullString();
    }

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