Add AddComment method to handle cell comments in Worksheets

Implement the AddComment method in the Worksheet class to add or update comments in specific cells. This method ensures compatibility with existing comments, managing the worksheet's comment part and authors list dynamically. It supports adding new comments if none exist or updating existing ones, thereby enhancing the functionality of our Excel handling utility.

This update is crucial for features that rely on providing feedback or notes directly within the Excel documents, such as automated report generation or data validation processes.

- Ensure the worksheet comments part exists before adding or updating comments.
- Manage authors list to avoid duplicates and use existing indices where possible.

Author: Fahad Adeel

.DS_Store

@@ -1566,48 +1566,73 @@ private static string IndexToColumnLetter(int index)
         }
 
 
+        /// <summary>
+        /// Adds or updates a comment in a specified cell within the worksheet. If the cell already has a comment,
+        /// it updates the existing comment text. If there is no comment, it creates a new one.
+        /// </summary>
+        /// <param name="cellReference">The cell reference where the comment should be added, e.g., "A1".</param>
+        /// <param name="comment">The comment object containing the author and the text of the comment.</param>
+        /// <remarks>
+        /// This method ensures that the worksheet comments part exists before adding or updating a comment.
+        /// It also manages the authors list to ensure that each author is only added once and reuses the existing author index if available.
+        /// Usage of this method requires that the workbook and worksheet are properly initialized and that the worksheet part is correctly associated.
+        /// </remarks>
+
         public void AddComment(string cellReference, Comment comment)
         {
             // Ensure the comments part exists
             var commentsPart = _worksheetPart.GetPartsOfType<WorksheetCommentsPart>().FirstOrDefault();
+            CommentList commentList;
+            Authors authors;
+
             if (commentsPart == null)
             {
                 commentsPart = _worksheetPart.AddNewPart<WorksheetCommentsPart>();
                 commentsPart.Comments = new Comments();
-                commentsPart.Comments.AppendChild(new CommentList());
-                commentsPart.Comments.AppendChild(new Authors());
+
+                // Initialize new CommentList and Authors only if a new comments part is created
+                commentList = new CommentList();
+                authors = new Authors();
+                commentsPart.Comments.AppendChild(commentList);
+                commentsPart.Comments.AppendChild(authors);
+            }
+            else
+            {
+                // Retrieve existing CommentList and Authors
+                commentList = commentsPart.Comments.Elements<CommentList>().First();
+                authors = commentsPart.Comments.Elements<Authors>().First();
             }
 
             // Ensure the author exists
-            var authors = commentsPart.Comments.Elements<Authors>().First();
             var author = authors.Elements<Author>().FirstOrDefault(a => a.Text == comment.Author);
             if (author == null)
             {
                 author = new Author() { Text = comment.Author };
-                authors.Append(author);
+                authors.AppendChild(author);  // Use AppendChild to add to the XML structure
             }
             uint authorId = (uint)authors.Elements<Author>().ToList().IndexOf(author);
 
             // Add or update the comment
-            var commentList = commentsPart.Comments.Elements<CommentList>().First();
             var existingComment = commentList.Elements<DocumentFormat.OpenXml.Spreadsheet.Comment>().FirstOrDefault(c => c.Reference == cellReference);
             if (existingComment == null)
             {
                 var newComment = new DocumentFormat.OpenXml.Spreadsheet.Comment() { Reference = cellReference, AuthorId = authorId };
-                newComment.Append(new CommentText(new DocumentFormat.OpenXml.Spreadsheet.Text(comment.Text)));
-                commentList.Append(newComment);
+                newComment.AppendChild(new CommentText(new DocumentFormat.OpenXml.Spreadsheet.Text(comment.Text)));
+                commentList.AppendChild(newComment); // Ensure appending to commentList
             }
             else
             {
                 // Update the existing comment's text
-                existingComment.CommentText = new CommentText(new DocumentFormat.OpenXml.Spreadsheet.Text(comment.Text));
+                existingComment.Elements<CommentText>().First().Text = new DocumentFormat.OpenXml.Spreadsheet.Text(comment.Text);
             }
 
             // Save the changes
             commentsPart.Comments.Save();
             _worksheetPart.Worksheet.Save();
         }
 
+
+
         public void CopyRange(Range sourceRange, string targetStartCellReference)
         {
             var (targetStartRow, targetStartColumn) = ParseCellReference(targetStartCellReference);