This C# example illustrates how to add comments in Google Sheets / Excel Spread sheet using the FileFormat.Cells library.

add-comments-into-worksheet.cs

// Define the directory where files are stored
string outputDirectory = "Z:\\Downloads\\";

// Specify the path to the Excel file
string filePath = "Z:\\Downloads\\testFile1.xlsx";

// Open the workbook located at the specified file path
using (Workbook wb = new Workbook(filePath))
{
    // Access the first worksheet in the workbook
    Worksheet firstSheet = wb.Worksheets[0];

    // Define the cell reference where the comment will be added
    string cellReference = "B10";
    // Retrieve the cell at the specified reference
    Cell cell = firstSheet.Cells[cellReference];
    
    // Set a new value for the cell
    cell.PutValue("randomValue");

    // Create a new comment with author and text
    Comment myComment = new Comment("John Doe", "This is a test comment.");

    // Add the comment to the specified cell in the worksheet
    firstSheet.AddComment(cellReference, myComment);

    // Save the workbook with all changes to the same file
    wb.Save(filePath);
}