public class Comment : Annotation 

using( var workbook = Workbook.Create( "AddComment.xlsx" ) ) 
  { 
    // Gets the first Worksheet; a Workbook always contains at least 1 Worksheet. 
    var worksheet = workbook.Worksheets[ 0 ]; 

    // Gets the annotations collection. 
    var annotations = worksheet.Annotations; 

    // Adds a Comment in Cell B1 and update its text. 
    var comment = annotations.AddComment( "This is the title", "B1" ); 
    comment.Text += " of the document"; 

    // Saves Workbook to disk; 
    workbook.Save(); 
  }