No quotef for first value in csv

I am trying to make a very simple CSV file, everything works except one ti开发者_StackOverflow中文版ny thing : when the file get opened in Excel, the very first value of each entry isn't quoted, e.g :

username,"fullname","fullname@company.com"

And here is the code :

StringBuilder sb = new StringBuilder();

// Get user info
foreach (User u in listUsers)
{
    sb.AppendFormat("\"{0}\",\"{1}\",\"{2}\"\r\n", u.Username, u.Fullname, u.EMail);
}

// Generate csv
Response.Clear();
Response.ContentType = "text/csv";
Response.AddHeader("content-disposition", "attachment; filename=\"" + "UsersExport.csv" + "\"");
Response.Write(sb.ToString());
Response.End();

Any idea?