How to use bracket notation to access model property inside the loop in c#

Question

Here what I trying to achieve :

var userList = _db.users.ToList();
StringBuilder sb = new StringBuilder();
foreach (var item in userList)
{
    sb.AppendLine($"{item["ActivityName"]}"); //Want to access property like that.
    sb.AppendLine($"{item.ActivityName}"); //Working fine
}