Iterating System.Collection.Generic.List.Add Does Not Work As Expected?

I was building an application for a project with .NET 3.5 and I noticed a weird behaviour of the List.Add method:

I have built my own class to organize data pulled from a database, and I use a while cycle to iterate through it.

However, when I List.Add(item), the whole content of the list is substituted with the last content pulled.

An example:

Suppose you have 3 users in a DB, each one identified with an ID and a username:

| ID | username |
| 1 | John |
| 2 | Fred |
| 3 | Paul |

and you have a "Users" class defined as

public class Users
{
private Int32 iD;
private String username;
public Int32 ID
{
get { return iD; }
set { iD = value; }
}
public String Username
{
get { return username; }
set { username = value; }
}
}

So you write this function:

[... SQL definitions - sdr is a SqlDataReader ...]
List<Users> userlist = new List<Users>();
if (sdr.HasRows) //There are users
{
Users user = new Users();
while (sdr.Read())
{
user.ID = sdr.GetInt32(0);
user.username = sdr.GetString(1);
userlist.Add(user);
}
}

What you expect (I expect) is userlist containing:

| ID | username |
| 1 | John |
| 2 | Fred |
| 3 | Paul |

What I actually get is, instead

| ID | username |
| 3 | Paul |
| 3 | Paul |
| 3 | Paul |