将对象添加到C＃中的列表

Question

When adding to a list in C#, I am getting the same object:

List<Group> group = new List<Group>();
        Group groupclass=new Group();
        if (this.OpenConnection() == true)
        {
            sql_cmd = new MySqlCommand("listofgroup", sql_con);
            sql_cmd.CommandType = CommandType.StoredProcedure;
            sql_cmd.Parameters.AddWithValue("@userid", userid).Direction = ParameterDirection.Input;
            MySqlDataReader sdr = sql_cmd.ExecuteReader();
            while (sdr.Read())
            {
                groupclass.groupName = sdr.GetString("group_name");
                groupclass.groupID = sdr.GetString("ID");
                group.Add(groupclass);
            }
            this.CloseConnection();
        }
        return group;

Answer 1

As your code stands you only create the object once. When you set the properties inside the while you are setting the values on the original object.

Instead you need to create the object inside the while

while (sdr.Read())
{
    Group groupclass = new Group();
    groupclass.groupName = sdr.GetString("group_name");
    groupclass.groupID = sdr.GetString("ID");
    group.Add(groupclass);
}

Answer 2

You're only creating one Group object, overwriting the values in it, and adding the same object to the list each time. You need to move your allocation inside the while loop:

while (sdr.Read())
{
    Group groupclass = new Group();
    groupclass.groupName = sdr.GetString("group_name");
    groupclass.groupID = sdr.GetString("ID");
    group.Add(groupclass);
}

Answer 3

You are reusing the same Group instance and re-adding it. Create a new Group instance in the while loop.