How to use aggregate function MAX in c#?

  public string GetMaxUserID()
    {
        string ConnString = GetConnString();
        string sqlString = "SELECT MAX(User.ID) FROM [User]";
        using (OleDbConnection conn = new OleDbConnection(ConnString))
        {
            using (OleDbCommand cmd = new OleDbCommand(sqlString, conn))
            {
                cmd.CommandType = CommandType.Text;
                conn.Open();
                if (cmd.ExecuteScalar() != null)
                {
                    object obj = cmd.ExecuteScalar();
                    conn.Close();
                    conn.Dispose();
                    return obj.ToString();
                }
                else
                {
                    conn.Close();
                    conn.Dispose();
                    return string.Empty;
                }
            }
        }
    }

 public static string GetConnString()
    {
       return ConfigurationManager.AppSettings["ConnectionString"].ToString();
    }