「標準表達式中的數據類型不匹配」在C＃中

我在字符串變量中有一個數字「userID」。我將它轉換爲int以從數據庫中獲取具有AutoNumber數據類型的「user_ID」字段的「用戶名」和「角色」。轉換後的用戶ID分配給變量「值」。但我在該行獲得「標準表達式中數據類型不匹配」：「標準表達式中的數據類型不匹配」在C＃中

OleDbDataReader roleReader = command.ExecuteReader();

下面是完整的代碼

class HomeClass 
{ 
    string userID; 
    string role; 
    string username; 

    public HomeClass(string myUserID) 
    { 
     userID = myUserID; 
    } 

    public string checkRole() 
    { 
     int value; 
     value = Int32.Parse(userID); 
     string query = "SELECT role FROM userAccounts WHERE user_ID = '" + value + "'"; 
     ConnectDatabaseString myConnectionString = new ConnectDatabaseString(); 
     OleDbConnection connection = new OleDbConnection(); 
     connection.ConnectionString = myConnectionString.connect(); 
     connection.Open(); 

     OleDbCommand command = new OleDbCommand(); 
     command.Connection = connection; 
     command.CommandText = query; 
     OleDbDataReader roleReader = command.ExecuteReader(); 
     roleReader.Read(); 
     role = roleReader["roles"].ToString(); 
     return role; 
    } 

    public string getUsernameToBeShown() 
    { 
     int value; 
     value = Int32.Parse(userID); 
     string query = "SELECT username FROM userAccounts WHERE user_ID = '" + value + "'"; 
     ConnectDatabaseString myConnectionString = new ConnectDatabaseString(); 
     OleDbConnection connection = new OleDbConnection(); 
     connection.ConnectionString = myConnectionString.connect(); 
     connection.Open(); 

     OleDbCommand command = new OleDbCommand(); 
     command.Connection = connection; 
     command.CommandText = query; 
     OleDbDataReader usernameReader = command.ExecuteReader(); 
     usernameReader.Read(); 
     username = usernameReader["username"].ToString(); 
     return username; 
    } 
}

這是什麼錯誤的根本原因？

來源

2013-07-07 Amvin Cinco