坑：關於 MySqlClientFactory 沒實現 CreateDataAdapter() 方法

情景

在使用數據庫通用類 DBHelper 操做 MySql 獲取 DataTable 時，DataTable 恆爲 null

過程

先看看 DBHelper 獲取 DataTable 的代碼。

public DataSet ExecuteDataSet(DbCommand cmd)
{
    var dbfactory = DbProviderFactories.GetFactory(DbProviderName);
    var dbDataAdapter = dbfactory.CreateDataAdapter();

    if (dbDataAdapter == null) return null;

    dbDataAdapter.SelectCommand = cmd;
    var ds = new DataSet();
    dbDataAdapter.Fill(ds);
    return ds;
}

源碼中使用了 Factory.CreateDataAdapter() 獲取 DbDataAdapter，再由 DbDataAdapter 將數據填充到 DataTable，並返回 DataTable。

跟蹤 DbFactory 和 DbDataAdapter，發現 DbDataAdapter 爲 null。

var factory = DbProviderFactories.GetFactory("MySql.Data.MySqlClient");
var da = factory.CreateDataAdapter();

var f_mysql = MySqlClientFactory.Instance;
var da_mysql = f_mysql.CreateDataAdapter();

額，DbDataAdapter 爲 null ？騙人的吧，既然爲 null，CreateDataAdapter() 還有啥用？

對比下 CreateDataAdapter() 分別在 Oracle 和 Sqlite 中的狀況。

var factory = DbProviderFactories.GetFactory("MySql.Data.MySqlClient");
var da = factory.CreateDataAdapter();

var f_mysql = MySqlClientFactory.Instance;
var f_mssql = SqlClientFactory.Instance;
var f_sqlite = SQLiteFactory.Instance;
var f_oracle = OracleClientFactory.Instance;

var da_mysql = f_mysql.CreateDataAdapter();
var da_mssql = f_mssql.CreateDataAdapter();
var da_sqlite = f_sqlite.CreateDataAdapter();
var da_oracle = f_oracle.CreateDataAdapter();

看看反編譯代碼，惟獨MySQLClientFactory沒實現 CreateDataAdapter() 方法

結論

<p style="text-decoration: underline;">MySqlDataAdapter>MySqlDataAdapter 不能由 MySqlClientFactory.CreateDataAdapter() 實例化。</p> 其實只是 config 文件沒配置好……

其餘

public DataSet ExecuteDataSet(DbCommand cmd)
{
    var dbfactory = DbProviderFactories.GetFactory(DbProviderName);
    var dbDataAdapter = dbfactory.CreateDataAdapter();

    if (dbDataAdapter == null)
    {
        if ("MySql.Data.MySqlClient" == DbProviderName)
        {
            dbDataAdapter = (DbDataAdapter) Assembly.Load("MySql.Data")
                .CreateInstance("MySql.Data.MySqlClient.MySqlDataAdapter");
        }
        else
        {
            return null;
        }
    }

    if (dbDataAdapter == null)
        return null;

    dbDataAdapter.SelectCommand = cmd;
    var ds = new DataSet();
    dbDataAdapter.Fill(ds);
    return ds;
}

參考

• 通用類 DBHelper
• C# CreateDataAdapter 建立 DbDataAdapter，以及用DataTable的Load方法獲取數據-布布扣-bubuko.com