为什么我需要 Odbc 读取器实例，但现在需要 Odbc 连接？
在类级别，我声明：
System::Data::Odbc::OdbcConnection conn;
System::Data::Odbc::OdbcDataReader datareader; //doesnt work
System::Data::Odbc::OdbcDataReader^ datareader; //works
但是，dataReader 必须声明为 OdbcDataReader^。我不明白为什么。
On the class level, I have declared:
System::Data::Odbc::OdbcConnection conn;
System::Data::Odbc::OdbcDataReader datareader; //doesnt work
System::Data::Odbc::OdbcDataReader^ datareader; //works
However, the dataReader has to be declared as OdbcDataReader^. I dont understand why.
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使用
OdbcConnection conn;，您可以直接（在堆栈上）实例化连接对象。不确定这是否是一个好主意，但这是可能的。您不能使用
OdbcDataReader datareader;来执行此操作，因为该类仅具有隐藏（私有/内部）构造函数。根据设计，您不应该直接创建 DataReader，而是通过调用ExecuteReader()来获取它们。并且 ExecuteReader 返回OdbcDataReader^。请参阅 MSDN。With
OdbcConnection conn;you are instantiating a connection object directly (on the stack). Not sure if that is a good idea but it is possible.You cannot do that with a
OdbcDataReader datareader;because that class only has hidden (private/internal) constructors. That is by design, you are not supposed to create DataReaders directly but instead get them from a call toExecuteReader(). And ExecuteReader returnsOdbcDataReader^. See MSDN.我不确定这是否回答了您的问题，但这里是：
Connection 和 DataReader 类在 .NET Framework 上成对出现，具体取决于所使用的数据库技术。所以你有
OdbcConnection和OdbcDataReader，还有SqlConnection和SqlDataReader等。你总是成对使用它们。无论如何请注意，所有这些都实现了通用接口IDataConnection和IDataReader。编辑。好吧，我完全误解了这个问题。 :-/
我通常使用C#，而不是C++，但我认为这是因为你可以直接创建一个新的
OdbcConnection实例，但是对于OdbcDataReader，你需要通过对相应的OdbcCommand执行ExecuteReader方法来获取实例。 ExecuteReader 返回指向新 OdbcDataReader 对象的指针。I'm not sure if this answers your question but here goes:
Connection and DataReader classes come in pairs on the .NET Framework, depending on the database technology used. So you have
OdbcConnectionandOdbcDataReader, alsoSqlConnectionandSqlDataReader, etc. You have always to use them in pairs. Note anyway that all of these implement the common interfacesIDataConnectionandIDataReader.EDIT. Fine, I completely misunderstood the question. :-/
I usually use C#, not C++, but I think that it is because you can directly create a new instance of
OdbcConnection, but as for theOdbcDataReader, you need to obtain an instance by executing theExecuteReadermethod on the correspondingOdbcCommand. ExecuteReader returns a pointer to a new OdbcDataReader object.