ATLMTO: Dynamically Acquires Column Definitions

Click to open or copy the ATLMTO project files.

The ATLMTO sample demonstrates a provider that dynamically acquires its column definitions (meta data). Its purpose is to enable two-tier clients to operate in a three-tier environment. The provider does so by delegating its work to a COM object that either resides on the local machine or at a location accessible by DCOM. The provider then receives a special cookie from the COM object (third-tier) that contains connection information and column meta data. Once the provider acquires the meta data, it calls through a dual interface to acquire a BSTR representation of the rowset data. With the stream (BSTR) of data and the column meta data, the provider is capable of carrying on a two-tier OLE DB conversation with an OLE DB template consumer or with an ADO client.

This provider will override the following: GetColumnInfo of its user record (CRemoteRecord), and the ArrayType parameter of IRowsetImpl. Further, it demonstrates the process of holding column data in a CSimpleArray<ATLCOLUMNINFO> to enable dynamic determination of the column meta data.

The provider can only work with COM objects that support a special "hidden" BSTR on their callable methods and that expose the IMTOData interface. A sample consumer called TestMTO, a COM object that supports the needed interface and parameter, is included as subproject of the ATLMTO sample.

Building and Running the Sample

• The sample is set up so that when you build TestMTO, it builds both TestMTO and ATLMTO in the correct order.
  
  
• A small database (Accounts.mdb) is included with the sample. To use the database with the sample, you need to register it as an ODBC data source. Use the MS Access 97 Database driver and Accounts as the data source name and point it to Accounts.mdb. You can do this from the 32bit ODBC icon in Control Panel.
  
  
• After building the sample, run Qurydemo.exe and choose the "MtoProv OLE DB Provider" entry in the Initialize dialog box to get a new command. (Qurydemo.exe is installed when you install the OLE DB SDK. There is also an SDK sample called Qurydemo, which has the source code for Qurydemo.exe.)
  
  
• In the command window, type the following.

  call GetAccounts (50)
  

  and press the Play button. You will get a result set that includes all accounts with more than $50 in them.