Dynamically Binding Columns in Your Provider

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The latest version of this topic can be found at Dynamically Binding Columns in Your Provider.

Make sure you really need dynamic column binding. You might need it because:

• Your rowset columns are not defined at compile time.

• You support an element such as bookmarks that adds columns.

To implement dynamic column binding

1. Remove any PROVIDER_COLUMN_MAPs from your code.

2. In the user record (your structure), add the following declaration:

   static ATLCOLUMNINFO* GetColumnInfo(void* pThis, ULONG* pcCols);  
   

3. Implement the GetColumnInfo function. This function lays out how the information is stored. You might need to get properties or other information for this function. You might want to create a macro, similar to the COLUMN_ENTRY macro, to add your own information.

   The following example shows a GetColumnInfo function.

   // Check the property flag for bookmarks, if it is set, set the zero  
   // ordinal entry in the column map with the bookmark information.  
   CAgentRowset* pRowset = (CAgentRowset*) pThis;  
   CComQIPtr<IRowsetInfo, &IID_IRowsetInfo> spRowsetProps = pRowset;  
   
   CDBPropIDSet set(DBPROPSET_ROWSET);  
   set.AddPropertyID(DBPROP_BOOKMARKS);  
   DBPROPSET* pPropSet = NULL;  
   ULONG ulPropSet = 0;  
   HRESULT hr;  
   
   if (spRowsetProps)  
      hr = spRowsetProps->GetProperties(1, &set, &ulPropSet, &pPropSet);  
   
   if (pPropSet)  
   {  
      CComVariant var = pPropSet->rgProperties[0].vValue;  
      CoTaskMemFree(pPropSet->rgProperties);  
      CoTaskMemFree(pPropSet);  
   
      if (SUCCEEDED(hr) && (var.boolVal == VARIANT_TRUE))  
      {  
         ADD_COLUMN_ENTRY_EX(ulCols, OLESTR("Bookmark"), 0, sizeof(DWORD), DBTYPE_BYTES,   
            0, 0, GUID_NULL, CAgentMan, dwBookmark, DBCOLUMNFLAGS_ISBOOKMARK)  
         ulCols++;  
      }  
   }  
   
   // Next, set up the other columns.  
   ADD_COLUMN_ENTRY(ulCols, OLESTR("Command"), 1, 256, DBTYPE_STR, 0xFF, 0xFF,   
      GUID_NULL, CAgentMan, szCommand)  
   ulCols++;  
   ADD_COLUMN_ENTRY(ulCols, OLESTR("Text"), 2, 256, DBTYPE_STR, 0xFF, 0xFF,   
      GUID_NULL, CAgentMan, szText)  
   ulCols++;  
   
   ADD_COLUMN_ENTRY(ulCols, OLESTR("Command2"), 3, 256, DBTYPE_STR, 0xFF, 0xFF,   
      GUID_NULL, CAgentMan, szCommand2)  
   ulCols++;  
   ADD_COLUMN_ENTRY(ulCols, OLESTR("Text2"), 4, 256, DBTYPE_STR, 0xFF, 0xFF,   
      GUID_NULL, CAgentMan, szText2)  
   ulCols++;  
   
   if (pcCols != NULL)  
      *pcCols = ulCols;  
   
   return _rgColumns;  
   }  
   

See Also

Working with OLE DB Provider Templates