Thursday, October 09, 2008

Updating rows with BLOBs

As I have previously discussed, we need to make some substantial changes to our code if we want to update rows with BLOB columns. So how exactly do we update these things? The first thing we need is to implement an object that behaves like an ISequentialStream COM object. Our code will create one such object per BLOB column, store the data in this object and then store a pointer to this object in the rowset buffer. When updating data, the OLE DB provider will read it (just like our code reads the storage objects coming from the provider) and then releases it. To make our life easier, our implementation of this object should delete itself when the reference count reaches zero. This way we will not have to worry about deleting it later.

After setting all the row data (BLOB and otherwise), we can insert or update it. If you want to delete a row it is pointless to write data to it. If you remember, I said that when using BLOBs with SQL Compact we must use the deferred update mode by requesting the IRowsetUpdate interface. Here's how the code changes for each operation (insert, update or delete):

  1. Call IRowsetChange::InsertRow - this sets the non-BLOB data;
  2. Call IRowsetChange::SetData for each BLOB accessor;
  3. Call IRowsetUpdate::Update to commit the changes.
  1. Call IRowsetChange::SetData for each accessor.
  2. Call IRowsetUpdate::Update to commit the row changes.

  1. Call IRowsetChange::DeleteRow.
  2. Call IRowsetUpdate::Update.
Note that if any of these processes fails before calling Update, you should call Undo to revert any changes made to the row. I'll show you the code in the next post.

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