Implementing SQL Compact Command Parameter BLOBs

After much ramblings, tests and misconceptions, I finally closed the chapter on SQL Compact command parameters. The sample code (see below) illustrates how to use a BLOB command parameter using the "variable length" binding buffer. Whenever the total size of the columns exceeds the allocated binding buffer size, the binding process is repeated increasing the binding buffer size. Although this is not an optimal solution (being able to use storage objects for this purpose would be much better), it frees you from having to specify the maximum size for BLOB parameters - the code calculates this for you.

Before you can execute a command with parameters, you must prepare it by calling the Prepare method (failing to do so will cause a run-time error). Here the code determines the number of existing parameters and allocates memory for a "user buffer". This user buffer is an array of CDbValue objects, a VARIANT-like class whose sole purpose is to store parameter values. When the user sets a parameter it is first written to one of these objects and upon execution it is copied to the data buffer. Meanwhile, the code determines if the new set of parameters on the CDbValue array requires rebinding, does so if needed and finally copies the values to the data buffer. This way you just have to set the parameters and execute the command (see the sample code).

As a side-effect of all these travails, I ended up creating a new class - CDbValueRef - from which CDbValue derives that I use as the "engine" that sets and gets data from various types, performing the necessary data type conversions. As you can now see, the column data access on the CRowset class is performed through an array of such objects that directly reference the bound data buffer, and they know how to handle storage object BLOBs. Now you can access your CRowset data using a type-safe interface.

Sample: InsertBlob.zip (152 KB)