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DB_E_CANNOTFREE
Meaning
Windows documents DB_E_CANNOTFREE as “Provider has ownership of this tree”. Here, a consumer attempts to free or take ownership of a command tree that remains owned by the OLE DB provider.
Relevant OLE DB contract
This result must be interpreted against this contract: an OLE DB command moves through explicit states: command definition, properties and parameters are established before optional preparation and execution; dialect, query-tree and optimizer contracts are provider-specific and should be inferred only from reported capabilities.
Evidence to collect before changing the system
When it involves command text, parameter values, object names and query-plan details, record types, lengths, hashes or redacted identifiers instead of secrets or full business data.
- Evidence 1: the method that returned the tree and its documented ownership contract.
- Evidence 2: the allocator, node addresses and ownership flags.
- Evidence 3: the command object's lifetime and any provider callback retaining the tree.
Specific conditions that produce it
- Cause 1: the tree was returned under provider ownership rules.
- Cause 2: a clone or translation call did not transfer ownership as the consumer expected.
- Cause 3: the consumer mixes allocator or release conventions from a different command-tree API.
Retry and recovery policy
Retry rule: retry is not appropriate until ownership is corrected; repeated freeing risks memory corruption even if the first call merely returns it.
Corrective actions
- Action 1: leave provider-owned trees untouched and use the documented release path.
- Action 2: clone data into consumer-owned storage when modification or independent lifetime is required.
- Action 3: centralize command-tree ownership in one wrapper to prevent double-free attempts.
Practical incident
A query inspector calls a generic tree destructor on a provider-owned tree obtained for diagnostics; switching to a read-only view prevents it.
Difference from related HRESULT values
DB_E_CANTTRANSLATE concerns representing a tree as text, while DB_E_CANNOTFREE concerns who owns and may release the tree.
Official Microsoft references
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