Thread safety of Microsoft.Data.SqlClient.SqlConnection

[kentcb]

Sorry to ask such a mundane question, but I cannot find this documented anywhere, nor asked by anyone else.

Is it safe to use a single Microsoft.Data.SqlClient.SqlConnection instance from multiple threads? Each thread creates its own SqlCommand against the SqlConnection.

Ostensibly, this works. But I'm trying to figure out whether it's working by accident or whether it's a ticking time-bomb.

[kentcb]

Answering my own Q: yes with MARS enabled (MultipleActiveResultSets). That was the source of my confusion.

[roji]

That is wrong, SqlConnection doesn't provide thread-safety, even when MARS is enabled, as per the docs. What MARS provides is the ability to have multiple resultsets open concurrently, but the SqlConnection instance itself must be accessed one thread at a time.

[kentcb]

@roji Interesting because in the code base in question, the same SqlConnection is (was, I've fixed it now) being used across multiple threads (each thread creating its own SqlCommand) with no error manifesting. Disabling MARS quickly revealed that multiple commands were in-flight at the same time, and debugging revealed that the access was occurring from distinct threads. So perhaps it's not officially supported, but can seemingly work depending on the specifics...?

[roji]

Things could by chance work, until one they they don't - that's generally the case when you concurrently use a type that isn't thread-safe. The situation is inherently non-deterministic. In any case, SqlConnection is not thread-safe.

[kentcb]

@roji Yes, that's exactly what I suspected when I posted this question, but got confused by use of MARS (which I had never encountered prior). Thanks for confirming!