Are Dapper QueryMultiple/Query/Execute methods thread safe?
All of the internal meta-programming parts of dapper should indeed be thread-safe, and it sees enough heavy load on sites like stackoverflow that I'm pretty confident this is fine.
However, if you perform multiple concurrent operations on the same connection instance - then that comes down to whether a single ADO.NET connection is thread-safe; and in most cases, no it is not, AFAIK. So: don't do that. Multiple connections with their own operations should be fine though.