Singleton implementation of a database connection
As per the docs

https://msdn.microsoft.com/en-us/library/system.data.sqlclient.sqlconnection(v=vs.90).aspx


  Any public static (Shared in Visual Basic) members of this type are thread safe. Any instance members are not guaranteed to be thread safe.


This means that while the Singleton instance can be technically correct, it is not correct in this specific context of the SqlConnection class if you plan to access instance members from different threads (and you possibly do).