Entity Exception : the underlying provider failed to open?

I would consider checking one of two things.

Most likely the problem is that you are opening the database with Visual Studio and your application at the same time. The connection string explicitely configures AttachDbFilename=... AttachDBFilename spins up a user instance of SQL Express attached to a specific DB Filename for single user mode. In single user mode, only one application can open the MDF at a time.

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