Changes To Cloned Database Are Affecting The Original

I am trying to use SQL Express to support unit testing of functionality which requires database access.

I created an express database (.mdf) which contains a database snapshot which is the database environment that each test expects to see when it starts. Through the execution of the test the database will be modified arbitrarily and I need to reset the database to the original state before the next test executes.

Here's what I do today:

Before Test:

The original database is copied to a unique location in the temp directory
A unique database name is generated (GUID-based)
I connect to the new database file with:

Server=.SQLEXPRESS;AttachDbFilename="PathToTempFile";Database="GuidDBName";Initial Catalog="GuidDBName";Trusted_Connection=Yes;User Instance=true

During the test all database access is routed to the temporary database GuidDBName

After Test:

Close all active database connections and clear the connection pool
Connect to the master database and detach the database:

Server=.SQLEXPRESS;Database=master;Initial Catalog=master;Trusted_Connection=Yes;User Instance=true - sp_detach_db 'GuidDBName'


Delete the temporary file

When I do this, everything seems to work correctly but when I re-open the original database it has changed unexpectedly and includes the modifications that were made by the test as it ran.

Can anyone explain why this is happening and/or provide a solution?