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You likely get the exception "Connections to SQL Server files (*.mdf) require SQL Server Express 2005 to function properly.��� when you are trying to add a Database file to a project and don’t have SQLExpress explicitly defined in options.
Go to Tools>Options>Database Tools>Data Connections
Enter SQLEXPRESS in the “SQL Server Instance Name” textbox.
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Click OK
Try to add again, should work.
HTH,
Jon