ConnectionProperties accepts a server, database, and optionally a username and password. It does not support attaching an MDF file. Omitting the username and password will force integrated Windows authentication, but you cannot specify an account with Windows auth, and them's the rules.
As a workaround, you can try adding the attachdb parameter into the server name, which will effectively let you attach an MDF to the SQLEXPRESS instance so that you can compare to it. The method for formatting the server connection string is described here