Connection String to Connect to .MDF

If you're using the *.mdf file in the App_Data folder of an ASP.NET app you can use this.

Server=.\SQLExpress;AttachDbFilename=|DataDirectory|mydbfile.mdf;Database=dbname; Trusted_Connection=Yes;

If it's not an ASP.NET application don't use the DataDirectory syntax and just use the full c:\... path.