Connect to LocalDB existing in App_Data Folder in ASP.net Project Solution: <connectionStrings> <add name="ConnectionName" connectionString="Data Source=(LocalDB)\MSSQLLocalDB; AttachDbFilename=|DataDirectory|DatabaseName.mdf; Integrated Security=True;" providerName="System.Data.SqlClient" /> </connectionStrings> And please don't use  this (User Instance=True;) in your connection string. Use above connection string as it is, just change your connection name to as you want to name it and Data base file name to your database file name (.mdf). It will work fine, Thanks... Please visit to our YouTube Channel for .net and other tech tutorial Thanks