SqlConnection myConnectionString = new SqlConnection(@"Data Source=(localdb)abcde;AttachDbFilename=C:Usersqqqdatabasename.mdf;Initial Catalog=databasename;Integrated Security=True");

SqlConnection myConnectionString = new SqlConnection(@"Data Source=(localdb)abcde;AttachDbFilename=|DataDirectory|databasename.mdf;Initial Catalog=databasename;Integrated Security=True");

<?xml version="1.0" encoding="utf-8" ?>
    <configuration>
        <startup>
            <supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.6.1" />
        </startup>
    </configuration>