Database reader generates exception An attempt to attach an auto-named database for file .. failed

Question

Console.WriteLine("++++++++ Fun with Data Readers AutoLot++++++++++++\n");

// create and open connection 
using (SqlConnection connect = new SqlConnection())
{
    connect.ConnectionString = @"Data Source=(LocalDB)\MSSQLLocalDB;AttachDbFilename=
                      C:\Users\serge\AutoLot.mdf;Integrated Security=True;Connect Timeout=30";

    connect.Open();

    // create object SQL command 
    string sql = "Select * From Inventory";

    SqlCommand myCommand = new SqlCommand(sql, connect);

    // get object of data reading with ExecuteREader () 
    using (SqlDataReader myDataReader = myCommand.ExecuteReader())
    {
        // iterate loop results 
        while (myDataReader.Read())
        {
             Console.WriteLine($"-> Make : {myDataReader["MakeId"]},\t " +
                        $"PetName : {myDataReader["PetName"]},\t" +
                        $" Color : {myDataReader["Color"]}.");
        }
    }
}

Console.WriteLine();
Console.WriteLine("============= Fun with Data Readers Northwind DB =============\n");

using (SqlConnection connection = new SqlConnection())
{
    connection.ConnectionString = @"Data Source=(LocalDB)\MSSQLLocalDB;AttachDbFilename=
                C:\Users\serge\AppData\Local\Microsoft\Microsoft SQL Server
                Local DB\Instances\MSSQLLocalDB\northwnd.mdf;Integrated Security=True;Connect Timeout=30";

    connection.Open();

    string sql = "Select * From Products ";

    SqlCommand myCommand1 = new SqlCommand(sql, connection);

    /*     SqlConnection nwrdConnection = new SqlConnection(
                              ConfigurationManager.ConnectionStrings["NorthwindDB"].ConnectionString);
             nwrdConnection.Open();
             SqlCommand myCommand1 = new SqlCommand(sql, nwrdConnection);*/

    using (SqlDataReader myDataReader1 = myCommand1.ExecuteReader())
    {
        while (myDataReader1.Read())
        {
            Console.WriteLine($"===> ProductName : {myDataReader1["ProductName"]},\t" +
                        $"CategoryId : {myDataReader1["CategoryID"]},\t" +
                        $"UnitsInStock : {myDataReader1["UnitsInStock"]}");
        }
    }
}
Console.ReadLine();

The second query generates an exception :

Unhandled Exception: System.Data.SqlClient.SqlException: An attempt to attach an auto-named database for file C:\Users\serge\AppData\Local\Microsoft\Microsoft SQL Server Local DB\Instances\MSSQLLocalDB\northwnd.mdf failed. A database with the same name exists, or specified file cannot be opened, or it is located on UNC share.

While using debug mod it throws an exception in method:

connection.Open(); 

PS: uncommenting the other version of connection with adding connection string in app.config works fine.

I really don't understand why because connection to the first query works fine without any changes in app.config.