I try to build the Database App in this Microsoft Walkthrough:(Simple Object Model and Query (C#)
In fact when I debug it it said: System.Data.SqlClient.SqlException: '在与 SQL Server 建立连接时出现与网络相关的或特定于实例的错误。未找到或无法访问服务器。请验证实例名称是否正确并且 SQL Server 已配置为允许远程连接。 (provider: SQL Network Interfaces, error: 26 - 定位指定的服务器/实例时出错)' like this: 
I try to add database on VS2019 and see the Troubleshooting section of Learning by Walkthroughs . but it still not works. I opened SQL Server(MSSQLSERVER), and also open TCP/IP and Names Pipes. I also make a rules to allow use 1433 on firewall.
I have no idea to solve this problem, any suggestions? Or I need to connectionString on App.config? Even if in LINQ to SQL? Is so here is my connectionString
and how to add it?
And I felt. Mircosoft docs missing some steps....
