using System;
using System.Threading;
using Npgsql;

class Test2
{
    static int Main(string[] args)
    {
        var connectionString = new NpgsqlConnectionStringBuilder {
            Host = "test-server",
            Database = "postgres",
            Username = "postgres",
            Password = "123456",
            Pooling = false,
            CommandTimeout = 0,
            KeepAlive = 30,
            TcpKeepAlive = true,
            TcpKeepAliveTime = 30
        };
        var connection = new NpgsqlConnection();
        connection.ConnectionString = connectionString.ConnectionString;
        connection.Open();

        try {
            using (var c = new NpgsqlCommand("SELECT 'dummy'", connection)) {
                c.ExecuteNonQuery();
            }
            Console.Write("disconnect the this client from the network and press enter when done so...");
            // note: you can also work on localhost and stop the server instead of disconnecting the network
            Console.ReadLine();
            Console.WriteLine("waiting 1 minute.");
            Thread.Sleep(60_000);
            Console.WriteLine("BeginTransaction");
            connection.BeginTransaction();
        }
        catch (NpgsqlException ex) {
            Console.WriteLine("Expected Exception: " + Environment.NewLine + ex);
        }
        catch (Exception unexpected) {
            Console.WriteLine("Unexpected Exception:" + Environment.NewLine + unexpected);
        }

        return 0;
    }
}

System.InvalidOperationException: Connection is not open
   at Npgsql.ThrowHelper.ThrowInvalidOperationException(String message)
   at Npgsql.NpgsqlConnection.CheckReady()
   at Npgsql.NpgsqlConnection.BeginTransaction(Boolean async, IsolationLevel level, CancellationToken cancellationToken)
   at Npgsql.NpgsqlConnection.BeginTransaction(IsolationLevel level)
   at Npgsql.NpgsqlConnection.BeginTransaction()
   at Test2.Main(String[] args) in Test2.cs:line 35