using (var context = new BloggingContext())
        {
            using (var transaction = context.Database.BeginTransaction())
            {
                try
                {
                    context.Blogs.Add(new Blog { Url = "//blogs.msdn.com/dotnet" });
                    context.SaveChanges();

                    context.Blogs.Add(new Blog { Url = "//blogs.msdn.com/visualstudio" });
                    context.SaveChanges();

                    var blogs = context.Blogs
                        .OrderBy(b => b.Url)
                        .ToList();

                    // Commit transaction if all commands succeed, transaction will auto-rollback
                    // when disposed if either commands fails
                    transaction.Commit();
                }
                catch (Exception)
                {
                    // TODO: Handle failure
                }
            }
        }

Cross-context transaction (relational databases only)

You can also share a transaction across multiple context instances. This functionality is only available when 
using a relational database provider because it requires the use of DbTransaction and DbConnection, 
which are specific to relational databases.

       using (var context1 = new BloggingContext(options))
        {
            using (var transaction = context1.Database.BeginTransaction())
            {
                try
                {
                    context1.Blogs.Add(new Blog { Url = "//blogs.msdn.com/dotnet" });
                    context1.SaveChanges();

                    using (var context2 = new BloggingContext(options))
                    {
                        context2.Database.UseTransaction(transaction.GetDbTransaction());

                        var blogs = context2.Blogs
                            .OrderBy(b => b.Url)
                            .ToList();
                    }

                    // Commit transaction if all commands succeed, transaction will auto-rollback
                    // when disposed if either commands fails
                    transaction.Commit();
                }
                catch (Exception)
                {
                    // TODO: Handle failure
                }
            }
        }

           var options = new DbContextOptionsBuilder<DemoContext>()
                          .UseMySql("Server=localhost;DataBase=test2;UID=root;Password=root;charset=utf8;port=3306;SslMode=None")
                          .Options;
            using (var context = new DemoContext(options))
            {
                using (var transaction = context.Database.BeginTransaction())
                {
                    var user = new UserInfo
                    {
                        AddTime = DateTime.Now,
                        Email = "test@sina.com",
                        UserName = "test"
                    };
                    context.UserInfo.Add(user);
                    context.SaveChanges();
                    using (var context2 = new TaxContext())
                    {
                        context2.Database.UseTransaction(transaction.GetDbTransaction());
                        context2.UserInfo.Add(new UserInfo { AddTime = DateTime.Now, Email = "tax_test", UserName = "tax" });
                        context2.SaveChanges();
                    }
                    transaction.Commit();
                }
            }

System.InvalidOperationException:“The specified transaction is not associated with the current connection.
 Only transactions associated with the current connection may be used.”