Func<Invoice, bool> func = x => x.State == InvoiceState.Created;
Expression<Func<Invoice, bool>> expression = x => x.State == InvoiceState.Created;

            using (var db = new CrmDbContext())
            {
                var q1 = db.Invoices.Where(func); //全部查询出来后筛选
                var q2 = db.Invoices.Where(expression); //直接延迟到Count时执行

                var query = db.Invoices.AsQueryable();
                var qq1 = query.Where(func);//全部查询出来后筛选
                var qq2 = query.Where(expression); //直接延迟到Count时执行

                Console.WriteLine($"q1:{q1.Count()}");
                Console.WriteLine($"q2:{q2.Count()}");
                Console.WriteLine($"qq1:{qq1.Count()}");
                Console.WriteLine($"qq2:{qq2.Count()}");
            }