单条件：

var query = from person in people

join pet in pets on person equals pet.Owner

select new { OwnerName = person.FirstName, PetName = pet.Name };

多条件：

IEnumerable<string> query = from employee in employees

join student in students

on new { employee.FirstName, employee.LastName }

equals new { student.FirstName, student.LastName }

select employee.FirstName + " " + employee.LastName;

https://www.cnblogs.com/ybtools/p/6559292.html