var results = from c in db.Companies
              join cn in db.Countries on c.CountryID equals cn.ID
              join ct in db.Cities on c.CityID equals ct.ID
              join sect in db.Sectors on c.SectorID equals sect.ID
              where (c.CountryID == cn.ID) && (c.CityID == ct.ID) && (c.SectorID == company.SectorID) && (company.SectorID == sect.ID)
              select new { country = cn.Name, city = ct.Name, c.ID, c.Name, c.Address1, c.Address2, c.Address3, c.CountryID, c.CityID, c.Region, c.PostCode, c.Telephone, c.Website, c.SectorID, Status = (ContactStatus)c.StatusID, sector = sect.Name };


return results.ToList();

OperationDataContext odDataContext = new OperationDataContext();    
        var studentInfo = from student in odDataContext.STUDENTs
                          join course in odDataContext.COURSEs
                          on student.course_id equals course.course_id
                          select new { student.student_name, student.student_city, course.course_name, course.course_desc };

学生和课程表在哪里有主键和外键的关系

它是这样的:

from t1 in db.Table1
join t2 in db.Table2 on t1.field equals t2.field
select new { t1.field2, t2.field3}

为了更好的示例，最好为您的表提供合理的名称和字段。：）

更新

我认为对于你的疑问，这可能更合适:

var dealercontacts = from contact in DealerContact
                     join dealer in Dealer on contact.DealerId equals dealer.ID
                     select contact;

既然你要找的是联系人，而不是经销商。

一个最好的例子

表名:TBL_Emp和TBL_Dep

var result = from emp in TBL_Emp join dep in TBL_Dep on emp.id=dep.id
select new
{
 emp.Name;
 emp.Address
 dep.Department_Name
}


foreach(char item in result)
 { // to do}

试试这个吧，

var dealer = from d in Dealer
             join dc in DealerContact on d.DealerID equals dc.DealerID
             select d;

var list = (from u in db.Users join c in db.Customers on u.CustomerId equals c.CustomerId where u.Username == username
   select new {u.UserId, u.CustomerId, u.ClientId, u.RoleId, u.Username, u.Email, u.Password, u.Salt, u.Hint1, u.Hint2, u.Hint3, u.Locked, u.Active,c.ProfilePic}).First();

编写所需的表名，并初始化select以获得字段的结果。