c# - 多个左连接 LINQ-to-entities

Question

我有 3 个表:

Dealerships
------------
ID, Name, Website

Locations
------------
ID, DealershipID, Address, Ect.

Contacts
------------
ID, LocationID, Name, Ect.

因此，该关系显示我们的经销商有多个地点(例如:宾夕法尼亚州的 Weed Chevrolet、新泽西州的 Weed Chevrolet)，然后每个地点都有自己的联系人(例如:PA 地点的经理、新泽西州地点的经理)。我需要将 3 个表连接在一起。这是我的:

var results = from d in entities.dealerships
              join l in entities.locations on d.ID equals l.DealershipID
              join c in entities.contacts on l.ID equals c.LocationID
              select new
              {
                  Name = d.Name,
                  Website = d.Website,
                  Address = l.Address + ", " + l.City + ", " + l.State + " " + l.Zip,
                  Contact = c.FirstName + " " + c.LastName,
                  WorkPhone = c.WorkPhone,
                  CellPhone = c.CellPhone,
                  HomePhone = c.HomePhone,
                  Email = c.Email,
                  AltEmail = c.AltEmail,
                  Sells = l.Sells
               }

当我尝试将 results 绑定(bind)到 BindingSource，然后再绑定(bind)到 DataGridView 时，我收到以下错误:

Unable to cast the type 'System.Nullable`1' to type 'System.Object'.
LINQ to Entities only supports casting Entity Data Model primitive types.

它可以是什么？我是 LINQ 中的 JOIN 语句的新手，所以我确定我做错了什么。

编辑:数据库中有数据所以结果不应该为空，只是为了澄清

Answer 1

你很接近，但我发现你必须将它从 LINQ-To-Entities 转换为 LINQ-To-Objects。首先，我必须使用 AsEnumerable() 转换实体，然后使用 ToList()。这样我就可以使用 ToString() 和 String.Format() 等函数。感谢您引导我朝着正确的方向前进。这是最终代码:

var query = from d in entities.dealerships
            from l in entities.locations.Where(loc => loc.DealershipID == d.ID).DefaultIfEmpty()
            from c in entities.contacts.Where(cont => cont.LocationID == l.ID).DefaultIfEmpty()
            where d.Keywords.Contains(keywords) || l.Keywords.Contains(keywords) || l.Sells.Contains(keywords) || c.Keywords.Contains(keywords)
            select new
            {
                Dealership = d,
                Location = l,
                Contact = c
            };

var results = (from r in query.AsEnumerable()
               select new
               {
                   Name = r.Dealership.Name,
                   Website = r.Dealership.Website,
                   Contact = r.Contact.FirstName + " " + r.Contact.LastName,
                   Address = r.Location.Address + ", " + r.Location.City + ", " + r.Location.State + " " + r.Location.Zip,
                   WorkPhone = r.Contact.WorkPhone,
                   CellPhone = r.Contact.CellPhone,
                   Fax = r.Contact.Fax,
                   Email = r.Contact.Email,
                   AltEmail = r.Contact.AltEmail,
                   Sells = r.Location.Sells
               }).ToList();

bindingSource.DataSource = results;