我们学习的时候还是看看其生成的代码吧！
具体设置映射继承层次结构有如下几步：
- 根类添加TableAttribute属性。
- 为层次结构中的每个类添加InheritanceMappingAttribute属性，同样是添加到根类中。每个 InheritanceMappingAttribute属性，定义一个Code属性和一个Type属性。Code属性的值显示在数据库表的IsDiscriminator列中，用来指示该行数据所属的类或子类。Type属性值指定键值所表示的类或子类。
- 仅在其中一个InheritanceMappingAttribute属性上，添加一个IsDefault属性用来在数据库表中的鉴别器值在继承映射中不与任何Code值匹配时指定回退映射。
- 为ColumnAttribute属性添加一个IsDiscriminator属性来表示这是保存Code值的列。
下面是这张图生成的代码的框架（由于生成的代码太多，我删除了很多“枝叶”，仅仅保留了主要的框架用于指出其实质的东西）：
[Table(Name = "dbo.Contacts")]
[InheritanceMapping(Code = "Unknown", Type = typeof(Contact), IsDefault = true)]
[InheritanceMapping(Code = "Employee", Type = typeof(EmployeeContact))]
[InheritanceMapping(Code = "Supplier", Type = typeof(SupplierContact))]
[InheritanceMapping(Code = "Customer", Type = typeof(CustomerContact))]
[InheritanceMapping(Code = "Shipper", Type = typeof(ShipperContact))]
public partial class Contact : INotifyPropertyChanging, INotifyPropertyChanged
{
[Column(Storage = "_ContactID",IsPrimaryKey = true, IsDbGenerated = true)]
public int ContactID{ }
[Column(Storage = "_ContactType",IsDiscriminator = true)]
public string ContactType{ }
}
public abstract partial class FullContact : Contact{ }
public partial class EmployeeContact : FullContact{ }
public partial class SupplierContact : FullContact{ }
public partial class CustomerContact : FullContact{ }
public partial class ShipperContact : Contact{ }
1.一般形式日常我们经常写的形式，对单表查询。
var cons = from c in db.Contacts
select c;
foreach (var con in cons) {
Console.WriteLine("Company name: {0}", con.CompanyName);
Console.WriteLine("Phone: {0}", con.Phone);
Console.WriteLine("This is a {0}", con.GetType());
}
2.OfType形式这里我仅仅让其返回顾客的联系方式。
var cons = from c in db.Contacts.OfType<CustomerContact>()
select c;
初步学习，我们还是看看生成的SQL语句，这样容易理解。在SQL语句中查询了ContactType为Customer的联系方式。
SELECT [t0].[ContactType], [t0].[ContactName], [t0].[ContactTitle], [t0].[Address],
[t0].[City], [t0].[Region], [t0].[PostalCode], [t0].[Country], [t0].[Fax],
[t0].[ContactID], [t0].[CompanyName], [t0].[Phone] FROM [dbo].[Contacts] AS [t0]
WHERE ([t0].[ContactType] = @p0) AND ([t0].[ContactType] IS NOT NULL)
-- @p0: Input NVarChar (Size = 8; Prec = 0; Scale = 0) [Customer]
3.IS形式这个例子查找一下发货人的联系方式。
var cons = from c in db.Contacts
where c is ShipperContact
select c;
生成的SQL语句如下：查询了ContactType为Shipper的联系方式。大致一看好像很上面的一样，其实这里查询出来的列多了很多。实际上是Contacts表的全部字段。
SELECT [t0].[ContactType], [t0].[ContactID], [t0].[CompanyName], [t0].[Phone],
[t0].[HomePage], [t0].[ContactName], [t0].[ContactTitle], [t0].[Address],
[t0].[City], [t0].[Region], [t0].[PostalCode], [t0].[Country], [t0].[Fax],
[t0].[PhotoPath], [t0].[Photo], [t0].[Extension] FROM [dbo].[Contacts] AS [t0]
WHERE ([t0].[ContactType] = @p0) AND ([t0].[ContactType] IS NOT NULL)
-- @p0: Input NVarChar (Size = 7; Prec = 0; Scale = 0) [Shipper]
4.AS形式
这个例子就通吃了，全部查找了一番。
var cons = from c in db.Contacts
select c as FullContact;
生成SQL语句如下：查询整个Contacts表。
SELECT [t0].[ContactType], [t0].[HomePage], [t0].[ContactName], [t0].[ContactTitle],
[t0].[Address], [t0].[City], [t0].[Region], [t0].[PostalCode], [t0].[Country],
[t0].[Fax], [t0].[ContactID], [t0].[CompanyName], [t0].[Phone], [t0].[PhotoPath],
[t0].[Photo], [t0].[Extension] FROM [dbo].[Contacts] AS [t0]
5.Cast形式使用Case形式查找出在伦敦的顾客的联系方式。
var cons = from c in db.Contacts
where c.ContactType == "Customer" &&
((CustomerContact)c).City == "London"
select c;
生成SQL语句如下，自己可以看懂了。
SELECT [t0].[ContactType], [t0].[ContactID], [t0].[CompanyName], [t0].[Phone], [t0].[HomePage],
[t0].[ContactName], [t0].[ContactTitle], [t0].[Address], [t0].[City], [t0].[Region],
[t0].[PostalCode], [t0].[Country], [t0].[Fax], [t0].[PhotoPath], [t0].[Photo], [t0].[Extension]
FROM [dbo].[Contacts] AS [t0] WHERE ([t0].[ContactType] = @p0) AND ([t0].[City] = @p1)
-- @p0: Input NVarChar (Size = 8; Prec = 0; Scale = 0) [Customer]
-- @p1: Input NVarChar (Size = 6; Prec = 0; Scale = 0) [London]
6.UseAsDefault形式当插入一条记录时，使用默认的映射关系了，但是在查询时，使用继承的关系了。具体看看生成的SQL语句就直截了当了。
//插入一条数据默认使用正常的映射关系
Contact contact = new Contact() { ContactType = null,
CompanyName = "Unknown Company",
Phone = "333-444-5555" };
db.Contacts.InsertOnSubmit(contact);
db.SubmitChanges();
//查询一条数据默认使用继承映射关系
var con = (from c in db.Contacts
where c.CompanyName == "Unknown Company" &&
c.Phone == "333-444-5555"
select c).First();
生成SQL语句如下：
INSERT INTO [dbo].[Contacts]([ContactType], [CompanyName], [Phone])
VALUES (@p0, @p1, @p2)
SELECT TOP (1) [t0].[ContactType], [t0].[ContactID], [t0].[CompanyName], [t0].[Phone],
[t0].[HomePage], [t0].[ContactName], [t0].[ContactTitle], [t0].[Address], [t0].[City],
[t0].[Region], [t0].[PostalCode], [t0].[Country], [t0].[Fax], [t0].[PhotoPath],
[t0].[Photo], [t0].[Extension] FROM [dbo].[Contacts] AS [t0]
WHERE ([t0].[CompanyName] = @p0) AND ([t0].[Phone] = @p1)
-- @p0: Input NVarChar (Size = 15; Prec = 0; Scale = 0) [Unknown Company]
-- @p1: Input NVarChar (Size = 12; Prec = 0; Scale = 0) [333-444-5555]
7.插入新的记录这个例子说明如何插入发货人的联系方式的一条记录。
//1.在插入之前查询一下，没有数据
var ShipperContacts = from sc in db.Contacts.OfType<ShipperContact>()
where sc.CompanyName == "Northwind Shipper"
select sc;
//2.插入数据
ShipperContact nsc = new ShipperContact() {
CompanyName = "Northwind Shipper", Phone = "(123)-456-7890" };
db.Contacts.InsertOnSubmit(nsc);
db.SubmitChanges();
//3.查询数据，有一条记录
ShipperContacts = from sc in db.Contacts.OfType<ShipperContact>()
where sc.CompanyName == "Northwind Shipper"
select sc;
//4.删除记录
db.Contacts.DeleteOnSubmit(nsc);
db.SubmitChanges();
生成SQL语句如下：
SELECT COUNT(*) AS [value] FROM [dbo].[Contacts] AS [t0]
WHERE ([t0].[CompanyName] = @p0) AND ([t0].[ContactType] = @p1)
AND ([t0].[ContactType] IS NOT NULL)
-- @p0: Input NVarChar (Size = 17; Prec = 0; Scale = 0) [Northwind Shipper]
-- @p1: Input NVarChar (Size = 7; Prec = 0; Scale = 0) [Shipper]
INSERT INTO [dbo].[Contacts]([ContactType], [CompanyName], [Phone])
VALUES (@p0, @p1, @p2)
-- @p0: Input NVarChar (Size = 7; Prec = 0; Scale = 0) [Shipper]
-- @p1: Input NVarChar (Size = 17; Prec = 0; Scale = 0) [Northwind Shipper]
-- @p2: Input NVarChar (Size = 14; Prec = 0; Scale = 0) [(123)-456-7890]
SELECT COUNT(*) AS [value] FROM [dbo].[Contacts] AS [t0]
WHERE ([t0].[CompanyName] = @p0) AND ([t0].[ContactType] = @p1)
AND ([t0].[ContactType] IS NOT NULL)
-- @p0: Input NVarChar (Size = 17; Prec = 0; Scale = 0) [Northwind Shipper]
-- @p1: Input NVarChar (Size = 7; Prec = 0; Scale = 0) [Shipper]
DELETE FROM [dbo].[Contacts] WHERE ([ContactID] = @p0) AND
([ContactType] = @p1) AND ([CompanyName] = @p2) AND ([Phone] = @p3)
-- @p0: Input Int (Size = 0; Prec = 0; Scale = 0) [159]
-- @p1: Input NVarChar (Size = 7; Prec = 0; Scale = 0) [Shipper]
-- @p2: Input NVarChar (Size = 17; Prec = 0; Scale = 0) [Northwind Shipper]
-- @p3: Input NVarChar (Size = 14; Prec = 0; Scale = 0) [(123)-456-7890]
-- @p4: Input NVarChar (Size = 7; Prec = 0; Scale = 0) [Unknown]
-- @p5: Input NVarChar (Size = 8; Prec = 0; Scale = 0) [Supplier]
-- @p6: Input NVarChar (Size = 7; Prec = 0; Scale = 0) [Shipper]
-- @p7: Input NVarChar (Size = 8; Prec = 0; Scale = 0) [Employee]
-- @p8: Input NVarChar (Size = 8; Prec = 0; Scale = 0) [Customer]
