我们使用EF Core的实体类向SQL Server数据库表中插入数据后，如果数据库表中有自增列或默认值列，那么EF Core的实体对象也会返回插入到数据库表中的默认值。
下面我们通过例子来展示，EF Core是怎么获取数据库的默认值的。首先我们建立一个.NET Core控制台项目，然后引入EF Core所需的NuGet包。
获取自增列值
我们在SQL Server数据库中建立一个表Person，该表有一个自增列ID为主键：
CREATE TABLE [dbo].[Person]( [ID] [int] IDENTITY(1,1) NOT NULL, [Name] [nvarchar](50) NULL, [Age] [int] NULL, CONSTRAINT [PK_Person] PRIMARY KEY CLUSTERED ( [ID] ASC )WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY] ) ON [PRIMARY]
然后我们使用EF Core的DB First将该表映射为实体类Person，如下所示：
public partial class Person { public int Id { get; set; } public string Name { get; set; } public int? Age { get; set; } }
实体类Person生成的Fluent API代码如下所示：
modelBuilder.Entity<Person>(entity => { entity.Property(e => e.Id).HasColumnName("ID"); entity.Property(e => e.Name).HasMaxLength(50); });
然后我们在Program类的Main方法中，通过实体类Person向数据库Person表中插入一行数据：
class Program { static void Main(string[] args) { using (var dbContext = new DemoDBContext()) { Person person = new Person() { Name = "Tom", Age = 20 }; dbContext.Person.Add(person); dbContext.SaveChanges(); Console.WriteLine(person.Id);//输出：1 } Console.WriteLine("Press any key to end..."); Console.ReadKey(); } }
我们在dbContext.SaveChanges()这行代码执行后，通过EF Core的后台日志可以查看到生成的SQL语句如下：
=============================== EF Core log started ===============================
Executed DbCommand (95ms) [Parameters=[@p0=‘?‘ (DbType = Int32), @p1=‘?‘ (Size = 50)], CommandType=‘Text‘, CommandTimeout=‘30‘]
SET NOCOUNT ON;
INSERT INTO [Person] ([Age], [Name])
VALUES (@p0, @p1);
SELECT [ID]
FROM [Person]
WHERE @@ROWCOUNT = 1 AND [ID] = scope_identity();
=============================== EF Core log finished ===============================
可以看到EF Core最后使用了scope_identity()函数，来返回了刚刚插入的自增列ID的值。
获取默认值列
我们在SQL Server数据库中再建立一个表Book，该表有一个默认值列BookCode为主键，其默认值为函数newid()：
CREATE TABLE [dbo].[Book]( [BookCode] [nvarchar](50) NOT NULL, [BookName] [nvarchar](50) NULL, [BookDescription] [nvarchar](50) NULL, CONSTRAINT [PK_Book] PRIMARY KEY CLUSTERED ( [BookCode] ASC )WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY] ) ON [PRIMARY] GO ALTER TABLE [dbo].[Book] ADD CONSTRAINT [DF_Book_BookCode] DEFAULT (newid()) FOR [BookCode] GO
然后我们使用EF Core的DB First将该表映射为实体类Book，如下所示：
public partial class Book { public string BookCode { get; set; } public string BookName { get; set; } public string BookDescription { get; set; } }
实体类Book生成的Fluent API代码如下所示：
modelBuilder.Entity<Book>(entity => { entity.HasKey(e => e.BookCode); entity.Property(e => e.BookCode) .HasMaxLength(50) .HasDefaultValueSql("(newid())"); entity.Property(e => e.BookDescription).HasMaxLength(50); entity.Property(e => e.BookName).HasMaxLength(50); });
然后我们在Program类的Main方法中，通过实体类Book向数据库Book表中插入一行数据：
class Program { static void Main(string[] args) { using (var dbContext = new DemoDBContext()) { Book book = new Book() { BookDescription = "English book", BookName = "English" }; dbContext.Add(book); dbContext.SaveChanges(); Console.WriteLine(book.BookCode);//输出：B5AD3E33-3730-4545-8D45-9BD6DFCD5063 } Console.WriteLine("Press any key to end..."); Console.ReadKey(); } }
我们在dbContext.SaveChanges()这行代码执行后，通过EF Core的后台日志可以查看到生成的SQL语句如下：
=============================== EF Core log started ===============================
Executed DbCommand (116ms) [Parameters=[@p0=‘?‘ (Size = 50), @p1=‘?‘ (Size = 50)], CommandType=‘Text‘, CommandTimeout=‘30‘]
SET NOCOUNT ON;
DECLARE @inserted0 TABLE ([BookCode] nvarchar(50), [_Position] [int]);
MERGE [Book] USING (
VALUES (@p0, @p1, 0)) AS i ([BookDescription], [BookName], _Position) ON 1=0
WHEN NOT MATCHED THEN
INSERT ([BookDescription], [BookName])
VALUES (i.[BookDescription], i.[BookName])
OUTPUT INSERTED.[BookCode], i._Position
INTO @inserted0;
SELECT [t].[BookCode] FROM [Book] t
INNER JOIN @inserted0 i ON ([t].[BookCode] = [i].[BookCode])
ORDER BY [i].[_Position];
=============================== EF Core log finished ===============================
我们发现EF Core实际上是用SQL Server的Merge语句往数据库Book表中插入了数据，并且使用了Merge语句的OUTPUT关键字，来返回Book表中列BookCode插入的默认值。
EF Core中，通过实体类向SQL Server数据库表中插入数据后，实体对象是如何得到数据库表中的默认值的
原文：https://www.cnblogs.com/OpenCoder/p/10584317.html