Linq To Sql-主键语法

Question

I am using LINQ To SQL and creating my database using CreateDatabase method. I understand from MSDN that the syntax to create an auto generated primary key field is

[Column(Storage="_ID", IsPrimaryKey=true, AutoSync=AutoSync.OnInsert, DbType="Int NOT NULL IDENTITY", IsDbGenerated=true)]

Can someone explain me what does it mean by AutoSync.OnInsert and what is the usage of other members of AutoSync enum? .ie. AutoSync.Always, AutoSync.Default etc.

And what's the significance of the word 'Identity' in DbType, when I have already specified the column as primary key?

Answer 1

Identity means that the ID column has an autogenerated value that will be inserted automatically on any insert. When you insert a row the first time the value for ID will be 1 the next time 2 and so on.

AutoSync Instructs the common language runtime (CLR) to retrieve the value after an insert or update operation.

Options: Always, Never, OnUpdate, OnInsert.

Source

Never: You will never get back an ID in your entity (as long as you won't load the Entity from Database). Always means that after every insert or update Linq2Sql operation it will retrieve the value from the Database.

Source