7.3 数据注解特性之ConcurrencyCheck特性【Code-First系列】

ConcurrencyCheck特性可以应用到领域类的属性中。当EF执行更新操作的时候，Code-First将列的值放在where条件语句中，你可以使用这个CurrencyCheck特性，使用已经存在的列做并发检查，而不是使用单独的TimeStamp列来做并发检查。

看下面的代码：

using System;
using System.Collections.Generic;
using System.ComponentModel.DataAnnotations;
using System.ComponentModel.DataAnnotations.Schema;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace EF2
{
    [Table("StudentInfo")]
   public class Student
    {
        [Key]
        [Column(Order=)]
        public int StudentKey1 { get; set; }

        [Key]
        [Column(Order=)]
        public int StudentKey2 { get; set; }

        [Column("Name",TypeName="ntext")]
        [MaxLength()]
        [ConcurrencyCheck]
        public string StudentName { get; set; }

        [NotMapped()]
        public int? Age { get; set; }

        public int StdId { get; set; }

        [ForeignKey("StdId")]
        public virtual Standard Standard { get; set; }

        [Timestamp]
        public byte[] RowVersion { get; set; }

    }
}

然后我们来修改一下Main函数测试的代码：

using System;
using System.Collections.Generic;
using System.Data.Entity;
using System.Data.Entity.Infrastructure;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace EF2
{
    class Program
    {
        static void Main(string[] args)
        {
            Student stuModel1 = null;
            Student stuModel2 = null;

                using (var db = new DbContextClass())
                {
                    stuModel1 = db.Students.Where(s => s.StudentKey1 ==  && s.StudentKey2 == ).Single();
                }

                using (var db = new DbContextClass())
                {
                    stuModel2 = db.Students.Where(s => s.StudentKey1 ==  && s.StudentKey2 == ).Single();
                }

                stuModel1.StudentName = "Test Only For one";
                stuModel2.StudentName = "Test Only For twos";

                try
                {
                    using (var db = new DbContextClass())
                    {
                        db.Entry(stuModel1).State = EntityState.Modified;
                        db.SaveChanges();
                    }
                }
                catch (DbUpdateConcurrencyException ex)
                {
                    Console.WriteLine(ex.Message);
                }

                try
                {
                    using (var db = new DbContextClass())
                    {
                        db.Entry(stuModel2).State = EntityState.Modified;
                        db.SaveChanges();
                    }
                }
                catch (DbUpdateConcurrencyException ex)
                {
                    Console.WriteLine(ex.Message);
                }

            Console.ReadKey();
        }
    }
}

然后错误并发消息是：

exec sp_executesql N'UPDATE [dbo].[StudentInfo]
SET [StudentName] = @, [StdId] = @
WHERE ((([StudentKey1] = @) AND ([StudentKey2] = @)) AND ([StudentName] = @))
 nvarchar(),@    nvarchar()',@0=N'Test Only For one',@1=1,@2=1,@3=1,@4=N'Test Only For one'

请注意：

Note that TimeStamp attribute can only be applied to a single byte array property in a class, whereas ConcurrencyCheck attribute can be applied to any number of properties with any datatype.

TimeStamp特性只能够被用到单字节属性的类中，但是ConcurrencyCheck特性可以被用到任何数量，任何类型的属性中。