异常类型：System.Data.Entity.Infrastructure.DbUpdateConcurrencyException
实体属性配置如上例所示。
操作代码：
using (CustomDbContext db = new CustomDbContext())
{
User user = new User
{
Id = 1,
Email = "test@132.com",
};
DbEntityEntry<User> entry = db.Entry<User>(user);
entry.State = EntityState.Unchanged;
entry.Property(t => t.Email).IsModified = true;
User user1 = new User
{
Id = 1,
Email = "test@132.com",
};
DbEntityEntry<User> entry1 = db.Entry<User>(user1);
entry1.State = EntityState.Unchanged;
entry1.Property(t => t.Email).IsModified = true;
int num = db.SaveChanges();
}
执行操作
涉及到两次修改操作，两次操作构造了两个实例，但是实例的属性Id有相同的值。
如果两次操作的是同一个实例，而不是不同的实例，那么不会抛出异常，代码如下：
using (CustomDbContext db = new CustomDbContext())
{
User user = new User
{
Id = 1,
Email = "test@132.com",
};
DbEntityEntry<User> entry = db.Entry<User>(user);
entry.State = EntityState.Unchanged;
entry.Property(t => t.Email).IsModified = true;
DbEntityEntry<User> entry1 = db.Entry<User>(user);
entry1.State = EntityState.Unchanged;
entry1.Property(t => t.Email).IsModified = true;
int num = db.SaveChanges();
}
3 未给主键赋值或赋给主键一个不存在的值，抛出异常
System.Data.Entity.Infrastructure.DbUpdateConcurrencyException
操作代码如下，其中Id=1这条语句被注掉，Id是主键：
using (CustomDbContext db = new CustomDbContext())
{
User user = new User
{
//Id = 1,
Email = "test@132.com",
};
DbEntityEntry<User> entry = db.Entry<User>(user);
entry.State = EntityState.Unchanged;
entry.Property(t => t.Email).IsModified = true;
int num = db.SaveChanges();
}
运行上述代码，抛出异常信息如下，注意异常类型居然是System.Data.Entity.Infrastructure.DbUpdateConcurrencyException，看上去像是并发问题，但实际却不是！
Message:
Store update, insert, or delete statement affected an unexpected number of rows (0). Entities may have been modified or deleted since entities were loaded. Refresh ObjectStateManager entries.
