无法在ef中插入标识列,尽管从未更改该值

我正在尝试使用EntityFramework更新表行, 不更改我的代码中的ID列 . 为什么它仍然告诉我不能插入显式值?

当IDENTITY“INSERT”设置为“off”时,无法在表“TbllicenseGroups”中插入标识列的显式值。

我的密码

//mytbl context is initialized as _gc
var mymodel=_gc.mytbl.Where(x=>x.ID.ToString()=="1").FirstorDefault();
mymodel.otherfield=22;//sets a new value
_gc.Add(mymodel);
_gc.SaveChanges();

EF模型

    [Key]
    [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
    public int ID { get; set; }

    public int otherfield { get; set; }

创建SQL

CREATE TABLE [dbo].[tblLicenseGroups](
    [ID] [int] IDENTITY(1,1) NOT NULL,
    ....
 CONSTRAINT [PK_tblSiteParentIDs] 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]
GO