LINQ to SQL attach entity issue

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C#3.5

I'am trying to create an update method in a generic repository as a data access layer
i have an entity like this:

[Table]
public class Product
{
    [Column(IsPrimaryKey = true, IsDbGenerated = true, DbType = "Int NOT NULL IDENTITY")]
    public int Id { get; private set; }
    [Column(UpdateCheck = UpdateCheck.Never)]
    public string Name { get; set; }
    ....
}

i set `Update Check = true` for all the fields exept for the id and i set the `asModified` propery in the attach method to true which i found as following:

    public void Update(T entity)
    {
        _db.GetTable<t>().Attach(entity, true); 
        _db.SubmitChanges();
    }
</t>

but i keep getting the same exception:

An entity can only be attached as modified without original state if it declares a version member or does not have an update check policy.

so what's the problem ???
or Do you recommend any other approches to create an update method in a generic repository except createing a timestamp column as a version number.

Thanks in Advance;