这样的实现看起来很美，却远远不够。我们不能附加一个修改过的实体，除非该实体所对应的表中具有TimeStamp列（或者主键字段的IsVersion为true）。因此，我们需要在数据库中添加列，使其类型为Timestamp，或者在LINQ to SQL设计器中将ID属性的IsVersion属性设置为true。我的建议是为所有数据表创建一个TimeStamp列，它可以提高系统的性能，因为在处理并发的时候，系统只需要检查主键和TimeStamp是否有变化，而不需要检查所有的字段。
事实上，我们也可以通过泛型传递原始的实体值，这种方法就不需要为每个表添加TimeStamp列，方法是使用Action<T> 委托，代码段如下所示：
public void Update(TEntity originalEntity, Action<TEntity> update)
{
InitDataContext();
try
{
m_context.GetTable<TEntity>().Attach(originalEntity);
update(originalEntity);
m_context.SubmitChanges();
}
catch (ChangeConflictException)
{
m_context.ChangeConflicts.ResolveAll(RefreshMode.KeepCurrentValues);
m_context.SubmitChanges();
}
}
{
InitDataContext();
try
{
m_context.GetTable<TEntity>().Attach(originalEntity);
update(originalEntity);
m_context.SubmitChanges();
}
catch (ChangeConflictException)
{
m_context.ChangeConflicts.ResolveAll(RefreshMode.KeepCurrentValues);
m_context.SubmitChanges();
}
}
现在，我们可以通过传递Lambda表达式来调用该方法：
[TestMethod()]
public void UpdateWithAction()
{
LinqSampleDataContext context = new LinqSampleDataContext ();
EmployeeAccessor accessor = new EmployeeAccessor();
Employee employee = context.Employees.Single(e => e.EmployeeID == 1);
accessor.Update(employee, t => { t.FirstName = "First"; t.LastName = "Last"; });
}
{
LinqSampleDataContext context = new LinqSampleDataContext ();
EmployeeAccessor accessor = new EmployeeAccessor();
Employee employee = context.Employees.Single(e => e.EmployeeID == 1);
accessor.Update(employee, t => { t.FirstName = "First"; t.LastName = "Last"; });
}
遗憾的是，这样的测试用例有时却无法通过，会抛出NotSupportedException异常，异常的信息如下：
An attempt has been made to Attach or Add an entity that is not new, perhaps having been loaded from another DataContext. This is not supported.
