使用实体框架时不会引发DbUpdateConcurrencyException

Question

我在我的action方法中为DbUpdateConcurrencyException定义了一个catch语句，我在我的数据库和模型类中包含了一个timestamp列。我设置了concurrency = Fixed。但如果两个用户试图编辑同一个对象，则不会引发DbUpdateConcurrencyException！

以下是代码：

[HttpPost]
public ActionResult Edit(int id, FormCollection collection) 
{ 
    Class c = repository.GetClass(id); 
    try 
    { 
        if (TryUpdateModel(c)) 
        { 
            epository.Save(); 
            return RedirectToAction("Details", new { id = c.ClassID }); 
        } 
    } 
    catch (DbUpdateConcurrencyException ex) 
    { 
        var entry = ex.Entries.Single(); 
        var databaseValues = (Class)entry.GetDatabaseValues().ToObject(); 
        var clientValues = (Class)entry.Entity; 
        if (databaseValues.ClassName != clientValues.ClassName) 
            ModelState.AddModelError("Name", "Current value: " 
            + databaseValues.ClassName); 

        ModelState.AddModelError(string.Empty, "The record you attempted to edit " 
        + "was modified by another user after you got the original value. The " 
        + "edit operation was canceled and the current values in the database " 
        + "have been displayed. If you still want to edit this record, click " 
        + "the Save button again. Otherwise click the Back to List hyperlink."); 
        c.Timestamp = databaseValues.Timestamp; 
    } 
    catch (DataException) 
    { 
        //Log the error (add a variable name after Exception) 
        ModelState.AddModelError(string.Empty, "Unable to save changes. Try again, and if the problem persists contact your system administrator."); 
    } 

    return View(c);
}