Entity Framework in ASP.NET MVC Application (二)

In the previous tutorial you created an MVC application that stores and displays data using the Entity Framework and SQL Server Compact. In this tutorial you will review and customize the CRUD (create, read, update, delete) code that the MVC scaffolding automatically creates for you in controllers and views.

The scaffolded code for the Index page left out the Enrollments property, because that property holds a collection. In the Details page you will display the contents of the collection in an HTML table.

In Controllers\StudentController.cs, the action method for the Details view resembles the following example:

Creating a Create Page 
In Controllers\StudentController.cs, replace the HttpPost Create action method with the following code to add a try-catch block to the scaffolded method:
Data validation works by default.? Enter names and an invalid date and click Create to see the error message.
In this case you're seeing client-side validation that's implemented using JavaScript. But server-side validation is also implemented. Even if client validation failed, bad data would be caught and an exception would be thrown in server code.
Change the date to a valid value such as 9/1/2005 and click Create to see the new student appear in the Index page.
Creating an Edit Page 
In Controllers\StudentController.cs, the HttpGet Edit method (the one without the HttpPost attribute) uses the Find method to retrieve the selected Student entity, as you saw in the Details method. You don't need to change this method. 
However, replace the HttpPost Edit action method with the following code to add a try-catch block:
Change some of the data and click Save. You see the changed data in the Index page.
Creating a Delete Page 
In Controllers\StudentController.cs, the template code for the HttpGet Delete method uses the Find method to retrieve the selected Student entity, as you saw in the Details and Edit methods. However, to implement a custom error message when the call to SaveChanges fails, you will add some functionality to this method and its corresponding view.
As you saw for update and create operations, delete operations require two action methods. The method that is called in response to a GET request displays a view that gives the user a chance to approve or cancel the delete operation. If the user approves it, a POST request is created. When that happens, the HttpPost Delete method is called and then that method actually performs the delete operation. 
You will add a try-catch block to the HttpPost Delete method to handle any errors that might occur when the database is updated. If an error occurs, the HttpPost Delete method calls the HttpGet Delete method, passing it a parameter that indicates that an error has occurred. The HttpGet Delete method then redisplays the confirmation page along with the error message, giving the user an opportunity to cancel or to try again.
Replace the HttpGet Delete action method with the following code, which manages error reporting:
Click Delete. The Index page is displayed without the deleted student. (You'll see an example of the error handling code in action in the Handling Concurrency tutorial later in this series.)
Ensuring that Database Connections Are Not Left Open
To make sure that database connections are properly closed and the resources they hold freed up, you should see to it that the context instance is disposed. That is why you will find a Dispose method at the end of the StudentController class in StudentController.cs, as shown in the following example:
protected override void Dispose(bool disposing){    db.Dispose();    base.Dispose(disposing);}
The base Controller class already implements the IDisposable interface, so this code simply adds an override to the Dispose(bool) method to explicitly dispose the context instance.
You now have a complete set of pages that perform simple CRUD operations for Student entities. In the next tutorial you'll expand the functionality of the Index page by adding sorting and paging.
Links to other Entity Framework resources can be found at the end of the last tutorial in this series.