MVC Basic CRud Functionality

In this tutorial, you'll create the following web pages:

Create a Details Page

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

In Controllers\StudentController.cs, the action method for the Details view uses the Find method to retrieve a single Student entity.

public ActionResult Details(int? id)
{
    if (id == null)
    {
        return new HttpStatusCodeResult(HttpStatusCode.BadRequest);
    }
    Student student = db.Students.Find(id);
    if (student == null)
    {
        return HttpNotFound();
    }
    return View(student);
}

The key value is passed to the method as the id parameter and comes from route data in the Details hyperlink on the Index page.

Route data

Route data is data that the model binder found in a URL segment specified in the routing table. For example, the default route specifies controller, action, and id segments:

 routes.MapRoute(
    name: "Default",
    url: "{controller}/{action}/{id}",
    defaults: new { controller = "Home", action = "Index", id = UrlParameter.Optional }
);

In the following URL, the default route maps Instructor as the controller, Index as the action and 1 as the id; these are route data values.

http://localhost:1230/Instructor/Index/1?courseID=2021

"?courseID=2021" is a query string value. The model binder will also work if you pass the id as a query string value:

http://localhost:1230/Instructor/Index?id=1&CourseID=2021

The URLs are created by ActionLink statements in the Razor view. In the following code, the id parameter matches the default route, so id is added to the route data.

 @Html.ActionLink("Select", "Index", new { id = item.PersonID  })

In the following code, courseID doesn't match a parameter in the default route, so it's added as a query string.

@Html.ActionLink("Select", "Index", new { courseID = item.CourseID }) 

1. Open Views\Student\Details.cshtml. Each field is displayed using a DisplayFor helper, as shown in the following example:

   <dt>
       @Html.DisplayNameFor(model => model.LastName)
   </dt>
   <dd>
       @Html.DisplayFor(model => model.LastName)
   </dd>

2. After the EnrollmentDate field and immediately before the closing </dl> tag, add the highlighted code to display a list of enrollments, as shown in the following example:

           <dt>
               @Html.DisplayNameFor(model => model.EnrollmentDate)
           </dt>
   
           <dd>
               @Html.DisplayFor(model => model.EnrollmentDate)
           </dd>
           <dt>
               @Html.DisplayNameFor(model => model.Enrollments)
           </dt>
           <dd>
               <table class="table">
                   <tr>
                       <th>Course Title</th>
                       <th>Grade</th>
                   </tr>
                   @foreach (var item in Model.Enrollments)
                   {
                       <tr>
                           <td>
                               @Html.DisplayFor(modelItem => item.Course.Title)
                           </td>
                           <td>
                               @Html.DisplayFor(modelItem => item.Grade)
                           </td>
                       </tr>
                   }
               </table>
           </dd>
       </dl>
   </div>
   <p>
       @Html.ActionLink("Edit", "Edit", new { id = Model.ID }) |
       @Html.ActionLink("Back to List", "Index")
   </p>

   If code indentation is wrong after you paste the code, press CTRL-K-D to correct it.

   This code loops through the entities in the Enrollments navigation property. For each Enrollment entity in the property, it displays the course title and the grade. The course title is retrieved from the Course entity that's stored in the Course navigation property of the Enrollments entity. All of this data is retrieved from the database automatically when it's needed. (In other words, you are using lazy loading here. You did not specify eager loading for the Courses navigation property, so the enrollments were not retrieved in the same query that got the students. Instead, the first time you try to access the Enrollments navigation property, a new query is sent to the database to retrieve the data. You can read more about lazy loading and eager loading in the Reading Related Data tutorial later in this series.)

3. Run the page by selecting the Students tab and clicking a Details link for Alexander Carson. (If you press CTRL+F5 while the Details.cshtml file is open, you'll get an HTTP 400 error because Visual Studio tries to run the Details page but it wasn't reached from a link that specifies the student to display. In that case, just remove "Student/Details" from the URL and try again, or close the browser, right-click the project, and click View, and then click View in Browser.)

   You see the list of courses and grades for the selected student:

   

Update the Create Page

1. In Controllers\StudentController.cs, replace the HttpPost Create action method with the following code to add a try-catch block and remove ID from the Bind attribute for the scaffolded method:

   [HttpPost]
   [ValidateAntiForgeryToken]
   public ActionResult Create([Bind(Include = "LastName, FirstMidName, EnrollmentDate")]Student student)
   {
       try
       {
           if (ModelState.IsValid)
           {
               db.Students.Add(student);
               db.SaveChanges();
               return RedirectToAction("Index");
           }
       }
       catch (DataException /* dex */)
       {
           //Log the error (uncomment dex variable name and add a line here to write a log.
           ModelState.AddModelError("", "Unable to save changes. Try again, and if the problem persists see your system administrator.");
       }
       return View(student);
   }

   This code adds the Student entity created by the ASP.NET MVC model binder to the Students entity set and then saves the changes to the database. (Model binder refers to the ASP.NET MVC functionality that makes it easier for you to work with data submitted by a form; a model binder converts posted form values to CLR types and passes them to the action method in parameters. In this case, the model binder instantiates a Student entity for you using property values from the Form collection.)

   You removed ID from the Bind attribute because ID is the primary key value which SQL Server will set automatically when the row is inserted. Input from the user does not set the ID value.

   Security Note: The ValidateAntiForgeryToken attribute helps prevent cross-site request forgery attacks. It requires a corresponding Html.AntiForgeryToken() statement in the view, which you'll see later.

   The Bind attribute is one way to protect against over-posting in create scenarios. For example, suppose the Student entity includes a Secret property that you don't want this web page to set.

      public class Student
      {
         public int ID { get; set; }
         public string LastName { get; set; }
         public string FirstMidName { get; set; }
         public DateTime EnrollmentDate { get; set; }
         public string Secret { get; set; }
   
         public virtual ICollection<Enrollment> Enrollments { get; set; }
      }

   Even if you don't have a Secret field on the web page, a hacker could use a tool such as fiddler, or write some JavaScript, to post a Secret form value. Without the Bind attribute limiting the fields that the model binder uses when it creates a Student instance, the model binder would pick up that Secret form value and use it to create the Student entity instance. Then whatever value the hacker specified for the Secret form field would be updated in your database. The following image shows the fiddler tool adding the Secret field (with the value "OverPost") to the posted form values.

   
   

   The value "OverPost" would then be successfully added to the Secret property of the inserted row, although you never intended that the web page be able to set that property.

   It's a security best practice to use the Include parameter with the Bind attribute to whitelist fields. It's also possible to use the Exclude parameter to blacklist fields you want to exclude. The reason Include is more secure is that when you add a new property to the entity, the new field is not automatically protected by an Exclude list.

   You can prevent overposting in edit scenarios is by reading the entity from the database first and then calling TryUpdateModel, passing in an explicit allowed properties list. That is the method used in these tutorials.

   An alternative way to prevent overposting that is preferrred by many developers is to use view models rather than entity classes with model binding. Include only the properties you want to update in the view model. Once the MVC model binder has finished, copy the view model properties to the entity instance, optionally using a tool such as AutoMapper. Use db.Entry on the entity instance to set its state to Unchanged, and then set Property("PropertyName").IsModified to true on each entity property that is included in the view model. This method works in both edit and create scenarios.

   Other than the Bind attribute, the try-catch block is the only change you've made to the scaffolded code. If an exception that derives from DataException is caught while the changes are being saved, a generic error message is displayed. DataException exceptions are sometimes caused by something external to the application rather than a programming error, so the user is advised to try again. Although not implemented in this sample, a production quality application would log the exception. For more information, see the Log for insight section in Monitoring and Telemetry (Building Real-World Cloud Apps with Azure).

   The code in Views\Student\Create.cshtml is similar to what you saw in Details.cshtml, except that EditorFor and ValidationMessageFor helpers are used for each field instead of DisplayFor. Here is the relevant code:

   <div class="form-group">
       @Html.LabelFor(model => model.LastName, new { @class = "control-label col-md-2" })
       <div class="col-md-10">
           @Html.EditorFor(model => model.LastName)
           @Html.ValidationMessageFor(model => model.LastName)
       </div>
   </div>

   Create.chstml also includes @Html.AntiForgeryToken(), which works with the ValidateAntiForgeryToken attribute in the controller to help prevent cross-site request forgery attacks.

   No changes are required in Create.cshtml. 

2. Run the page by selecting the Students tab and clicking Create New.

3. Enter names and an invalid date and click Create to see the error message.

   

   This is server-side validation that you get by default; in a later tutorial you'll see how to add attributes that will generate code for client-side validation also. The following highlighted code shows the model validation check in the Create method.

   if (ModelState.IsValid)
   {
       db.Students.Add(student);
       db.SaveChanges();
       return RedirectToAction("Index");
   }

4. Change the date to a valid value and click Create to see the new student appear in the Index page.

   

Update the Edit HttpPost Method

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:

[HttpPost, ActionName("Edit")]
[ValidateAntiForgeryToken]
public ActionResult EditPost(int? id)
{
    if (id == null)
    {
        return new HttpStatusCodeResult(HttpStatusCode.BadRequest);
    }
    var studentToUpdate = db.Students.Find(id);
    if (TryUpdateModel(studentToUpdate, "",
       new string[] { "LastName", "FirstMidName", "EnrollmentDate" }))
    {
        try
        {
            db.SaveChanges();

            return RedirectToAction("Index");
        }
        catch (DataException /* dex */)
        {
            //Log the error (uncomment dex variable name and add a line here to write a log.
            ModelState.AddModelError("", "Unable to save changes. Try again, and if the problem persists, see your system administrator.");
        }
    }
    return View(studentToUpdate);
}

These changes implement a security best practice to prevent overposting,  The scaffolder generated a Bind attribute and added the entity created by the model binder to the entity set with a Modified flag. That code is no longer recommended because the Bind attribute clears out any pre-existing data in fields not listed in the Include parameter. In the future, the MVC controller scaffolder will be updated so that it doesn't generate Bind attributes for Edit methods.

The new code reads the existing entity and calls TryUpdateModel to update fields from user input in the posted form data. The Entity Framework's automatic change tracking sets the Modified flag on the entity. When the SaveChanges method is called, the Modified flag causes the Entity Framework to create SQL statements to update the database row. Concurrency conflicts are ignored, and all columns of the database row are updated, including those that the user didn't change. (A later tutorial shows how to handle concurrency conflicts, and if you only want individual fields to be updated in the database, you can set the entity to Unchanged and set individual fields to Modified.)

As a best practice to prevent overposting, the fields that you want to be updateable by the Edit page are whitelisted in the TryUpdateModel parameters. Currently there are no extra fields that you're protecting, but listing the fields that you want the model binder to bind ensures that if you add fields to the data model in the future, they're automatically protected until you explicitly add them here.

As a result of these changes, the method signature of the HttpPost Edit method is the same as the HttpGet edit method; therefore you've renamed the method EditPost. 

Entity States and the Attach and SaveChanges Methods

The database context keeps track of whether entities in memory are in sync with their corresponding rows in the database, and this information determines what happens when you call the SaveChanges method. For example, when you pass a new entity to the Add method, that entity's state is set to Added. Then when you call the SaveChanges method, the database context issues a SQL INSERT command.

An entity may be in one of the following states:

• Added. The entity does not yet exist in the database. The SaveChanges method must issue an INSERT statement.
• Unchanged. Nothing needs to be done with this entity by the SaveChanges method. When you read an entity from the database, the entity starts out with this status.
• Modified. Some or all of the entity's property values have been modified. The SaveChanges method must issue an UPDATE statement.
• Deleted. The entity has been marked for deletion. The SaveChanges method must issue a DELETE statement.
• Detached. The entity isn't being tracked by the database context.

In a desktop application, state changes are typically set automatically. In a desktop type of application, you read an entity and make changes to some of its property values. This causes its entity state to automatically be changed to Modified. Then when you call SaveChanges, the Entity Framework generates a SQL UPDATE statement that updates only the actual properties that you changed.

The disconnected nature of web apps doesn't allow for this continuous sequence. The DbContext that reads an entity is disposed after a page is rendered. When the HttpPost Edit action method is called,  a new request is made and you have a new instance of the DbContext, so you have to manually set the entity state to Modified. Then when you call SaveChanges, the Entity Framework updates all columns of the database row, because the context has no way to know which properties you changed.

If you want the SQL Update statement to update only the fields that the user actually changed, you can save the original values in some way (such as hidden fields) so that they are available when the HttpPost Edit method is called. Then you can create a Student entity using the original values, call the Attach method with that original version of the entity, update the entity's values to the new values, and then call SaveChanges. For more information, see Entity states and SaveChanges and Local Data in the MSDN Data Developer Center.

The HTML and Razor code in Views\Student\Edit.cshtml is similar to what you saw in Create.cshtml, and no changes are required.

Run the page by selecting the Students tab and then clicking an Edit hyperlink.

Change some of the data and click Save. You see the changed data in the Index page.

Updating the 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'll 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'll 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 try again.

1. Replace the HttpGet Delete action method with the following code, which manages error reporting:

   public ActionResult Delete(int? id, bool? saveChangesError=false)
   {
       if (id == null)
       {
           return new HttpStatusCodeResult(HttpStatusCode.BadRequest);
       }
       if (saveChangesError.GetValueOrDefault())
       {
           ViewBag.ErrorMessage = "Delete failed. Try again, and if the problem persists see your system administrator.";
       }
       Student student = db.Students.Find(id);
       if (student == null)
       {
           return HttpNotFound();
       }
       return View(student);
   }

   This code accepts an optional parameter that indicates whether the method was called after a failure to save changes. This parameter is false when the HttpGet Delete method is called without a previous failure. When it is called by the HttpPost Delete method in response to a database update error, the parameter is true and an error message is passed to the view.

2. Replace the HttpPost Delete action method (named DeleteConfirmed) with the following code, which performs the actual delete operation and catches any database update errors.

   [HttpPost]
   [ValidateAntiForgeryToken]
   public ActionResult Delete(int id)
   {
       try
       {
           Student student = db.Students.Find(id);
           db.Students.Remove(student);
           db.SaveChanges();
       }
       catch (DataException/* dex */)
       {
           //Log the error (uncomment dex variable name and add a line here to write a log.
           return RedirectToAction("Delete", new { id = id, saveChangesError = true });
       }
       return RedirectToAction("Index");
   }

   This code retrieves the selected entity, then calls the Remove method to set the entity's status to Deleted. When SaveChanges is called, a SQL DELETE command is generated. You have also changed the action method name from DeleteConfirmed to Delete. The scaffolded code named the HttpPost Delete method DeleteConfirmed to give the HttpPost  method a unique signature. ( The CLR requires overloaded methods to have different method parameters.) Now that the signatures are unique, you can stick with the MVC convention and use the same name for the HttpPost and HttpGet delete methods.

   If improving performance in a high-volume application is a priority, you could avoid an unnecessary SQL query to retrieve the row by replacing the lines of code that call the Find and Remove methods with the following code:

   Student studentToDelete = new Student() { ID = id };
   db.Entry(studentToDelete).State = EntityState.Deleted;

   This code instantiates a Student entity using only the primary key value and then sets the entity state to Deleted. That's all that the Entity Framework needs in order to delete the entity.

   As noted, the HttpGet Delete method doesn't delete the data. Performing a delete operation in response to a GET request (or for that matter, performing any edit operation, create operation, or any other operation that changes data) creates a security risk. For more information, see ASP.NET MVC Tip #46 — Don't use Delete Links because they create Security Holes on Stephen Walther's blog.

3. In Views\Student\Delete.cshtml, add an error message between the h2 heading and the h3 heading, as shown in the following example:

   <h2>Delete</h2>
   <p class="error">@ViewBag.ErrorMessage</p>
   <h3>Are you sure you want to delete this?</h3>

   Run the page by selecting the Students tab and clicking a Delete hyperlink:

   

4. 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 concurrency tutorial.)

Closing Database Connections

To close database connections and free up the resources they hold as soon as possible, dispose the context instance when you are done with it. That is why the scaffolded code provides 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.

Handling Transactions

By default the Entity Framework implicitly implements transactions. In scenarios where you make changes to multiple rows or tables and then call SaveChanges, the Entity Framework automatically makes sure that either all of your changes succeed or all fail. If some changes are done first and then an error happens, those changes are automatically rolled back. For scenarios where you need more control -- for example, if you want to include operations done outside of Entity Framework in a transaction -- see Working with Transactions on MSDN.

Summary

You now have a complete set of pages that perform simple CRUD operations for Student entities. You used MVC helpers to generate UI elements for data fields. For more information about MVC helpers, see Rendering a Form Using HTML Helpers (the page is for MVC 3 but is still relevant for MVC 5).

In the next tutorial you'll expand the functionality of the Index page by adding sorting and paging.