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7 Data Entry in ASP.NET MVC
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Note Regarding repeated code, you might be surprised to see that I mentioned the OAOO principle instead of the most popular Don t Repeat Yourself (DRY). Both come from the world of Extreme Programming (XP) and are two of the 12 common XP practices. What s the difference between the two DRY refers to storing data in one place and with one unambiguous representation. OAOO refers to implementing a given behavior once and only once. It turns out that OAOO is more difficult to achieve and often only a driving vector. OAOO is the ideal goal of any refactoring attempt, and it s similar to normalization, as you might know it from the theory of relational databases.
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The Post-Redirect-Get Pattern
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The purpose of the pattern is self-explanatory. It essentially teaches you a way to reuse the same code to serve both GET and POST requests for the same resource or, as in ASP.NET MVC, for the same action. You start by fully writing your code for the GET scenario, as shown here:
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[ActionName("Edit"), AcceptVerbs(HttpVerbs.Get)] public ActionResult EditViaGet(string id) { var data = new CustomerViewModel(); data.Customers = ...; data.CustomerBeingEdited = ...; return View("Edit", data); }
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Next, you refactor the POST method so that it first does its own things (if any) and then redirects to the GET action. Here s the new version of the POST action:
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[ActionName("Edit"), AcceptVerbs(HttpVerbs.Post)] public ActionResult EditViaPost(string listCustomers) { string customerId = listCustomers; return RedirectToAction("Edit", new RouteValueDictionary(new { id = customerId })); }
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If the user types the URL directly in the address bar, the selected customer is edited and the view and URL are in full sync. If the user picks up a customer from the drop-down list and then posts to edit it, a redirection occurs and the Edit action receives an HTTP GET request. The PRG pattern is helpful in keeping the URL and view in sync, but it s even more useful for keeping update code and the view neatly separated, as you ll see in a moment. In this regard, the PRG pattern saves you from the nasty F5 problem (page refresh requests from
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Part III Programming Features
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the client). If the user refreshes the currently displayed page (that is, he hits the F5 button) after an update, no POST is repeated and no message pops up to announce your intent to resend data. In fact, when F5 is pressed the browser repeats its latest action. When using PRG, the browser repeats a GET action, not a POST action, when from the user s perspective, their last action was a POST.
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Saving Data
After the input form is displayed, the user enters any valid data and then presses the button that posts the current content of the form. Here s a typical form that posts changes:
<% Html.BeginForm("Update", "Customer", new {id = ViewData.Model.CustomerID}); %> ... <% Html.EndForm() %>
The resulting URL for a customer ID of ALFKI is the following:
http://yourserver/customer/update/alfki
The content of the form is uploaded with the request and packaged into the Request object. However, as you saw in 6 a controller s method has various options for binding posted data to its own parameters.
Binding Input Data
Let s examine a few possible signatures for the Update action that is ultimately responsible for saving changes on the edited customer:
public ActionResult Update(string string string string string string id, companyName, contactName, country, city, address);
The id parameter is resolved through route data, whereas all the other parameters are resolved in the presence of input elements with matching IDs. This signature also allows you to pass in fixed data for testing purposes. Another signature is the following:
public ActionResult Update(string id);
In this case, the remaining input data is resolved using the TryUpdateModel on the controller class that uses model binders internally.
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