I've inherited a code base written in ASP.Net MVC 4. Every post method takes a FormCollection. Aside from annoyance of having to access the values through quoted strings, it also leads to drawbacks such as not being able to use things like ModelState.IsValid, or [AllowHtml] attributes on my ViewModel properties. They actually did create ViewModel classes for each of their views, (though they are pretty much just direct wrappers around the actual Entity Framework Model classes), but they are only used for the GET methods.
Is there anything I'm missing about FormCollection that gives a reason why this may have actually been a good idea? It seems to only have drawbacks. I'd like to go through and "fix" it by using ViewModels instead. This would take a good bit of work because the ViewModels have properties that are interfaces and not concrete classes, which means either writing a custom binder or changing the ViewModels.
But perhaps there's something I'm missing where it makes sense to use FormCollection?
Is there any good reason to use FormCollection instead of ViewModel?
No. I have following issues.
Issue - 1
In case FormCollection is being used...It will be mandatory to Type Cast the Primitive Type Values un-necessarily because while getting the entry of specific Index of the System.Collections.Specialized.NameValueCollection, value being returned is of type String. This situation will not come in case of Strongly Typed View-Models.
Issue - 2
When you submit the form and goes to Post Action Method, and View-Model as Parameter exists in the Action method, you have the provision to send back the Posted Values to you View. Otherwise, write the code again to send back via TempData/ViewData/ViewBag
View-Models are normal classes, created to bind data to-from Views
Issue - 3
We have Data Annotations that can be implemented in View Model or Custom Validations.
ASP.Net MVC simplifies model validatons using Data Annotation. Data Annotations are attributes thyat are applied over properties. We can create custom validation Attribute by inheriting the built-in Validation Attribute class.
Issue - 4
Example you have the following HTML
<input type="text" name="textBox1" value="harsha" customAttr1 = "MyValue" />
Question : How can we access the value of customAttr1 from the above eg from inside the controller
Answer : When a form get posted only the name and value of elements are posted back to the server.
Alternatives : Use a bit of jQuery to get the custom attribute values, and post that along with the form values to action method
Another option is to rather put what you got in your custom attributes in hidden controls
That's the reason, I would always prefer to use View-Models
The only advantage I can think of is if you want to use the automatically generated controller provided when you don't specify a EF model to be strongly typed to. In that case, your Create and Edit actions will use the FormCollection object as it is a reliable, pre-existing artifact of the framework to work with for this purpose. Perhaps the previous developer chose this option while creating his controllers, and stuck with it since Visual Studio must know what it's doing :)
But, in reality, I would never recommend this headstart of a few seconds. It's always better to build out viewmodels, I would recommend looking at the effort to move in that direction if only for maintenance purposes. With model binding and strongly typed views and html helpers, you are much more likely to reduce the number of run-time errors as a result of changing some magic string and not realizing it until your page blows up.
Ok, I see the general consensus here is that it isn't liked. To offer another perspective, I've always liked using the formcollection passed into the controller on POST actions. It offers the use of the TryUpdateModel method from the controller which will map the collection to your strongly typed class. TryUpdateModel also has overloads that allow you to white list the properties of the model that you want to allow to be updated.
if (TryUpdateModel(viewModel, new string[] { "Name" }))
{
//Do something
}
It still allows all the model binding you want, but helps to keep anything other than the "Name" property on my viewmodel from being updated.
You can see more about the TryUpdateModel method here:
http://msdn.microsoft.com/en-us/library/system.web.mvc.controller.tryupdatemodel(v=vs.108).aspx
There are always workarounds for getting away from a FormCollection lol.. you can have hidden fields bound to your view model variables in the form to your heart's content.
Form collections mostly emerge from the laziness of creating a view model but still end up taking time trying to get figure out how to get the values out of it in your controller :P
I think it was simply created in the very beginning of MVC as an alternative to using strongly typed views when having very simple forms - back in the days when everyone used ViewBag :) ... and once hey had it in there they couldn't just take it out as simple as that.
Maybe you can use it if you are absolutely sure your view will never have more than one form input? Probably still a bad idea though..
I cant find any recent articles talking about any advantages of form collections.. while strongly typed views are everywhere.
Yes. Sometimes, it can be useful. Here's an example:
Let's say we have in our db "date_and_time_field".
In Razor View, we want to use two form fields. The first one "Date" (maybe with jQuery UI Datepicker). The second one "Hour".
In the Controller Action, we compose the "date_and_time_field" by means of Request.Form["Date"] and Request.Form["Hour"].
There are other scenarios where it can be useful:
A cross-table (with checkBoxes in Razor view)
The collection Request.Unvalidated().Form (maybe this is not part of your question: I don't wanna be off-topic)
The default model binder will do almost everything you need it to do. I resorted to the FormCollection once - only to later figure out how to bind arrays of elements into a collection on the ViewModel.
Just go ViewModel. Better all around, for every reason enumerated.
With form collection you will be able to get all the values inside the form. There can be situations where you may need to pass some additional values from the form which may not be part of your view model.
Just take an example of passing 10 hidden values from the form. The form collection makes sense.
The only difficulty that you may face is type casting. All form collection items that you get will be string; you may need to type cast based on your requirement.
Also model state validation is another area where you may face a challenge.
You can always add the form collection properties to your method signatures. They will automatically be populated by form values with corresponding keys.
Well with Forms Collection you will find a quick way to get the values of a form. Otherwise you have to create a class that mimics the Form Fields and people are sometime lazy to create custom classes for less important/rarely used Forms.
No there is no extra benefit (in fact limited) of forms collection over a custom class as action parameters and it should be avoided whenever possible.
Responding to the title question: yes.
There are some situations that FormCollection needs to be used. For instance, suppose a ViewModel that has a property that implements the 1 to N relation (in concrete case, a TimesheetViewModel with ICollection<TimesheetEntryViewModel>), and the Controller has to perform a validation between the time entries to not get a time collision between the end time of an entry and the start time of the following entry. To mark a related entry with a validation error, how can be the line index be retrieved?
Well, with the default model binding, the index value is lost in the Controller logic. Fortunately, FormController stores the index you used in the View and a more specific validation can be done.
There are type of SPA apps where you have no idea about your model (there is no ViewModel at all and views are created dynamically (for short ;))), so FormCollection is your only choice where you implement custom post validation having entire page input values...
If your view has a knowledge about the model then, of course, you can use your concrete ViewModel object. That's easy ;)
I have tried to follow this post in order to create a 3 page form wizard that passes data to each page.
He uses the HTML helper serialize, to serialize an object in the view.
#Html.Serialize("wizard", Model)
However this HTML helper isn't available in MVC 5 it seems.
I found another related post to this here where he suggests using the following to serialize the object.
#Html.Hidden("otherComplexData", new Microsoft.Web.Mvc.MvcSerializer().Serialize(complexObject))
But I then get the following error
There is no argument given that corresponds to the required formal parameter 'mode' of 'MvcSerializer.Serialize(object, SerializationMode)'
It seems to want a SerializationMode, however the documented one doesn't. https://msdn.microsoft.com/en-us/library/microsoft.web.mvc.mvcserializer.serialize(v=vs.118).aspx
What direction can I go in now?
Thanks.
Here's the Serialization option you need:
https://github.com/ASP-NET-MVC/ASP.NET-Mvc-3/blob/master/mvc3/src/MvcFutures/Mvc/SerializationMode.cs
Options are Signed or EncryptedAndSigned.
You can try that and see if it will work.
There's multiple ways to encode data that will work for you. You could put the values in a hidden input using Json.Encode for the view, and Json.Decode on the server side.
Let's say I have one view model. It has one required Name property. And I have disabled client-side validation. I have this code in my action method:
if (!ModelState.IsValid)
{
return View(model);
}
So, everything works fine. It will highlight the required field after post. But, I can't understand that which jQuery validaion function do this process? How, jQuery Validation detects that the form has been submitted once?
I want to find that code, because I want to change it slightly. For example, I have my own helpers, which has custom validation logic. For now, my custom helper validation are not showing after invalid Post. And, I want to add my logic to the built-in function, which I CAN NOT FIND ANYWHERE.
Firstly, if you have disabled client side validation, jquery validation has nothing to do with it (you have disabled it!). To briefly explain what happens when you post and return the view.
The DefaultModelBinder initializes a new instance of you model
The DefaultModelBinder then reads the form data (name/value pairs)
and if a property name matches one of the form data values, its
property is set (assuming its valid) and its value is also added to
ModelState. If the value is not valid, the property is not set but
its value is added to ModelState (the attemptedValue) along with
a ModelState error
When you return the view, your #Html.ValidationMessageFor() method
reads the ModelState values and if there is an error associated
with the property, the error message is added to the html generated
by the ValidationMessageFor() method and the relevant class name
(which highlights it) is added
You can inspect the source code for the DefaultModelBinder and ValidationExtensions if you want to see more detail of how these work.
As for "I want to find that code, because I want to change it slightly", then DONT. You have not indicated what you trying to do, or shown any code for your html helper extension method, but html helpers do not (and should not) contain validation logic. They are responsible for generating html based on a property and the validation attributes applied to that property.
If you have custom validation logic for a property, then you create an attribute that inherits from ValidationAttribute (and if you also want client side validation then it also needs to implement IClientValidatable). A good guide for creating your own validation attributes is this article.
Mvc has its own validation that is not server side and works with the data annotations you set on your model. On post it simply goes to the controller then checks the modelstate errors if its valid it runs your code in the function, if not it returns the model with its errors. If you had jquery validation it would never go to the controller in the first place. When server side validation is enabled the validation is done before the form is sent to the controller. Without jquery it is validated at the controller. If not what you're looking for please let me know
I am using ASP.NET MVC 3 and have a need to detect if the form field has been changed on the server side. I know about using tricks with hidden fields, but I was wondering if there is a way to do it by just using the API?
Basically, I have edit screen for my model and one of the fields is an optional id that can be specified. If the field is specified, I have to insure it is unique (no other model has it). So on the edit controller, I want to run the validation but only if that field has been changed.
Please note, I don't need to know previous value vs. new value, just if the field value has changed.
You will have to keep a copy of the old value somewhere, and do the comparison. You may store it in your View Model.
There is indeed no 'dirty' flag - MVC actually is closer to "the way the web works" to reuse that statement. All that is sent over are name value pairs. nothing else. MVC's model binder just matches those names to your object - so in order to truly detect a change you have to either validate against the true data source upon post or compare values passed in on the form - in which case - it is best to hash to avoid forgery.
I have been adding error and business validation to my app, and when I test using a view that was strongly typed to one model, let's say locations, I get the validation summary as well as the validation messages for each field that didn't pass, plus my css highlights the appropriate field as expected...
...when I try this with a view that's strongly typed to a custom view model, let's say I passed it location - so the user can enter a new one, as well as
IEnumerable<Location> locations
, so it will list out all of the existing locations below the new location form. When I do this I get the validation summary, but the message for each field is not displaying, nor is the css applying the * and the highlighting to each incorrect field.
I am using Linq to SQL, so I have added all of my validation as partial classes, so in this case all of the validation comes from the partial class location, which to my understanding will compile with the designer file that linq to sql created and add my business validation. Is this problem happening because I am not passing the view MyApp.Models.Location, and instead passing it MyApp.Models.MyCustomViewModel? if so, what's my best approach so that I can use just one form for create and list?
I was able to get this working - I changed the return value in my POST method to return
return View(new MyCustomViewModel(location) { });
instead of
return RedirectToAction(new MyCustomViewModel(location) { });
hope this helps if anyone stumbles across the same problem I did - which was my own misunderstanding of how View works differently than RedirectToAction.