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 ;)
please help me with this:
I have model with property of Enum type. This property is required, but on View initial value must be empty. Will show watermark..
If I understand correct than model will have not nullable property. But viewModel will have nullable. Is this right way?
Looks like in this case:
need create viewModel
viewModel and model will have different data sometimes
viewModel need logic for set and get correct values to/from model or get null in first time and leave as is model
viewModel need one more validation rule
Other way is:
Model will have nullable property and required validation:
can bind to model
model does not "correspond to reality"
we need always expose data from nullable enum and forgot about null? or handle it (convert to DTO or something like this)
nullable property is only UI requirement and can be changed in future
Having the (domain) model use a non-nullable enum suggest that the value MUST have a valid value for use case and / or data integrity.
Having the requirement on the UI to allow a non-valid value (initial or otherwise) will result in 1 of 2 things:
When the View posts a null value (via a dedicated ViewModel) and checks are performed to ensure its not null (I assume that the user selects the "Please Select" value from a select box) the server will respond with an error message saying something like "Please select a valid value blah blah".
Or, the server will allow the null from the View Model and default the domain model property to some predetermined sensible value.
In my experience this kind of setup leads to a frustrating user experience because having the UI present an invalid selection option that only feedsback upon submit (or using some frontend / client validation) is rather annoying (especially on a long form, Etc).
Perhaps you would be better rethinking that requirement so the UI only presents valid enum values for selection and optionally make the default one a sensible value.
Having said that if you really want the UI to not have a valid initial value then make a view model with a nullable version and validate on the server as you suggest. As far as changing the (domain) model with a nullable, you already seem concerned that it doesn't "correspond to reality" which is a good impulse considering you already mentioned that this is a UI only requirement, thus place the detail in that layer and keep your domain modals as close to reality as is needed to satisfy your use cases and / or business requirements.
In C# we can use DebuggerBrowsable attribute with DebuggerBrowsableState.Never option to hide it from debugger.
This helps me for my View Model classes which have a lot of properties which causes me to scroll for what I need to check. I always see items in the debugger like:
...
Id = 5
id = 5
...
And I don't want to see properties in most cases. Is it possible to automatically set state for properties as DebuggerBrowsableState.Never?
One method to automate this is to add a code snippet for generating the the DebuggerBrowsable attribute with property generation.
Yet if possible I would like to do it the other way (without too much performance loss somehow).
Automatically set all property states to DebuggerBrowsableState.Never - keep fields as they are. For exceptional cases I would specifically change state to DebuggerBrowsableState.Collapsed.
This would result in much cleaner code.
Any suggestions?
I get the whole idea of the fact that ViewBag is dynamic, and that new properties can be "added" to the ViewBag by using and initializing those properties for the first time without compilation issues.
However, I'm looking for a way to get all the properties that have been initialized on the ViewBag. ViewBag (according to MSDN) is declared as an object in the ControllerBase class, and there appears to be no method on how to get the properties of the object dynamically.
Perhaps reflection, but then again, I wouldn't know where to start.
The reason I want to do this is that I am converting a huge website from MVC to MVVM architecture. For that purpose I want to make tests that make sure that the ViewBag is not used anywhere, and that no properties on the ViewBag have been set after a controller's method has been run.
Where do I start?
Check ViewData - it contains everything in ViewBag in a form of a Dictionary.
I am attempting to put auditing in my application based on properties that have changed. I was attempting to follow this example however the Entity Framework only keeps track of scalar and complex type properties, not navigation properties as described here.
So in a different approach, I figured if I could find which FormCollection values had been changed then I could use those. Is there anyway to easily determine if a value has changed between being loaded in a form and being submitted?
I guess you want to check this in the controller.
The easiest way should be to reload the data (the same you did to send the data to the view) and compare this with the values you got. Aside from this you could use a lot of Hidden-Fields in your form and check those (initialise to the same values as your inputs) - but I don't really like this.