I have an MVC web app that I am porting from .net framework to .net core. Part of the app has a view model with a number of properties, one of which is of type "object".
In .net framework when I post this view model data the controller receives it with no problem. However, in .net CORE all the properties contain their expected values except the one that is object.
If I change the type to string most work fine (but it means if the value is ever an int (for example) it will be the string representation of an int).
Does anybody know how I can get values for view model properties that are object types to work or if this just isn't supported in Core?
I really dont feel like this would work.
Model binding uses reflection to map request data, route data and query strings into the parameter of your action method.
Request data is essentially all string data and so the defaultmodelbinder is able to map the request into any simple value type such as string, integer or bool by checking to see if the string can be converted into an int or bool without failure.
Complex types, like a Customer object, which contain simple value types (like string and int) can then also be mapped using this same methodology. An example of a complex type like this is shown here:
public class Customer
{
public string Name {get;set}
public bool IsLongTermCustomer {get;set;}
}
Because the defaultmodelbinder uses reflection, I really dont see how it would know how to map to a property of type "object" which could be anything.
An object could be a Customer, an int ,a string etc and so i feel like this would fail no matter what version of MVC you use becaue i dont see how the defaultmodelbinder would know if the data can be ported into the object data type. Here is a detailed article on model binding specifically in MVC core and how it works.
https://learn.microsoft.com/en-us/aspnet/core/mvc/models/model-binding?view=aspnetcore-2.2
As an extra check, I would also look to see if you are running the mvc default model binder in your older application and that someone is not using a custom model binder which may be allowing this binding to occur in your legacy application.
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'm using the Google ReCaptcha element on a form. ReCaptcha adds a parameter to the POSTed parameters named g-recaptcha-response, which the controller needs to retrieve. Since this parameter name is not a legal C# variable name, it seems that the only way to retrieve its value is via Request.Params["g-recaptcha-response"] (rather than via model binding to a property in the view model).
Now, my problem is that elsewhere in the POSTed parameters I have a couple of form fields that may contain HTML markup. I have annotated the associated properties in my view model with [AllowHtml], which prevents the model binder from throwing an HttpRequestValidationException if the user enters HTML markup into the form. But [AllowHtml] apparently only works in the model binder. If the user has entered HTML markup into the form, then I get an HttpRequestValidationException when I reference Request.Params to fetch the ReCaptcha response.
As near as I can tell, the only way I can fetch the ReCaptcha response while still allowing HTML markup in selected POST parameters is to use go through the pain of writing a custom model binder (e.g. https://stackoverflow.com/a/4316327/1637105) to allow me to bind a property in my view model to an alias (in my case, a property name that is not a valid C# variable name).
The point of this question is just to confirm that I really do need to go to the pain of implementing a custom model binder.
Any suggestions or alternate solutions are more than welcome!
EDIT:
It occurs to me that another solution would be to figure out a way to fetch the value from the POSTed parameters without triggering (or while handling) the HttpRequestValidationException.
You can use the Unvalidated property of Request to access values without triggering Request Validation. for example
var captcha = Request.Unvalidated.Form["g-recaptcha-response"];
I'm sending the following model to my view.
return View(new { url = returnUrl });
In the view, I'm don't want to specify any particular class for my object (since I wish to keep it flexible and elastic for now). So, the #Model is the apparently an object and as such, it's got no url property. Hence the error.
Additional information: 'object' does not contain a definition for 'url'
Now, I do know that the thing inside the object has url property. I have assigned it to it and I also see it when watching the variable as the exception's been thrown.
My question is how to access the field. Is my only option declaring a class and type the model using #model Something? I can't use as keyword to type it to var...
In "plain" C# we can do something like this.
var some = new {thing = "poof"};
string output = some.thing;
How do I do the equivalent of it in CSHTML file under Razor?
Strongly-typed view models are the way to go. Create a type that suits the needs of the view and treat reusability/duplication as a secondary concern.
However, let me explain why your attempt did not work.
It is legal to pass an anonymous type--even between assemblies[1]--as long as it is cast to object. In fact, the MVC framework assemblies consume anonymous types in many helper methods. Those anonymous types are then evaluated using reflection (optimized by caching).
1: I believe there are some caveats to this, and it certainly isn't good practice in most cases.
A view is compiled into a class so that it can be executed. Part of the class's contract is the type of model it expects, as indicated by #model in your view markup.
This presents a problem with anonymous types, as you cannot indicate their type in your view and type object doesn't expose the properties you set when declaring the type. Thus, you end up with a view that only knows that its model is an object.
Again, use strongly-typed models, or the ViewBag if you truly only need one or two values.
However, to prove that the anonymous type can be passed to the view, look at this (horrible) example:
Controller
return View( new { property1 = "hello world"} );
View
#model object
#{
var rvd = new RouteValueDictionary( Model );
}
#rvd["property1"]
We passed an anonymous type to the view as an object, and then read the object's properties using RouteValueDictionary.
You can use ViewData and ViewBag to send objects to the view page, in your case you can write in the controller something like this:
ViewData["url"] = url ; //Or whatever
return View();
Now in the view you can simply use your object example:<div>#ViewData["url"]</div>
But mainly in MVC it is more recommended to use strongly typed View Models
You may want to look into using the dynamic type in C#. See https://msdn.microsoft.com/en-us/library/dd264736.aspx for details.
While the standard would be to use a strongly-typed view model, there are some scenarios where you might want to use dynamic as your model type (or as a property of your strongly-typed view model), such as in a CMS where the properties are built dynamically by the CMS Provider.
Example view:
#model dynamic
<p>
Url: #Model.url
</p>
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.