I've just spent quite some time tracking down a defect where a JSON model parameter to an MVC action method was always null. The cause is different to those raised in other related questions, hence the new question.
My action looks something like this:
[HttpPost]
public ActionResult SendDeviceDetails(DeviceModel model)
{
model.DoStuffHere();
}
And my model class looks like this:
public class DeviceModel
{
public string Manufacturer { get; set; } // e.g. "Asus"
public string Model { get; set; } // e.g. "Nexus 7"
// etc.
}
The model object is posted as a JSON string from a mobile device.
However, the controller action always throws a NullReferenceException because model is always null. I spent some time verifying that the data is sent correctly from the client end before starting to strip down my model class and build it back up again. (In reality it has many more properties than I've shown here.)
What I found was this: if the model class has a property called Model, you will always get a null reference passed to your action method. If I rename that property (e.g. to ModelName), everything works perfectly.
Is this really as insane as it seems to me? Am I missing some good reason for this restriction or is it just a flat-out defect in MVC? And is there any way to work around it if I really want to have a property named Model?
The problem was caused by the property name (DeviceModel.Model) matching the action parameter name (model). Renaming either of them solves the problem. So actually it's OK to have a Model property in your model class, as long as the parameter you pass it as is not called model.
Crazy!
Related
I've been working with MVC for quite a while. I made a form to submit my data using an entity model and as per requirement had to add tags too so I updated the view and the actionmethod to use a viewmodel instead. Like this:
[HttpPost]
[ValidateAntiForgeryToken]
public ActionResult Create(PostwTagsVM post)
{
}
Surprisingly, the model was null.I couldn't find out why but then decided to rename the object as below:
[HttpPost]
[ValidateAntiForgeryToken]
public ActionResult Create(PostwTagsVM model)
{
}
Surprisingly, I get the data in the model now.
I know I can work like this but if i really needed to name by model object something else other than 'model'. Whats happening here?
It does not have to be named model.
If in the first case, your model is null, its because your PostwTagsVM model contains a property named post.
The parameter can be named whatever you want, except that it cannot be the same name as one of the properties in your model.
The reason is that your form would be sending back a name/value pair that is (say) post=someValue. The DefaultModelBinder looks for a matching name, sets the value of the property named Post to someValue, but then also finds a parameter named post and tries to set that to someValue, which fails (because you cannot do PostwTagsVM post = "someValue";), and the model becomes null.
I cannot figure out why my model is not being populated. All the data posted is in the Request.Form, but the model is actually turning out to be null.
According to this answer on model with collection not populating on postback
In other words, (...) If any required fields are missing, or if the values are
submitted in such a way that they cannot be converted to the type of a required field, then the entire object will be left null
I have changed several value types, but I cannot get it to work.
Here is my model:
public class AddModel
{
//Get properties
public Vehicle vehicle;
//Post properties
[Required(ErrorMessage = "Please enter a start date")]
public DateTime StartDate;
public int? StatusCode;
public SelectList StatusCodes()
{
...
}
}
Can you think of why it's not being populated?
Making AddModel members Properties - adding get; set; to fields should solve your problem
As per #archil's response, you should make your public variables properties. Although this may work with types you will run into problems as soon as you add complexity. Certainly for classes but possible for nullable types too.
The model binders use reflection to parse the form fields into the model, and reflection works differently on properties to public variables - in the case of these models, the differences will likely be causing the failure here.
Hope that helps - and hat tip to #archil for answering this (probably) correctly, much sooner than me!
I'm testing some concepts in ASP.NET MVC multisteps (Style Wizards) with a small application which allow me to records organizations in a database.
To make things easier, I've a class OrganizationFormModelView that contains an object of class Organization and a property called ParentOrgList of SelectList type. The only purpose of the selectList property is to be used by a DropDownList.
I've also serialize OrganizationFormModelView to get the multisteps Wizard effect.
In my first view (or first step), I use a dropdownlist helper to assign a value to one of the the Organization's property called ParentOrganization, which draws data from the ParentOrgList.
...
<% = Html.DropDownList("Organization.ParentOrganization", Model.ParentOrgList)%>
...
The first time the page loads, I'm able to make a choice. And, my choice is reflected in my object Model all along the wizard' steps(see Visual studio in debugging mode).
But, when any time I'm redirected back to the first view (first step), I get the following error message:
"The ViewData item with the key 'Organization.ParentOrganization' is of type 'System.String' but needs to be of type 'IEnumerable'."
Thanks for helping
After considering carefully my code, I understand now what's going on. OrganizationFormModelView is the class that is being serialized, and here's its definition.
[Serializable]
public class OrganizationFormViewModel
{
public Organization Organization { get; set; }
[NonSerialized]
public SelectList ParentOrgList = null;
public OrganizationFormViewModel(Organization organization, SelectList cList)
{
Organization = organization ?? new Organization();
ParentOrgList = pList;
}
}
From that, I've concluded that, After each serialization process, ParentOrgList is set to null, so I need to find a way of re-assigning value to it. So, below is what I did:
public ActionResult CreateOrganization(string nextButton)
{
//Omitted for brievety
if (formViewModel.ParentOrgList == null)
formViewModel.ParentOrgList = repository.CommuneList;
//Omitted for brievety
}
I also, modified the View so that, even if the value of the ParentOrgList is continuously re-assigned, but the DropDownList keeps the user's choice. So, I choose an Helper overload with default value.
...
<% = Html.DropDownList("Organization.ParentOrganization", Model.ParentOrgList,
Model.Organization.ParentOrganization)%>
...
Now, everything is working perfectly.
However, If someone knows how to proceed differently with the Serialization business, it'd be helpful to share.
Thanks
Suppose I want to allow to select our entity (from a dropdown, etc) on a page, let's say Product. As a result I may receive this:
public ActionResult SelectedAction(Guid productId)
{
}
But, I want to use model binders power, so instead I write model binder to get my product from repository and instead use
public ActionResult SelectedAction(Product product)
{
if (ModelState.IsValid) {} else {}
}
My model binder will set model state to false if product is invalid.
Now, there're problems with this approach:
It's not always easy to use strongly-typed methods like Html.ActionLink(c => c.SelectedAction(id)) since we need to pass Product, not id.
It's not good to use entities as controller parameters, anyway.
If model state is invalid, and I want to redirect back and show error, I can't preserve selected product! Because bound product is not set and my id is not there. I'd like to do RedirectToAction(c => c.Redisplay(product)) but of course this is not possible.
Now, seems like I'm back to use "Guid productId" as parameter... However, there's one solution that I'd like to present and discuss.
public class EntityViewModel<T> where T : BaseEntity
{
public EntityViewModel(Guid id)
{
this.Id = id;
}
public static implicit operator EntityViewModel<T>(T entity)
{
return new EntityViewModel<T>(entity.Id);
}
public override string ToString()
{
return Id.ToString();
}
public Guid Id { get; set; }
public T Instance { get; set; }
}
Now, if I use
public ActionResult SelectedAction(EntityViewModel<Product> product)
{
if (ModelState.IsValid) {} else {}
}
all the problems are solved:
I can pass EntityViewModel with only Id set if I have only Id.
I don't use entity as parameter. Moreover, I
can use EntityViewModel as property inside another ViewModel.
I can pass EntityViewModel back to RedirectToController and it will keep its Id value, which will be
redisplayed to user along with the validation messages (thanks to MVCContrib and ModelStateToTempData / PassParametersDuringRedirect).
The model binder will get Instance from the repository and will set model state errors like "Not found in database" and so on. And I can use things like ActionLink(c => c.Action(Model.MyProductViewModelProperty)).
The question is, are there any drawbacks here? I can't see anything bad but I'm still new to MVC and may miss some important things. Maybe there're better and approved ways? Maybe this is why everybody uses entity IDs as input parameters and properties?
Overall that looks like a good appoach to me...
As an alternative, you could use POCO for your viewmodel then I think all 3 problems would be solved automatically. Have you seen the Automapper project that allows an Entity to DTO approach? This would give you more flexibility by separating you ViewModel from your EntityModel, but really depends on the complexity of you application you are building.
MVC's ViewDataExtensions might also be useful instead of creating custom containers to hold various viewmodel objects as you mention in number 2.
MVCContrib's ModelStateToTempData should work for any serializable object (must be serializable for any out of process sessionstate providers eg. SQL, Velocity etc.), so you could use that even without wrapping your entity classes couldn't you?
I am just at the brink of going "Ah HA!" when it comes to coding Domain Driven Design. The question is How and where to define a MVC ActionMethod parameter class that involves an entity, a value object, and a few extra fields? The entity and value object classes are defined in my repository.
Do I:
Create a custom class in the repository implementing the other classes (to get the properties of all in a single class), and add a few more properties for the extra fields?
Create the entity / value object poco classes in a repository and create a composite class referencing these objects in my controller class, then use this as the ActionMethod parameter type?
Something else?
The request form simply collects a few Customer class fields, a mailing address, and a few form specifics like how they found us. The content is not important, only that it holds information from multiple pocos.
I know MVC will match the fields of the posted form to the properties of the Poco in the ActionMethod parameter like the following:
[AcceptVerbs(HttpVerbs.Get)]
public ActionResult RequestCatalog()
[AcceptVerbs(HttpVerbs.Post)]
public ActionResult RequestCatalog(Customer customer)
So customer.firstName is bound to firstName in the posted form automatically.
I have the following:
[AcceptVerbs(HttpVerbs.Post)]
public ActionResult RequestCatalog(string fname, string lname, string address,
string city, string state, string zip, string phone, string howTheyFoundUs)
But want to have something like:
[AcceptVerbs(HttpVerbs.Post)]
public ActionResult RequestCatalog( RequestForm requestForm)
Any thoughts?
First of all you must realize that the Domain Model must be defined independently of the run-time framework (MVC). Imagine that in the future you must be able to expose the Domain Model as a WCF service, or a WPF/Silverlight rich client, or a batch job, or something else...
This means that all modeling must be unconstrained by technical details. How you store your data and the specifics of how the Domain Objects are consumed must be (largely) ignored at this stage of the design process.
You must always ask yourself: Does this class make sense as a part of the pure domain?
In you specific case, most of your input sounds like it belongs on a Customer class with a structure similar to this
public class Customer
{
public string FirstName { get; set; }
public string LastName { get; set; }
public string Phone { get; set; }
public Address Address { get; set; }
}
and the Address class is defined similarly.
Now the question you need to ask yourself (or any Domain Experts you are working with): Does the howTheyFoundUs belong on the Customer class? Or is is a Domain concept in its own right? Or is it really just an application-specific piece of data that doesn't have anything to do with the Domain Model at all? The answer to such questions will guide how you end up modeling your input.
As an aside, the Phone property above looks dubious to me, but is a pretty good example on how implementation details (in this case ASP.NET MVC) may leak into the model.
A phone number should really be a Value Object in its own right (see this blog post for a related treatment of this subject), but the default ModelBinder of ASP.NET requres a primite type. A better option would be to implement a custom ModelBinder that can parse a phone number into a proper object.
There is nothing to stop you using Request.Form in your action method.
E.g.
[AcceptVerbs(HttpVerbs.Post)]
public ActionResult RequestCatalog(Customer customer)
{
var howTheyFoundUs = Request.Form["howTheyFoundUs"];
// ...
}