Form validation and passing model back to view - asp.net-mvc

Question

I have setup validation in my viewmodel such as the below:
[Required(ErrorMessage = "This field is required.")]
[StringLength(25, MinimumLength = 6)]
[DataType(DataType.Password)]
public string Password { get; set; }
[DataType(DataType.Password)]
[System.Web.Mvc.CompareAttribute("Password", ErrorMessage = "Password must be the same")]
public string ConfirmPassword { get; set; }
When I submit the form I check if ModelState.IsValid and if its not valid then return the original view but by doing this I lose the original data I had in my model.
[HttpPost]
public ActionResult Form(MemberAddViewModel viewModel, string returnUrl)
{
if (ModelState.IsValid)
{
...
}
return View("Form", viewModel);
}
I would have expected the viewModel to be passed back to the original View but it seems only model items populated in the view are. What is best practice for this? Hidden fields / Session data?

Answer 1

To understand why you have to rebuild parts of the model, you need to think about what's going on under the hood when the model binder is passed the data from your view. A SelectList is a perfect example of this.
Let's say you have a view model as follows:
public class EmployeesViewModel
{
public int EmployeeId { get; set; }
public SelectList Employees { get; set; }
// other properties
}
Here, EmployeeId represents the Id of the selected Employee from the SelectList. So let's assume you have a controller action like this, which populates the SelectList and passes the data to the view:
public ActionResult Index()
{
var model = new EmployeesViewModel();
model.Employees = new SelectList(/* populate the list */);
return View(model);
}
Now let's assume a user comes along, navigates to this view, chooses an employee from the list, and POSTs the data back to the server. When this happens, the only thing that gets submitted from that form, is the Id of the employee. There's no need for HTTP to transport all of the other options from the SelectList to the server, because a) they haven't been selected and b) you have that data on the server already.

Answer 2

Not sure for I undertandood your question... This is how i populate lost fields: i divide model population into 2 parts: for editable props (elements that being posted back to server) and non-editable (that are getting lost on postback)
// View model
public class MyModel
{
public MyModel() { }
public MyModel(Entity e, ContractTypes[] ct)
{
// populate model properties from entity
ContractTypeId = e.ContractTypeId;
// and call magic method that'll do the rest my model needs
PopulateNonEditableFields(ct);
}
public void PopulateNonEditableFields(
Dictionary<int, string> ContractTypes [] ct)
{
// populate dictionaries for DDLs
ContractTypesList = new SelectList(..., ct,...);
}
// model properties
public ContractTypeId { get; set; }
public SelectList ContractTypesList { get; set; }
}
// controller action
[HttpPost]
public ActionResult Action(MemberAddViewModel viewModel)
{
if (ModelState.IsValid)
{
...
}
// user input stays as-is but need to populate dictionaries and evrithing
// that was lost on postback
viewModel.PopulateNonEditableFields(context.ContractTypes.GetAll());
return View("Form", viewModel);
}

Answer 3

As I understand it, you have data in the view model which is not posted back through the form. There can be many valid reasons why this is the case.
Another alternative is to always create the view model manually, and then update it using the posted back values through a call to TryUpdateModel. Calling TryUpdateModel will do two things: set the model's public properties using the controller's value provider, then runs validation checks on the model.
[HttpPost]
public ActionResult Action(int id, string returnUrl)
{
MemberAddViewModel viewModel = CreateViewModel(id); // Populates with
// intial values
if(TryUpdateModel(viewModel))
{
// If we got here, it passed validation
// So we continue with the commit action
// ...
}
else // It failed validation
{
return View("Form", viewModel);
}
}