ModelState, TempData, ViewModel issue - new viewmodel value automatically gets overridden - asp.net-mvc

Question

This problem is best explained with code:
public async Task<ActionResult> Index()
{
if (TempData.ContainsKey("ModelState"))
{
ModelState.Merge((ModelStateDictionary)TempData["ModelState"]);
var viewModelA= new ViewModelA
{
FirstName = "John"
}
return View(viewModelA);
}
}
[HttpPost]
public async Task<ActionResult> IndexPostActionMethod(ViewModelB model)
{
if (ModelState.IsValid)
{
var result = await DoSomeStuff();
if (result.Succeeded)
{
DoSomeOtherStuff();
}
else
{
AddErrors(result);
}
}
TempData["ModelState"] = ModelState;
return RedirectToAction("Index");
}
The problem here is that when I post to IndexPostActionMethod and DoSomeStuff() returns false for some reason, the form/page is rediplayed by redirecting to Index and saving the ModelState in TempData during that redirect and then once the form/page is redisplayed, the value from ModelState (or where does it get the value from?) gets set as the value for a #Html.TextBoxFor(m => m.FirstName) and not the value that I set here: var viewModelA = new ViewModelA { FirstName = "John"}. Why can't I override the value and where does it get it from? It's like the framework is automatically updating viewModelA with whatever is in the ModelStatDictionary eventhough I create a new ViewModelA with a new value.

Answer 1

Don't do this. The way you should be handling a GET-POST-Redirect cycle is:
public async Task<ActionResult> Index()
{
var model = new ViewModel
{
FirstName = "John"
}
return View(model);
}
[HttpPost]
public async Task<ActionResult> Index(ViewModel model)
{
if (ModelState.IsValid)
{
var result = await DoSomeStuff();
if (result.Succeeded)
{
DoSomeOtherStuff();
}
else
{
AddErrors(result);
}
}
return View(model);
}
ModelState rules all. Regardless of what you pass to the view explicitly, if the value is present in ModelState it will take precedence. Using TempData to pass the ModelState to another view is such a code smell, "smell" doesn't even really cover it.
If your only goal is to use one view model for the GET action and another, different, view model for the POST action, then you should really just stop and ask yourself seriously why you need to do that. In all the years I've worked with MVC, I've never had to do that, never needed to do that, and I can't imagine a scenario where I would need to do that.
UPDATE
So, based on the scenario you laid out in the comments, I would design it like this:
public class IndexViewModel
{
public SomeTabViewModel SomeTab { get; set; }
public AnotherTabViewModel AnotherTab { get; set; }
public FooTabViewModle FooTab { get; set; }
}
public class SomeTabViewModel
{
[Required]
public string SomeRequiredField { get; set; }
}
public class AnotherTabViewModel
{
[Required]
public string AnotherRequiredField { get; set; }
}
public class FooTabViewModel
{
[Required]
public string FooRequiredField { get; set; }
}
Basically, you break up the individual POST pieces into view models that altogether make up one big view model. Then, model validation only follows actual internal view model instances of the container. So, for example if you postback to FooTab.FooRequiredField only, then only FooTab is instantiated. The other two view model properties will remain null and will not be validated internally (i.e. the fact that they have required fields doesn't matter).