MVC5 Binding issue - asp.net-mvc-5

Question

I'm trying to set up some basic navigation on a web site I'm rewriting and I've run into a brick wall and don't see why this is not working. I'm doing something similar in a half dozen other places but it just ain't working.
What I want to do is if my article has a next and or previous ID I want to show a navigation bar with appropriate forward/reverse navigation arrows or whatever to allow user to navigate pages.
The ViewModel
public class NavViewModel
{
public int NextID { get; set; }
public int PreviousID { get; set; }
public string NextString { get; set; }
public string PreviousString { get; set; }
public bool SelectedMode { get; set; }
public NavViewModel() { }
}
The View
#Html.HiddenFor(model => model.NavigationViewModel.PreviousID)
#Html.HiddenFor(model => model.NavigationViewModel.NextID)
<div class="post-nav">
#if (#Model.NavigationViewModel.PreviousString != null)
{
using (Html.BeginForm("SinglePost", "Article", FormMethod.Post, new { #nvm = Model.NavigationViewModel }))
{
<input type="submit" class="btn btn-default" value="#Model.NavigationViewModel.PreviousString" />
}
}
#if (#Model.NavigationViewModel.NextString != null)
{
using (Html.BeginForm("SinglePost", "Article", FormMethod.Post, new { nvm = #Model.NavigationViewModel }))
{
<input type="submit" class="btn btn-default" value="#Model.NavigationViewModel.NextString" />
}
}
</div>
and the Controller
[HttpPost]
public ActionResult SinglePost(NavViewModel nvm)
{
return RedirectToAction("SinglePost", "Article", new { postID = nvm.PreviousID });
}
I've tried passing back the bool, the IDs, the ViewModel and they all come null or containing null values.
I had this code in a PartialView and because it wasn't working I moved it up a level into the calling view and it has the same result.

Answer 1

You have stated you want to navigate to the next and previous items so using forms and inputs and submitting to a POST method is not appropriate. Instead use a link to navigate to a GET method, passing the ID of the previous or next item.
#if (#Model.NavigationViewModel.PreviousString != null)
{
#Html.ActionLink(Model.NavigationViewModel.PreviousString, "SinglePost", "Article", new { postID = Model.NavigationViewModel.PreviousID }, null)
}
#if (#Model.NavigationViewModel.NextString != null)
{
#Html.ActionLink(Model.NavigationViewModel.NextString , "SinglePost", "Article", new { postID = Model.NavigationViewModel.NextID }, null)
}
The reason your code does not work will be obvious when you inspect the html generated for the <form> tag. Your generating an attribute nvm="YourAssembly.NavigationViewModel" (not a route value). If you used the correct overload to generate route values, which would be
using (Html.BeginForm("SinglePost", "Article", new { #nvm = Model.NavigationViewModel }))
it will still fail because it will generate something similar to (depending on you routes) action="/Article/SinglePost?nvm=YourAssembly.NavigationViewModel" so when you post back, the DefaultModelBinder will try to assign the string "YourAssembly.NavigationViewModel" to parameter nvm, but nvm is a complex object, not a string, so binding will fail.
You could make the POST method work by using
using (Html.BeginForm("SinglePost", "Article", Model.NavigationViewModel))
however this is just degrading performance by posting back unnecessary data and if your model contained properties that were complex objects or collections, it would fail anyway, so don't do it.
Finally, if you want to make the link look like a button, then style it using css.

Answer 2

Try to move hidden inputs into the form
<div class="post-nav">
#if (#Model.NavigationViewModel.PreviousString != null)
{
using (Html.BeginForm("SinglePost", "Article", FormMethod.Post, new { #nvm = Model.NavigationViewModel }))
{
#Html.HiddenFor(model => model.NavigationViewModel.PreviousID)
<input type="submit" class="btn btn-default" value="#Model.NavigationViewModel.PreviousString" />
}
}
#if (#Model.NavigationViewModel.NextString != null)
{
using (Html.BeginForm("SinglePost", "Article", FormMethod.Post, new { nvm = #Model.NavigationViewModel }))
{
#Html.HiddenFor(model => model.NavigationViewModel.NextID)
<input type="submit" class="btn btn-default" value="#Model.NavigationViewModel.NextString" />
}
}
</div>