ASP.NET MVC: How to maintain TextBox State when your ViewModel is a Collection/List/IEnumerable
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18-09-2019 - |
Question
I am using ASP.NET MVC 2 Beta. I can create a wizard like workflow using Steven Sanderson's technique (in his book Pro ASP.NET MVC Framework) except using Session instead of hidden form fields to preserve the data across requests. I can go back and forth between pages and maintain the values in a TextBox without any issue when my model is not a collection. An example would be a simple Person model:
public class Person
{
public string Name { get; set; }
public int Age { get; set; }
public string Email { get; set; }
}
But I am unable to get this to work when I pass around an IEnumerable. In my view I am trying to run through the Model and generate a TextBox for Name and Email for each Person in the list. I can generate the form fine and I can submit the form with my values and go to Step2. But when I click the Back button in Step2 it takes me back to Step1 with an empty form. None of the fields that I previously populated are there. There must be something I am missing. Can somebody help me out?
Here is my View:
<% using (Html.BeginForm()) { %>
<% int index = 0;
foreach (var person in Model) { %>
<fieldset>
<%= Html.Hidden("persons.index", index.ToString())%>
<div>Name: <%= Html.TextBox("persons[" + index.ToString() + "].Name")%></div>
<div>Email: <%= Html.TextBox("persons[" + index.ToString() + "].Email")%></div>
</fieldset>
<% index++;
} %>
<p><input type="submit" name="btnNext" value="Next >>" /></p>
<% } %>
And here is my controller:
public class PersonListController : Controller
{
public IEnumerable<Person> persons;
protected override void OnActionExecuting(ActionExecutingContext filterContext)
{
persons = (Session["persons"]
?? TempData["persons"]
?? new List<Person>()) as List<Person>;
// I've tried this with and without the prefix.
TryUpdateModel(persons, "persons");
}
protected override void OnResultExecuted(ResultExecutedContext filterContext)
{
Session["persons"] = persons;
if (filterContext.Result is RedirectToRouteResult)
TempData["persons"] = persons;
}
public ActionResult Step1(string btnBack, string btnNext)
{
if (btnNext != null)
return RedirectToAction("Step2");
// Setup some fake data
var personsList = new List<Person>
{
new Person { Name = "Jared", Email = "test@email.com", },
new Person { Name = "John", Email = "test2@email.com" }
};
// Populate the model with fake data the first time
// the action method is called only. This is to simulate
// pulling some data in from a DB.
if (persons == null || persons.Count() == 0)
persons = personsList;
return View(persons);
}
// Step2 is just a page that provides a back button to Step1
public ActionResult Step2(string btnBack, string btnNext)
{
if (btnBack != null)
return RedirectToAction("Step1");
return View(persons);
}
}
Solution
As far as I can tell, this is not supported in ASP.NET MVC 2 Beta, nor is it supported in ASP.NET MVC 2 RC. I dug through the MVC source code and it looks like Dictionaries are supported but not Models that are IEnumerable<> (or that contain nested IEnumerable objects) and it's inheritors like IList<>.
The issue is in the ViewDataDictionary class. Particularly, the GetPropertyValue method only provides a way to retrieve property values from dictionary properties (by calling GetIndexedPropertyValue) or simple properties by using the PropertyDescriptor.GetValue method to pull out the value.
To fix this, I created a GetCollectionPropertyValue method that handles Models that are collections (and even Models that contain nested collections). I am pasting the code here for reference. Note: I don't make any claims about elegance - in fact all the string parsing is pretty ugly, but it seems to be working. Here is the method:
// Can be used to pull out values from Models with collections and nested collections.
// E.g. Persons[0].Phones[3].AreaCode
private static ViewDataInfo GetCollectionPropertyValue(object indexableObject, string key)
{
Type enumerableType = TypeHelpers.ExtractGenericInterface(indexableObject.GetType(), typeof(IEnumerable<>));
if (enumerableType != null)
{
IList listOfModelElements = (IList)indexableObject;
int firstOpenBracketPosition = key.IndexOf('[');
int firstCloseBracketPosition = key.IndexOf(']');
string firstIndexString = key.Substring(firstOpenBracketPosition + 1, firstCloseBracketPosition - firstOpenBracketPosition - 1);
int firstIndex = 0;
bool canParse = int.TryParse(firstIndexString, out firstIndex);
object element = null;
// if the index was numeric we should be able to grab the element from the list
if (canParse)
element = listOfModelElements[firstIndex];
if (element != null)
{
int firstDotPosition = key.IndexOf('.');
int nextOpenBracketPosition = key.IndexOf('[', firstCloseBracketPosition);
PropertyDescriptor descriptor = TypeDescriptor.GetProperties(element).Find(key.Substring(firstDotPosition + 1), true);
// If the Model has nested collections, we need to keep digging recursively
if (nextOpenBracketPosition >= 0)
{
string nextObjectName = key.Substring(firstDotPosition+1, nextOpenBracketPosition-firstDotPosition-1);
string nextKey = key.Substring(firstDotPosition + 1);
PropertyInfo property = element.GetType().GetProperty(nextObjectName);
object nestedCollection = property.GetValue(element,null);
// Recursively pull out the nested value
return GetCollectionPropertyValue(nestedCollection, nextKey);
}
else
{
return new ViewDataInfo(() => descriptor.GetValue(element))
{
Container = indexableObject,
PropertyDescriptor = descriptor
};
}
}
}
return null;
}
And here is the modified GetPropertyValue method which calls the new method:
private static ViewDataInfo GetPropertyValue(object container, string propertyName) {
// This method handles one "segment" of a complex property expression
// First, we try to evaluate the property based on its indexer
ViewDataInfo value = GetIndexedPropertyValue(container, propertyName);
if (value != null) {
return value;
}
// If the indexer didn't return anything useful, continue...
// If the container is a ViewDataDictionary then treat its Model property
// as the container instead of the ViewDataDictionary itself.
ViewDataDictionary vdd = container as ViewDataDictionary;
if (vdd != null) {
container = vdd.Model;
}
// Second, we try to evaluate the property based on the assumption
// that it is a collection of some sort (e.g. IList<>, IEnumerable<>)
value = GetCollectionPropertyValue(container, propertyName);
if (value != null)
{
return value;
}
// If the container is null, we're out of options
if (container == null) {
return null;
}
// Third, we try to use PropertyDescriptors and treat the expression as a property name
PropertyDescriptor descriptor = TypeDescriptor.GetProperties(container).Find(propertyName, true);
if (descriptor == null) {
return null;
}
return new ViewDataInfo(() => descriptor.GetValue(container)) {
Container = container,
PropertyDescriptor = descriptor
};
}
Again, this is in the ViewDataDictionary.cs file in ASP.NET MVC 2 RC. Should I create a new issue to track this on the MVC codeplex site?